Help with my 1978 Mark I

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One thing that puzzles me is all the reports of the early Mark 1 amps being bright sounding. I remember playing through a teachers Mark 1 amp and it was incredibly bright.

My Mark 1 wasn’t nearly as bright and with lower gain. Ironically, I’m getting more gain from my SOB which was converted to a Mark 1 (the solid state device was blown and I wasn’t overly impressed with it).

Regards

Mark

I would certainly not describe mine as bright, I always thought it tended to be dark. Of course with high presence settings or bright knob pulled it can become bright, but not in a good way, it is not the place where the amp needs to be to have a balanced sound. To me it seems for a certain setup, there is a sweet spot of the presence pot, not really a choice of how bright you like the tone (to me at least).

When I swapped the speaker for an EV12L I was able to get a brighter sound, but I would call that clarity rather than brightness.

My more recent tone discovery is to run a cable directly in the FX loop, with the loop pot turned fairly low (and master a bit higher to compensate volume), this adds brightness, and on top of that I use a cry baby wah fully pushed and I roll down the tone pot on the guitar (and the presence pot a bit) to balance things out. I can get a pretty nice low volume sound that way. All that helps get more defined brightness in the tone. Bass at pretty much zero to keep the distortion focused.

Now I can't wait to get my hand on two 1.5k 0.5W resistors to try that and see how it affects the sound!

Fred
 
Ok so yesterday I spent some time tracing the preamp circuit (compared to the available drawings of the Mark I) and highlighted the differences.

Also I traced my effect loop mod and researched the components a bit. The yellow caps are Philips Mepolesco series (sometime known as Philips chiclets) and are considered good vintage caps. The datasheet dates from 1974. I have not found when those caps stopped being made, but it seems like the FX loop mod was done a long time ago.

Here are pictures of my tracing, and below a list of things I'd like to figure out.

6NVOcTb.jpg


JTjQX35.jpg


1) With the Fetron specific circuit, what is the point of having the 22k resistor above the two 82k load resistors, instead of having just the standard 100k load resistors? If converting for 12AX7, should'nt I remove the 22k, and end up with a standard supply and 100k load, along with the 1.5k cathode resistors? I don't necessarily plan on doing that, I mostly want to understand the impact of the current balance of the selected components.
2) Presence pot is not at the same location as in the schematic?
3) I will post later, but my bias circuit is quite different from the schematic.

4) Fx loop: your thoughts are welcome. I was surprised to see no load resistors at the plate, and all resistors at the cathode. Interestingly there is a voltage divider to send a fixed signal when nothing connected to the FX loop return.
5) The FX loop in indeed always in circuit, with both stages in use, but the pot is out of the circuit.

I started to review the power amp section and rectifier etc but not done yet, and that becomes harder with some wiring not visible under the boards. I was mostly interested to verify the pre amp and FX loop anyways.

I would be very interested to see pictures of an effect loop mod by Mesa to see how different they were doing it and maybe see some differences or similarities in components.

Fred
 
Ok, the big filter ones as well as the bias ones are now replaced so that's good.

I will look for a replacement part, but if I replace it I will be careful with the old one and try to keep it, in case it affects the tone in a way I don't like. I thought a decoupling cap would have the signal go through it but I looked again at the Mk I schematic and there is a 60uF 350V cap shown (top right, just above the 100k load resistor of stage 2), so if it is not in the signal path I am less worried. I will check next time I open the amp if the location matches that schematic. Being located there, does'nt it just act like an additional power supply filter?

That's fair enough. Mine has the F&T 47uf 500v in that position. I can't imagine the difference between that and the stock 40uf was significant. It did sound noticeably better afterwards. I had the fetron circuitry removed at a later date and that made a difference as well.

Also, you might want to measure the pot values. It seems like it's common for them to be pretty far off from the stated values, based on the amps I've had Mesa service.

As for the FX loop mod, I know someone traced the one in their IIB and posted a schematic. It's in a multi page thread on here somewhere. I don't think it's exactly the same as the version installed on the earlier amps though.

One thing that puzzles me is all the reports of the early Mark 1 amps being bright sounding. I remember playing through a teachers Mark 1 amp and it was incredibly bright.

My Mark 1 wasn’t nearly as bright and with lower gain. Ironically, I’m getting more gain from my SOB which was converted to a Mark 1 (the solid state device was blown and I wasn’t overly impressed with it).

Regards

Mark

I get the impression they became brighter with more drive towards 1978.

I was surprised at the amount of drive the SOB has, but it doesn't really sound like a MK 1 at all. Did converting it require a complete rebuild?
 
Ok so yesterday I spent some time tracing the preamp circuit (compared to the available drawings of the Mark I) and highlighted the differences.

Also I traced my effect loop mod and researched the components a bit. The yellow caps are Philips Mepolesco series (sometime known as Philips chiclets) and are considered good vintage caps. The datasheet dates from 1974. I have not found when those caps stopped being made, but it seems like the FX loop mod was done a long time ago.

Here are pictures of my tracing, and below a list of things I'd like to figure out.

6NVOcTb.jpg


JTjQX35.jpg


1) With the Fetron specific circuit, what is the point of having the 22k resistor above the two 82k load resistors, instead of having just the standard 100k load resistors? If converting for 12AX7, should'nt I remove the 22k, and end up with a standard supply and 100k load, along with the 1.5k cathode resistors? I don't necessarily plan on doing that, I mostly want to understand the impact of the current balance of the selected components.
2) Presence pot is not at the same location as in the schematic?
3) I will post later, but my bias circuit is quite different from the schematic.

4) Fx loop: your thoughts are welcome. I was surprised to see no load resistors at the plate, and all resistors at the cathode. Interestingly there is a voltage divider to send a fixed signal when nothing connected to the FX loop return.
5) The FX loop in indeed always in circuit, with both stages in use, but the pot is out of the circuit.

I started to review the power amp section and rectifier etc but not done yet, and that becomes harder with some wiring not visible under the boards. I was mostly interested to verify the pre amp and FX loop anyways.

I would be very interested to see pictures of an effect loop mod by Mesa to see how different they were doing it and maybe see some differences or similarities in components.

Fred
H Fred

Thanks so much for tracing out the circuit diagram. The P.I. stage looks the same as my original Mark 1 amp.

1. The 22K is there to decouple the power supply. My SOB amp didn’t have it and when I converted it to a Mark 1 amp, I found the amp went into oscillation with the new P.I. stage etc. I put this stage into my SOB too.

2. The Presence control is in keeping with the Mark 1 reissue amps.

http://guiguijones.free.fr/Electron...is Effets etc/Amplis/Mesa Boogie/mkirga_1.jpg
3. No problem, look forward to seeing bias control.

4. The effects loop looks like a decent design, it has a cathode follower on the send side which is why there are no plate resistors, this is a good thing as it can do longer cable runs without loses.

5. The two 12K resistors in series are providing constant signal throughout the amp. The link has the Mesa Mark 1 effects loop circuit.

The Mesa loop does go completely out of circuit without the need for a switch, but there is switching in the send and return jacks.
 
That's fair enough. Mine has the F&T 47uf 500v in that position. I can't imagine the difference between that and the stock 40uf was significant. It did sound noticeably better afterwards. I had the fetron circuitry removed at a later date and that made a difference as well.

Also, you might want to measure the pot values. It seems like it's common for them to be pretty far off from the stated values, based on the amps I've had Mesa service.

As for the FX loop mod, I know someone traced the one in their IIB and posted a schematic. It's in a multi page thread on here somewhere. I don't think it's exactly the same as the version installed on the earlier amps though.



I get the impression they became brighter with more drive towards 1978.

I was surprised at the amount of drive the SOB has, but it doesn't really sound like a MK 1 at all. Did converting it require a complete rebuild?

I didn’t think the SOB’s were as flexible as the Mark 1 amp they didn’t have the pull bright or the presence control. I think the output of the P.I. wasn’t particularly strong as the grid leak/bias resistors were increased to 330K.

Converting didn’t require a complete rebuild, it just needed a few mods here and there.

I will take a few photos of it in the future.

Regards

Mark
 
43113F68-6406-465A-84DF-957EE1D11F90.png

EB41250D-4C08-4975-AE2C-316B63232977.jpeg


Above is the Mark 1 reissue amp and below is the diagram simplified.

When nothing is plugged in output of the reverb return stage goes directly to the P. I. stage which is out of circuit.

When leads are plugged in the reverb return line going to the P. I. is open circuit. The signal can only go through the send pot. The return jack sends the signal to input of the effects return stage. The output of the effects return stage goes to the switch in the send Jack which is now closed and sending the signal to the P. I. stage.

The switching is smart in that plugging into the send only allows you to drive another P.I. stage and power amp, but given that there is a line out off the output transformer that doesn’t seem to be the desired outcome.

An additional note Fred, if you are after more gain from your amp you can either replace the reverb return 220K resistor with a 470K or put an additional 220K in series with the existing 220K. That way you keep the original resistor. I tried it in the SOB and I went back to the 220K resistor.

Regards

Mark
 
That's fair enough. Mine has the F&T 47uf 500v in that position. I can't imagine the difference between that and the stock 40uf was significant. It did sound noticeably better afterwards. I had the fetron circuitry removed at a later date and that made a difference as well.

Also, you might want to measure the pot values. It seems like it's common for them to be pretty far off from the stated values, based on the amps I've had Mesa service.

As for the FX loop mod, I know someone traced the one in their IIB and posted a schematic. It's in a multi page thread on here somewhere. I don't think it's exactly the same as the version installed on the earlier amps though.



I get the impression they became brighter with more drive towards 1978.

I was surprised at the amount of drive the SOB has, but it doesn't really sound like a MK 1 at all. Did converting it require a complete rebuild?

Thanks James for the feedback about that big cap on the preamp board. I'll try to find a replacement along these lines. For the pots, I don't have a good enough multimeter to measure (does not measure high enough resistance). Do you usually get a good reading of the pots while they are in circuit? I would have thought you'd read a bunch of the other components at the same time, in parallel, lowering the reading maybe.

I'll try to find the fx loop schematic just for fun of comparing. I have no intention of modifying mine since it works good and gives me access to better tones, but I'm interested in learning.

Today I played quite a bit with a newly acquired Boss EQ pedal in the FX loop. That, combined with a very low level of the FX loop send pot (the trick I mentioned earlier), I got a rather nice high gain tone. I don't understand why yet (when looking at the schematic) but very low send level creates a very bright and not much bass sound, but sounds much more high gain. Then with the EQ I dial in more bass, and boost the signal too so it is almost back to a normal level, while retaining the clarity. So I don't know why, but:

- Increasing the send level (from the very low setting) progressively lose the clarity and bring back the bass towards normal levels.
- Increasing the low send signal with the pedal instead does not lose the clarity created with the low send signal, even if the final level is back to a normal level.

This seems to mean the clear sound comes from the send stage of the fx loop (not the return stage). Do you guys understand why, by looking at the schematic? I kind of wonder if I am putting stress on a component by doing that. And if not, it is interesting and I might try to recreate that trick in a more easily integrated way if I ever end up making a custom amp project :)
 
H Fred

Thanks so much for tracing out the circuit diagram. The P.I. stage looks the same as my original Mark 1 amp.

1. The 22K is there to decouple the power supply. My SOB amp didn’t have it and when I converted it to a Mark 1 amp, I found the amp went into oscillation with the new P.I. stage etc. I put this stage into my SOB too.

2. The Presence control is in keeping with the Mark 1 reissue amps.

http://guiguijones.free.fr/Electron...is Effets etc/Amplis/Mesa Boogie/mkirga_1.jpg
3. No problem, look forward to seeing bias control.

4. The effects loop looks like a decent design, it has a cathode follower on the send side which is why there are no plate resistors, this is a good thing as it can do longer cable runs without loses.

5. The two 12K resistors in series are providing constant signal throughout the amp. The link has the Mesa Mark 1 effects loop circuit.

The Mesa loop does go completely out of circuit without the need for a switch, but there is switching in the send and return jacks.

Glad I could help, were you able to get the values you were after for the phase inverter? I think I got that covered in the revision of the schematic. If there is no note, it is because the one indicated was correct.

Thank you for the answers, I will have to teach myself on cathode follower circuits I guess!

On my fx loop a switching also occurs in the send jack, but as you can see in the schematic, it just returns the sent signal to the return circuit, to the same place as the actual return comes from if you connect a cable in the return jack.

But what I read about the Mesa loop is that it does not go out of circuit (the FX loop done on the MK1 and 2 by Mike B, I think the mod is also called the deaf louie). It stays in circuit, and contrary to mine even the FX loop pot stays in circuit. I think it is because that FX loop mod by Mike B. does more than just solving fx loop issues, it seems to solve some other issues people were having with the high gain tone.

I finished tracing the bias circuit but have not snapped a picture yet, I'll post that tomorrow. Thanks again for your valuable feedback!

Fred
 
Thanks James for the feedback about that big cap on the preamp board. I'll try to find a replacement along these lines. For the pots, I don't have a good enough multimeter to measure (does not measure high enough resistance). Do you usually get a good reading of the pots while they are in circuit? I would have thought you'd read a bunch of the other components at the same time, in parallel, lowering the reading maybe.

I'll try to find the fx loop schematic just for fun of comparing. I have no intention of modifying mine since it works good and gives me access to better tones, but I'm interested in learning.

Today I played quite a bit with a newly acquired Boss EQ pedal in the FX loop. That, combined with a very low level of the FX loop send pot (the trick I mentioned earlier), I got a rather nice high gain tone. I don't understand why yet (when looking at the schematic) but very low send level creates a very bright and not much bass sound, but sounds much more high gain. Then with the EQ I dial in more bass, and boost the signal too so it is almost back to a normal level, while retaining the clarity. So I don't know why, but:

- Increasing the send level (from the very low setting) progressively lose the clarity and bring back the bass towards normal levels.
- Increasing the low send signal with the pedal instead does not lose the clarity created with the low send signal, even if the final level is back to a normal level.

This seems to mean the clear sound comes from the send stage of the fx loop (not the return stage). Do you guys understand why, by looking at the schematic? I kind of wonder if I am putting stress on a component by doing that. And if not, it is interesting and I might try to recreate that trick in a more easily integrated way if I ever end up making a custom amp project :)

I haven't measured them myself - just going by what Mike B told me when he worked on them.

I'll have to get some pictures of my IIA with the loop mod. It's the tube driven version like the MK 1. It's very busy inside though - six preamp tubes now.

GE-7? I'd definitely keep a low send level with one of those, especially if you're using it to boost frequencies. I'd have a look at the Mesa EQ pedal too - there's a separate level for both the input and output, plus it works on the same frequencies as the internal graphic EQs.

I'm not sure really. My first thought was a bright cap on one of the pots, but it doesn't appear to have one.

I'd be interested in how it sounds with an external volume control (just a 500k pot) in the loop. You should be able to turn both masters up that way, but limit the overall output.

Yes I've heard it called the Deaf Louie mod before. It definitely changes the sound - main reason I had it done. Some say it turns the IIB into a poor man's IIC+ but I'm not sure I agree with that description.
 
But what I read about the Mesa loop is that it does not go out of circuit (the FX loop done on the MK1 and 2 by Mike B, I think the mod is also called the deaf louie). It stays in circuit, and contrary to mine even the FX loop pot stays in circuit. I think it is because that FX loop mod by Mike B. does more than just solving fx loop issues, it seems to solve some other issues people were having with the high gain tone.

28504417-4511-447A-ACBA-A4D395148B24.png
This is the Mark IIC effects loop. Its a strange loop. Have you tried pushing as much signal as possible into the cathode follower stage of the loop. It has this great thickening effect, perhaps you’ve already done it and are use to the sound?

I wonder what issues with gain tone were resolved by the effects loop. I had been thinking of putting a effects loop into my SOB as the previous owner drilled holes for an effects loop. A FET effects loop is the easiest option, though there are a couple of good valve circuits about too.

I will have to post pics of the SOB in two weeks to show you what my amp looks like.

Regards

Mark
 
I haven't measured them myself - just going by what Mike B told me when he worked on them.

I'll have to get some pictures of my IIA with the loop mod. It's the tube driven version like the MK 1. It's very busy inside though - six preamp tubes now.

GE-7? I'd definitely keep a low send level with one of those, especially if you're using it to boost frequencies. I'd have a look at the Mesa EQ pedal too - there's a separate level for both the input and output, plus it works on the same frequencies as the internal graphic EQs.

I'm not sure really. My first thought was a bright cap on one of the pots, but it doesn't appear to have one.

I'd be interested in how it sounds with an external volume control (just a 500k pot) in the loop. You should be able to turn both masters up that way, but limit the overall output.

Yes I've heard it called the Deaf Louie mod before. It definitely changes the sound - main reason I had it done. Some say it turns the IIB into a poor man's IIC+ but I'm not sure I agree with that description.

Yes the EQ is the GE-7. I knew it was far from the best but it was pretty cheap used available locally so I thought I'd give it a try and see if I like the results of tweaking the tone with an EQ in the FX loop. While searching before buying the GE-7 I did see the Mesa EQ pedal and liked the idea a lot, but it is like 380$ CAD here so I thought I'd try the cheap Boss first to get an idea. I like the effect it has but it always seems like when I want to tweak something, it does not cover the frequencies I would like to tweak. Maybe I'm just not used to what is tweakable or not with an EQ. For instance the presence and bright pull knob both seems to affect frequencies far above what the Boss EQ does. Anyway I'm off topic here :D

About the brightness at very low fx pot setting, I can only see it being because of the 22-35 cap, or maybe some special dynamics of the tubes with very low signal? It is also worth mentioning when I put the FX pot completely minimum position, I still get a very thin signal. Basically the extreme point of the bright sound I achieve by this method. Then I roll the pot up very slowly until there is suddenly just a bit of bass and other frequencies and a tiny bit mode volume. Then I stop there, crank the master at like 2 instead of being below 1 (or boost the FX signal with the EQ to bring back to normal volume instead of cranking the master).

I have an Ernie Ball volume pedal but it is the 250k model made for passive pickups. Do you think it will be a problem when used in the loop? If I do as you suggest though (master up, and FX send up, lowering the signal with the pedal) I suspect I will not recreate the sound I am after, since it is the opposite of what I'm currently doing, but it will be an interesting test for sure.

Thanks for bringing the ideas and feeding the discussion! Seems like I keep learning stuff every time you guys reply hahaha

Fred
 
This is the Mark IIC effects loop. Its a strange loop. Have you tried pushing as much signal as possible into the cathode follower stage of the loop. It has this great thickening effect, perhaps you’ve already done it and are use to the sound?

I wonder what issues with gain tone were resolved by the effects loop. I had been thinking of putting a effects loop into my SOB as the previous owner drilled holes for an effects loop. A FET effects loop is the easiest option, though there are a couple of good valve circuits about too.

I will have to post pics of the SOB in two weeks to show you what my amp looks like.

Regards

Mark

The first times I tried to connect a cable directly in the send-return, this is exactly what I tried, I thought I'd either get more gain or more aggressive sound but I did not notice much change except for the higher volume. But I have to say I did not play often with that setting, I will try some more. I am not sure about the gain tone effect of the Mike B FX loop mod as I have never tried any other Mark amp except my Mark I.

Last night I played again for quite some time and even tried the amp with the standard sound, no FX loop trick. We previously discussed how some say the Mark I is crazy bright sounding, while I felt like it was a very dark amp. In fact, it can become crazy bright if you roll up the treble (required to get good gain) without reducing the presence.

Without any FX loop trick, the following is pretty much the only usable low volume lead sound for me (with various gain 1 and gain 2 settings): high treble (9-10), low bass (3), mid around 7, and most importantly presence adjusted to tame down the excess brightness. Usually bright boost not pulled, unless I play some cleaner sounds. This gets me pretty good low volume sounds, for higher volume (which I don't do very often) I have to adjust presence a bit, don't quite recall how much though.

Thanks for the brainstorming!

Fred
 
I added some details in my schematics revision, I left the original above, below are the updated ones. What I changed:

- "Mojo" wire going from pin 3 of one of the power tubes, through a 220k resistor, and wraps tied around the 3 signal wires on grids of stages 1-2-3. Not sure how that adds to the circuit given it is open ended. You can probably see it in the pictures at the beginning of this thread (red wire wrapped around the gray coax signal cables).
- 5.6k value between two of the big 30-500 caps (not sure if the original schematic says 5.6 or 6.6 so I wrote it down).
- 220-300 big caps instead of 60-350.
- The diode and 470 ohm are flipped on the actual amp (does not change anything in the circuit though).
- Big differences in the bias circuit. Not sure why, any input is welcome. Cap values written are the originals (50-75), but I recapped with 47-100.
- Added references to the drawing (date of amp, serial number etc)
- Power circuit looks to be all matching (except for the mojo wire) so no change there.


IDlIp7M.jpg


ymCTlwQ.jpg
 
I have just ordered replacement cap for the preamp board 60uF 350V cap, along with resistors to put 100k and 1.5k instead of the 82k and diodes.

In order to reach the minimum amount for the order I also added a second cap, higher value (100uF 350V) thinking I could maybe use it as an alternative instead of the 60uF. Given its position in the schematic, is it to smooth and stabilize the voltage supply for stages 1 and 2 ? If that is the case, increasing the value like that (60uF to 100uF) be ok and just provide a more solid voltage? Could that provide a tighter sound somehow?

@James , you said yours came with a 40uF while mine is 60uF, do you remember yours manufacturing date or serial number? Maybe the tendency of Mesa to progressively build tighter gain amps through the years (until they did the IIC+), if yours was older than mine it could match that "story". But I'm just speculating so feel free to correct me if I'm wrong on the purpose of the cap or if it's a very bad idea to increase that value further.

@mark1406 You said this cap was to stop oscillation, and possibly add decoupling to the power supply. I guess that means filtering out fluctuations that could occur on the supply. What are your thoughts on increasing its value, would it just result in better filtering, more stable supply?

Regards,

Fred
 
Ok so yesterday I spent some time tracing the preamp circuit (compared to the available drawings of the Mark I) and highlighted the differences.

Also I traced my effect loop mod and researched the components a bit. The yellow caps are Philips Mepolesco series (sometime known as Philips chiclets) and are considered good vintage caps. The datasheet dates from 1974. I have not found when those caps stopped being made, but it seems like the FX loop mod was done a long time ago.

Here are pictures of my tracing, and below a list of things I'd like to figure out.

6NVOcTb.jpg


JTjQX35.jpg


1) With the Fetron specific circuit, what is the point of having the 22k resistor above the two 82k load resistors, instead of having just the standard 100k load resistors? If converting for 12AX7, should'nt I remove the 22k, and end up with a standard supply and 100k load, along with the 1.5k cathode resistors? I don't necessarily plan on doing that, I mostly want to understand the impact of the current balance of the selected components.
2) Presence pot is not at the same location as in the schematic?
3) I will post later, but my bias circuit is quite different from the schematic.

4) Fx loop: your thoughts are welcome. I was surprised to see no load resistors at the plate, and all resistors at the cathode. Interestingly there is a voltage divider to send a fixed signal when nothing connected to the FX loop return.
5) The FX loop in indeed always in circuit, with both stages in use, but the pot is out of the circuit.

I started to review the power amp section and rectifier etc but not done yet, and that becomes harder with some wiring not visible under the boards. I was mostly interested to verify the pre amp and FX loop anyways.

I would be very interested to see pictures of an effect loop mod by Mesa to see how different they were doing it and maybe see some differences or similarities in components.

Fred
Fred,
Don’t get around much anymore or I would have piped in earlier…

Back in the day, Randy toyed with several PI/NFB presence circuits based on either a tweed bassman and Blackface Fender, which is not news of course. During the 70’s Mark 1 run either might show up in any given amp. Looks like you got the Blackface setup. Fixed NFB tuning that holds the output section together like a blackface Twin until you start peeling paint above 7-8, when the output section can’t keep up and you get that tasty output saturation, or the police shut you down, whichever comes first.

Your “presence” is not a presence control at all (it’s wise not to believe a knob’s label in any mark amp, at least before the mark IV). Trust but verify.

What you have is a treble gate that rolls of highs before the PI instead. You can find a couple of Mesa variations of this. The mark 1 Reissue schematic posted earlier shows one like yours. Sometime after Gil drew that one, Mesa added a small value bleed cap from the wiper to ground on subsequent Mark 1 Reissues. Mesa used the same “presence” solution on the heartbreaker and lone star (without and with the bleed cap respectively), but, they moved the circuit from in front of the master to post master. You can find those schematics online if you want a peek.
Obviously, you can play with those configurations to see what you like.

The FX loop story is much longer.

The loop in the reissue schematic is similar to a couple of other Mesa amps, however, quite different from the mark 2/3 series implementations.

The one in the reissue has an extra gain stage that is only active if you plug into the loop. If you run a straight cable from send to return, then the FX level controls the additional drive for that section’s gain. If you have an effect in the loop, you can set it for the effect input level. This controlled by some Jack-Switch trickery. No plugs in the loop equals total circuit bypass.

The Mark 2b was the first implementation of a loop in the Boogie. As mentioned previously, Randy implemented it as a cathode follower fo a low impedance output circuit. This “neutered” that stage of the mark 2A’s triode with effective unity gain… But it had some issues. Somewhere along the arc of the later Mark 2B, Mike came up with a fix—he converted that stage back to a gain stage and added a post loop volume, which can also serve as a master-master volume and the original master controlled the effects loop input.

This is a fix that was available by request, and Mike continued doing it after Mesa stopped. The other part of that package was moving the reverb from the classic topology to a spot inline with the loop.

This is known as the B+ or FX/Reverb mod in modern parlance. There is a lot of work to do it, as the function of two tube’s triode are electronically swapped and noise is a consideration. The result is more, and smother, gain structure, better interplay between rhythm and lead modes, the ability to crank and open up the preamp without shattering windows—for half-deaf geriatric players, virtually at bedroom levels. More total gain than a 2C, less than a C+. In fact, these are the architectural changes that Mike applied to the 2C that Randy had stuck with a low-Z implementation, albeit in a different location), which ultimately spawned the C+ and subsequent Mark architectures. (If you try the c+ FX loop test on a B+, it will behave like a +. )

These loops are serial and always active in the circuit whenever an effect is plugged in.

In the mark 1, a loop could be added to a non reverb amp by utilizing the space for the missing reverb tube. Mike did a number of these, but yours does not look like one to my eyes. But asking is the only definitive way to know about that. In a reverb amp, this would be a bit more surgery.

Hope that provided some context and clues—enjoy the journey!

Nice drawing BTW; something of a lost art.
 
Fred,
Don’t get around much anymore or I would have piped in earlier…

Back in the day, Randy toyed with several PI/NFB presence circuits based on either a tweed bassman and Blackface Fender, which is not news of course. During the 70’s Mark 1 run either might show up in any given amp. Looks like you got the Blackface setup. Fixed NFB tuning that holds the output section together like a blackface Twin until you start peeling paint above 7-8, when the output section can’t keep up and you get that tasty output saturation, or the police shut you down, whichever comes first.

Your “presence” is not a presence control at all (it’s wise not to believe a knob’s label in any mark amp, at least before the mark IV). Trust but verify.

What you have is a treble gate that rolls of highs before the PI instead. You can find a couple of Mesa variations of this. The mark 1 Reissue schematic posted earlier shows one like yours. Sometime after Gil drew that one, Mesa added a small value bleed cap from the wiper to ground on subsequent Mark 1 Reissues. Mesa used the same “presence” solution on the heartbreaker and lone star (without and with the bleed cap respectively), but, they moved the circuit from in front of the master to post master. You can find those schematics online if you want a peek.
Obviously, you can play with those configurations to see what you like.

The FX loop story is much longer.

The loop in the reissue schematic is similar to a couple of other Mesa amps, however, quite different from the mark 2/3 series implementations.

The one in the reissue has an extra gain stage that is only active if you plug into the loop. If you run a straight cable from send to return, then the FX level controls the additional drive for that section’s gain. If you have an effect in the loop, you can set it for the effect input level. This controlled by some Jack-Switch trickery. No plugs in the loop equals total circuit bypass.

The Mark 2b was the first implementation of a loop in the Boogie. As mentioned previously, Randy implemented it as a cathode follower fo a low impedance output circuit. This “neutered” that stage of the mark 2A’s triode with effective unity gain… But it had some issues. Somewhere along the arc of the later Mark 2B, Mike came up with a fix—he converted that stage back to a gain stage and added a post loop volume, which can also serve as a master-master volume and the original master controlled the effects loop input.

This is a fix that was available by request, and Mike continued doing it after Mesa stopped. The other part of that package was moving the reverb from the classic topology to a spot inline with the loop.

This is known as the B+ or FX/Reverb mod in modern parlance. There is a lot of work to do it, as the function of two tube’s triode are electronically swapped and noise is a consideration. The result is more, and smother, gain structure, better interplay between rhythm and lead modes, the ability to crank and open up the preamp without shattering windows—for half-deaf geriatric players, virtually at bedroom levels. More total gain than a 2C, less than a C+. In fact, these are the architectural changes that Mike applied to the 2C that Randy had stuck with a low-Z implementation, albeit in a different location), which ultimately spawned the C+ and subsequent Mark architectures. (If you try the c+ FX loop test on a B+, it will behave like a +. )

These loops are serial and always active in the circuit whenever an effect is plugged in.

In the mark 1, a loop could be added to a non reverb amp by utilizing the space for the missing reverb tube. Mike did a number of these, but yours does not look like one to my eyes. But asking is the only definitive way to know about that. In a reverb amp, this would be a bit more surgery.

Hope that provided some context and clues—enjoy the journey!

Nice drawing BTW; something of a lost art.

Thank you "CrustyBugger" for the precious history details and your input on my amp! Helps me understand it more and why it is that way, and how different it potentially is from other variations of the Mark 1. I really like the presence control as it is so I do not think I am going to play with changing it. I think presence as it is now is one of the most important knob on the whole amp.

I did not know there were some Mark 1 with PI inspired by Blackface vs Bassman, that is very interesting to know. I figured NFB is Negative Feedback, so yes I do have it (FB wire going from output to the PI circuit), but I am surprised some Mark 1 had different ones. In your opinion, what would be the effect of interrupting or altering that feedback loop? I searched a bit and read that the more NFB you have, the more abrupt and higher volume the power amp distortion starts, and less touch sensitive than amps with no or less NFB. That brings my next question (or next google search haha), how do you reduce the NFB effect? I guess it has to do with the 820 ohms and 100 ohms resistors at the PI?

Thanks a bunch for the help, greatly appreciated, you guys rock!

Edit: I just google search for blackface negative feedback loop and found threads discussing the 820 ohm and 100 ohm resistors and how the ratio of them determines the negative feedback level. That is awesome, having access to some of you guy's minds even a tiny bit seems to give access to a whole next level of information hahaha :D Especially for an amp circuit noob like me! It looks like I have a NFB ratio of 820/100 = 8.2 compared to the standard Mark 1 schematic with 56k/10k = 5.6.

Some examples I took from thegearpage thread below:
5F6A Bassman : 27K/5K = 5.4 (just as the first JTM 45 Marshalls used)
6G6 Bassman: 100K/4.7K = 21.3
MD. 1959T Marshall = 82K/5K = 16.4

Higher value means less NFB if I understand correctly, so both my current values and the Mark 1 schematic kind of fall into the higher range of NFB.

https://www.thegearpage.net/board/i...ce-fender-negative-feedback-circuits.1716833/http://www.aikenamps.com/index.php/designing-for-global-negative-feedback
The series feedback resistor, in conjunction with the resistor to ground, determines the amount of voltage being fed back. If you want to feed back more voltage, you make the series resistor smaller, or the shunt resistor larger, or you use a higher impedance tap on the output transformer.

The actual resistor values used in the feedback attenuator aren't that important, as their ratio determines the amount of feedback. The shunt resistor value is usually fixed by the phase inverter design requirements, and the series resistor is then sized according to the desired amount of feedback, given the voltage available at the output. Note that Marshall typically uses 100K/5K attenuator, while Fender uses a 820ohms/100ohms.

Fred
 
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In the mark 1, a loop could be added to a non reverb amp by utilizing the space for the missing reverb tube. Mike did a number of these,
If you don’t mind I would love to see the circuit of this effects loop as I wish to put a loop into my SOB.

Some examples I took from thegearpage thread below:
5F6A Bassman : 27K/5K = 5.4 (just as the first JTM 45 Marshalls used)
6G6 Bassman: 100K/4.7K = 21.3
MD. 1959T Marshall = 82K/5K = 16.4
I don’t think this is the right way to think of negative feedback as it fails to take into consideration the current of the negative feedback signal.

Wouldn’t you think that you have a total resistance from the transformer secondary of 27K + 5K given a total of 32K. This network won’t offer the same current flow as a Blackface negative feedback loop would.

Regards

Mark
 
I don’t think this is the right way to think of negative feedback as it fails to take into consideration the current of the negative feedback signal.

Wouldn’t you think that you have a total resistance from the transformer secondary of 27K + 5K given a total of 32K. This network won’t offer the same current flow as a Blackface negative feedback loop would.

Regards

Mark

I am still learning so I really am unsure, but what I understand from the article below is the two resistor determines the feedback voltage, which appears to be the important factor given how the tube acts as a voltage amplifier. But it also says that the transformer tap is important in the overall calculation of the actual gain (of negative feedback).

So you are right that we cannot just generalize and say this other amp has x factor of negative feedback (the formula is more complex than that). But other variables are being equal (for instance if we stay on the same amp) then yes changing the amount of negative feedback seems to be simply to change the series resistor, and the ratio of change determine how much less or more feedback you end up with. So I'll refrain from comparing with other amps, unless I can verify that the remaining is comparable. Thanks for the observation!

Fred


http://www.aikenamps.com/index.php/designing-for-global-negative-feedback
 
I am still learning so I really am unsure, but what I understand from the article below is the two resistor determines the feedback voltage, which appears to be the important factor given how the tube acts as a voltage amplifier. But it also says that the transformer tap is important in the overall calculation of the actual gain (of negative feedback).

So you are right that we cannot just generalize and say this other amp has x factor of negative feedback (the formula is more complex than that). But other variables are being equal (for instance if we stay on the same amp) then yes changing the amount of negative feedback seems to be simply to change the series resistor, and the ratio of change determine how much less or more feedback you end up with. So I'll refrain from comparing with other amps, unless I can verify that the remaining is comparable. Thanks for the observation!

Fred


http://www.aikenamps.com/index.php/designing-for-global-negative-feedback
Fred,

It’s easy to make an amp sound bad, much harder to make one sound good…

Rather than focus on the math, start by finding an amp you like and understand why it does.

Understanding why NFB is used/manipulated is key to knowing what approach gets you what you want.

To your earlier query, vintage Vox and matchless amps have output sections without it. Leo Fender was searching for clarity, and so, used it in the BF circuit to keep the output clean as long as possible as the volume increases.

For a variable NFB solution, the designer is picking frequencies to focus that control. For example, reducing lower frequency harmonic distortion in the output.

If you look at the progression of presence control circuits in the Mark amps you will see that Randy progressively got more frequency tuning going on in there—making something of an output section tone control going on.

The BF type PI/NFB circuit is a great one, so yeah, if you like it, stick with it. In fact, it is said that Santana preferred that mode on the King Snake, FWIW.

The “presence” pot on your diagram has the high-cut cap drawn backwards to Mesa’s implementation—have a look at Gil’s diagram—the cap straps the signal leg to the sweep leg, not sweep to ground. This also allows the added 250pf cap to be added from sweep to ground, if you want to knock off the top of the signal.

In high gain situations you EQ the preamp for upper frequencies to keep it from muddying up and farting out. This “presence” control (actually a post preamp tone control) allows the player to tone down this top end frequencies to warm the amp back up.

This is of even more use on the reissue which is more gained up than the original with more bottom end trash as the result. ( hint, don’t “update” your mk1 by blindly copying the RI circuit values.

The amp works a a system, always think in those terms rather than individual components or sections.

On the topic of caps… if you take the amp in to Petaluma, the first thing Mike will do is swap the electrolytics for known good Mesa parts. He used to leave good ones in the amps, but knowing that older ones will be degrading soon, if not already, and knowing that a lot of name brand caps are now of poor quality, he replaces them with parts built to Mesa’s specs.

Your power supply and bias caps can make a huge difference in the performance of your amp, bringing it back to how it was meant to sound /feel. Food for thought… YMMV.

Mark,

I don’t have a schematic in my notes for a Mesa retrofitted loop. I would suggest following the 2C+/mk3 circuit as a model. There was a guy a decade or decade and a half ago maybe who drafted the circuit that was in his B. I’ll see if I can find his name or post in my rats nest of notes. The B solution would likely be the one used to retrofit and the post effects volume is what would be called for in a mk1/modded SoB.

However, I am loath to post any creative or original content directly to the board, as the user agreement states that the company running it owns copyright to all posted content, unless it can be otherwise defended. If you want to argue it, go to Panama where such things are managed. ..not called Crusty for nothing!
 
Not sure if anyone answered your 12AT7 vs 12AX7 but yes, the 12AT7 is drop in replacement for the 12AX7. They are both twintriodes. The 12AX7 has an amplication factor of 100, and I think the 12AT7 is like 70. Also you can use the 7025 (ala Fender) or a 12AU7 (Also used in some Fender's), but I believe the gain is even lower in the AU7. Sorry if someone already answered this. I mainly just skimmed so far. Boring work day...
 

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