Help with my 1978 Mark I

[James]

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

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

Yeah I like the GE-7, but found it works best before distortion if you're boosting anything. A parametric EQ might be worth trying as well, though there are fewer options for those.

That's interesting. The MK 1 with the factory mod behaves in a very similar way - fairly bright and thin with the front master at low settings, but probably due to the bright cap. I know my IIA doesn't have one and is a bit more consistent at lower levels. I imagine there are a few variations of this mod - probably depending on the amp it's being installed into.

Yeah that's fine. The JHS volume box (which is specifically for this purpose) uses a 100k if I recall.

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

It's dated 9-77. Based on the ones I've owned/tried, I'd say that's correct. For what it's worth, I've seen one amp that Mesa serviced and it had the 30uf cap in that position.

I believe mine has 100uf mains caps too. I can't remember if that was the original value or if it was 60uf like the earlier amps. I'll have to see if I have any pictures of when it was stock.

 

[mark1406]

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

The negative feedback reduces gain, that can be good and bad. If you have a high gain amp and you are getting the sound via the preamp and you will want that power amp to stay tight. More feedback is good.

No feedback means more gain, but the bass response can be flabby sounding. Getting the P.I. stage to distort can make the gain sound more amp like, though some may not like it.

P.I. DC operation

P.I. AC operation

In my SOB to Mk 1 amp, I currently have two master volumes. One is the stock Mesa pre P.I. master volume and the other is the LarMar post P.I. master volume.
I can dial in how hard I drive the P.I. and the post P.I. Master volume has an effect of the negative feedback.

The original SOB doesn’t have negative feedback and it was a factor I didn’t like about the amp. The bass was too flabby and the amp sounded mushy.

I seem to remember trying the earlier P.I. circuit with the 56K negative feedback and the Tweed like P.I. stage, it added a stack of gain and the amp was less flexible.

My SOB has a bit of that going on. The Mark 1 I owned years ago was very flexible but I thought it lacked gain. I should have bought a Tubescreamer but I was indignant that I own a high gain amp so why should I buy an effect to add gain.

Regards

Mark

 

[mark1406]

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...

I dropped a 12AT7 in the P.I. stage, the stage is a bit brighter and there isn’t as much gain as a 12AX7.

Regards

Mark

 

[mark1406]

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!

No problem, if you had the circuit it would have been good to try it out.

Having the loop after the master volume seems to be the convention these days. This what I originally had in mind.

I can try my luck and ask Mesa, they can only say no at worst.

Regards

Mark

 

[mark1406]

I am still learning

Check out these links.

All the calculators you could need.

Regards

Mark

 

[mark1406]

I found this link where I’m discussing the rebuild of my amp as it’s going along.

Amp Garage post

I had a moment of clarity and realised that the effects loop across the 2.2M/20pF reverb network has two common cathode triodes due to the phase of the signal. It basically reverb circuit tweaked into an effects loop.

I will have to think about it before I come up with a circuit.

Regards

Mark

 

[mark1406]

To update, I received a reply from Mike Bendinelli and he states:

“I have installed a loop on a very few original MK1 Mesa amps. I inserted after the master, at the input to the phase inverter.”

In another thread there is talk of effects loops being inserted where the reverb circuit would have been. Mike Bendinelli states he has never implemented this sort of effects loop.
Mike didn’t give me a circuit as he said there were no records of the circuit. It’s fairly safe to assume it would be a cathode follower and a common cathode stage as used with Dumble amps.

I hope this is of assistance.

Fred, this is more or less what you already have in your amp.

Regards

Mark

 

[James]

That makes sense. I wonder if they have records of the amps that were modified. I believe they generally do, but it might be too long ago in this case.

Also, here's the thread with the IIB mod info that I mentioned earlier. Pictures are offline, but still worth a read.

https://boogieforum.com/threads/boogie-mkiib-rev-eq-60-100w-head.59554/

 

[bluebug]

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!

Thank you CrustyBooger for your input. I certainly don't want to fiddle with things and end up with a bad sounding amp, yes I want to understand why it sounds like it does, and if some reversible minor modifications can be done to explore new sounds, who knows, maybe I'll do some tests in the future. But maybe not on this amp

I will double check the way I draw the presence high-cut cap (backwards) just to make sure I did not make a mistake. That's good to know too. You description of the high gain use (as much high frequency in the preamp and tame down with the presence) is exactly what I do pretty much all the time since I got the amp, unless I play very low gain.

The amp is almost all re-capped now (all the big caps, and the bias ones), only waiting for the additional 60uF/350v on the preamp board. I used all Mesa caps, except for this last one on the preamp board because the Mesa store did not have it, but it is a Sprague Atom, so I think it compares ok with what Mesa sells. I am building up my tube amp knowledge slowly (well, quickly in a sense if I compare to what I knew a few weeks ago, but slowly compared to everything there is to possibly learn!), hopefully the capability to understand the amp as a system will start being a possibility at some point!

Fred

 

[bluebug]

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...

Thanks twintriode for the information! I did play with using all 12AX7 many years ago (instead of the 12AT7) but I did not feel like the difference was big. I could try that again though since I put more time on understanding the amp through the years.

Yeah I like the GE-7, but found it works best before distortion if you're boosting anything. A parametric EQ might be worth trying as well, though there are fewer options for those.

That's interesting. The MK 1 with the factory mod behaves in a very similar way - fairly bright and thin with the front master at low settings, but probably due to the bright cap. I know my IIA doesn't have one and is a bit more consistent at lower levels. I imagine there are a few variations of this mod - probably depending on the amp it's being installed into.

Yeah that's fine. The JHS volume box (which is specifically for this purpose) uses a 100k if I recall.



It's dated 9-77. Based on the ones I've owned/tried, I'd say that's correct. For what it's worth, I've seen one amp that Mesa serviced and it had the 30uf cap in that position.

I believe mine has 100uf mains caps too. I can't remember if that was the original value or if it was 60uf like the earlier amps. I'll have to see if I have any pictures of when it was stock.

I will try the EQ before the amp to see how I like it, really a different purpose, but still important (getting more control over what hits the preamp gain stages). Kind of how recently I like to put the wah wah fully pushed to get extra highs, but have to roll down the highs somewhere else to get a balanced tone. Thing is with the wah, it is quite extreme and makes a lot of frequencies disappear that I cannot make appear again later even with the Boss EQ.

I tried the volume pedal in the fx loop, and I still get the same behavior, as long as I keep the overall return volume low I get the bright and somewhat thin tone needed to get a pseudo tight high-gain sound. It did seem like with the volume pedal I was able to get higher volume without completely losing the bright effect, but I'll have to play with it more to experiment. Thanks for confirming the date and 30uF cap. Can you elaborate on what kind of improvement you remember having after replacing your old 30uF with the 47uF? Can't wait to try my my replacement 60uF (my factory value) that's in the mail now. Or even try the 100uF? What do you guys think about that idea? I am not going to blindly try that, I'd like to understand its purpose more and read what more experienced people have to say about it before even thinking of putting the higher value in.

Fred

 

[bluebug]

The negative feedback reduces gain, that can be good and bad. If you have a high gain amp and you are getting the sound via the preamp and you will want that power amp to stay tight. More feedback is good.

No feedback means more gain, but the bass response can be flabby sounding. Getting the P.I. stage to distort can make the gain sound more amp like, though some may not like it.

P.I. DC operation

P.I. AC operation

In my SOB to Mk 1 amp, I currently have two master volumes. One is the stock Mesa pre P.I. master volume and the other is the LarMar post P.I. master volume.
I can dial in how hard I drive the P.I. and the post P.I. Master volume has an effect of the negative feedback.

The original SOB doesn’t have negative feedback and it was a factor I didn’t like about the amp. The bass was too flabby and the amp sounded mushy.

I seem to remember trying the earlier P.I. circuit with the 56K negative feedback and the Tweed like P.I. stage, it added a stack of gain and the amp was less flexible.

My SOB has a bit of that going on. The Mark 1 I owned years ago was very flexible but I thought it lacked gain. I should have bought a Tubescreamer but I was indignant that I own a high gain amp so why should I buy an effect to add gain.

Regards

Mark

The more I read about NFB, the more I think yes I do like my amp as it currently is, but I also feel like there would be some interesting tones to try with a reduced NFB. If it ends up being a very reversible mod, maybe at some point I will give it a try, but I'm not ready for that yet. I have the type of personality that likes to do things correctly, but for some reason I also like to tackle complicated things. So things end up taking time

Check out these links.

All the calculators you could need.

Regards

Mark

Thats a shipload of calculators! Thank you, I bookmarked that and will probably soon have to use some of them give the project I just tackled. I just scored an old tube stereo console (the big furniture sized ones), full working, that I plan to use and repurpose as guitar amp to learn and tweak. There are already some known kit amps using the tubes in it, so I am slowly getting my ideas clearer about what I'd like to do with it. This will be a good cheap base to learn I think. It is from the early 60s I think. I posted in another forum where I noticed there was other threads about that, I will check if there is also a relevant forum section for that here, but being a boogie forum I did not initially think it would be the right place.

Edit: I also just created a thread here on the Boogie forum, in the Other stuff section:
https://boogieforum.com/threads/converting-sparton-tube-console-to-amp.86773/
https://www.thegearpage.net/board/index.php?threads/converting-sparton-tube-console-to-amp.2427540/

I found this link where I’m discussing the rebuild of my amp as it’s going along.

Amp Garage post

I had a moment of clarity and realised that the effects loop across the 2.2M/20pF reverb network has two common cathode triodes due to the phase of the signal. It basically reverb circuit tweaked into an effects loop.

I will have to think about it before I come up with a circuit.

Regards

Mark

I'm not sure I understand the thought process here, but I'll wait for your conclusions and see if I can learn a bit further from it

To update, I received a reply from Mike Bendinelli and he states:

“I have installed a loop on a very few original MK1 Mesa amps. I inserted after the master, at the input to the phase inverter.”

In another thread there is talk of effects loops being inserted where the reverb circuit would have been. Mike Bendinelli states he has never implemented this sort of effects loop.
Mike didn’t give me a circuit as he said there were no records of the circuit. It’s fairly safe to assume it would be a cathode follower and a common cathode stage as used with Dumble amps.

I hope this is of assistance.

Fred, this is more or less what you already have in your amp.

Regards

Mark

That's awesome you got to ask him, so it looks like mine being before the master confirms it is different for his fx loop work of the time. But I guess the rest of the FX loop maybe was of a similar design, anyways I like it how it is and it was nice to learn about the differences, even if the original mod is not clearly documented. Thanks for reaching out to him on this!

That makes sense. I wonder if they have records of the amps that were modified. I believe they generally do, but it might be too long ago in this case.

Also, here's the thread with the IIB mod info that I mentioned earlier. Pictures are offline, but still worth a read.

https://boogieforum.com/threads/boogie-mkiib-rev-eq-60-100w-head.59554/

Thanks for the link James, I tried to use the Way Back Machine to see if that page or schematic was saved there but apparently not, but an interesting read none the less!

Frederik

 

[James]

Thanks for the link James, I tried to use the Way Back Machine to see if that page or schematic was saved there but apparently not, but an interesting read none the less!

Frederik

Maybe send a message to that user and ask if he'd be willing to send it. He was active last month.

I will try the EQ before the amp to see how I like it, really a different purpose, but still important (getting more control over what hits the preamp gain stages). Kind of how recently I like to put the wah wah fully pushed to get extra highs, but have to roll down the highs somewhere else to get a balanced tone. Thing is with the wah, it is quite extreme and makes a lot of frequencies disappear that I cannot make appear again later even with the Boss EQ.

I tried the volume pedal in the fx loop, and I still get the same behavior, as long as I keep the overall return volume low I get the bright and somewhat thin tone needed to get a pseudo tight high-gain sound. It did seem like with the volume pedal I was able to get higher volume without completely losing the bright effect, but I'll have to play with it more to experiment. Thanks for confirming the date and 30uF cap. Can you elaborate on what kind of improvement you remember having after replacing your old 30uF with the 47uF? Can't wait to try my my replacement 60uF (my factory value) that's in the mail now. Or even try the 100uF? What do you guys think about that idea? I am not going to blindly try that, I'd like to understand its purpose more and read what more experienced people have to say about it before even thinking of putting the higher value in.

Fred

Exactly. I use it as a backwards boost of sorts. It's on most of the time - mostly cutting out some of the lower frequencies. I'd try a parametric EQ for that - you can dial it in to match the frequency of a wah pedal, but have it boost a wider range. The guitar's tone control could be useful too.

I actually found the old parts the other day. It was a 60uf like yours, rather than a 40uf. It's difficult to say, since it had the stock electrolytic caps and all were replaced at the same time. It had two 60uf caps for the mains as well, which were replaced with 100uf.

From what I recall, the overall sound didn't really change. It was definitely a lot noisier on idle and sounded a bit weak, but that was likely due to age and a previous repair in the bias circuit.

There was far more of a difference when Mike B optimized it for the 12AX7 in V1 a few years later. A bit thicker sounding with more drive and sustain.

 

[mark1406]

That's awesome you got to ask him, so it looks like mine being before the master confirms it is different for his fx loop work of the time. But I guess the rest of the FX loop maybe was of a similar design, anyways I like it how it is and it was nice to learn about the differences, even if the original mod is not clearly documented. Thanks for reaching out to him on this!
Frederik

I would leave the loop in place. It’s a good design. You may wish to see if it’s transparent by bypassing it with a switch (temporarily without drilling holes in the chassis.)

Regards

Mark

 

[mark1406]

Hi Fred

In your circuit diagram, I think there is one error. Vol 1 has an 0.047uF cap across it in red. If this cap were in the circuit the amp would either have no output when the Vol 1 was set to zero or full output when the volume control was turned up. The cap could also shunt signal on the clean channel depending on hot the Vol 1 is set.
I have included a picture of your amp and I can’t see the cap across the Vol 1 control. Possibly a pF value cap which are small, but no Orange Drop caps.

As far as I can remember, my Mark 1 amp didn’t have a cap across Vol 1, of course that’s only one example of the Mark 1 amp.

Regards

Mark

 

[bluebug]

Hi Fred

In your circuit diagram, I think there is one error. Vol 1 has an 0.047uF cap across it in red. If this cap were in the circuit the amp would either have no output when the Vol 1 was set to zero or full output when the volume control was turned up. The cap could also shunt signal on the clean channel depending on hot the Vol 1 is set.
I have included a picture of your amp and I can’t see the cap across the Vol 1 control. Possibly a pF value cap which are small, but no Orange Drop caps.

As far as I can remember, my Mark 1 amp didn’t have a cap across Vol 1, of course that’s only one example of the Mark 1 amp.

Regards

Mark

Hello Mark,

Regarding the FX loop, I have no intention of getting rid of it, I was mostly doing some archeology I was interested to know more about the story of my amp.

Regarding the cap, sorry for the delay, the devices I was on did not have the password memorized and I could not get back to you when I wanted, but also the amp was put back together, I just opened it again to double check. The cap is really there, I took some more pictures to show it clearly, it is between the cursor of the volume 1 pot, and the signal of input 2. The third pin (upper one) of volume 1 is to the ground, folded directly on the pot's body.

Regarding volume 1 and 2 and their fully zero position, you are correct, if either volume 1 or 2 is completely off, there is no sound. When plugged in input 2 (low gain) volume 1 has no effect.

 

[mark1406]

Thanks for opening the amp and taking the pictures of the first volume control. I couldn’t see in the pictures, but what value did you see written on the cap when you had the amp apart?

I still find it hard to believe that an 0.047uF cap would be in that position, I could believe a 470pF (471), or maybe a 4700pF (472) as those values would be tone shaping. I see a 0.047uF (473) affecting the taper of the pot making it almost reverse logarithmic.

Thanks again for all your assistance, I appreciate the effort you have gone to on my behalf.

Regards

Mark

 

[bluebug]

Thanks for opening the amp and taking the pictures of the first volume control. I couldn’t see in the pictures, but what value did you see written on the cap when you had the amp apart?

I still find it hard to believe that an 0.047uF cap would be in that position, I could believe a 470pF (471), or maybe a 4700pF (472) as those values would be tone shaping. I see a 0.047uF (473) affecting the taper of the pot making it almost reverse logarithmic.

Thanks again for all your assistance, I appreciate the effort you have gone to on my behalf.

Regards

Mark

Hello Mark,

I'm glad I could help you a bit with it. I looked again and the value is actually 47K, so if I understand correctly, 47K being 47pF it would be 0.047nF and not uF! Sorry about the mistake, I will update the schematic, below is a picture of the cap with the writing on it.

Fred

 

[mark1406]

Hello Mark,

I'm glad I could help you a bit with it. I looked again and the value is actually 47K, so if I understand correctly, 47K being 47pF it would be 0.047nF and not uF! Sorry about the mistake, I will update the schematic, below is a picture of the cap with the writing on it.

Fred

Again thank you so much for chasing this up for me. I think it would be 47pF and the K would refer to 10% tolerance.

I will have to try it out and see what it’s like. Usually 47pF is quite subtle. It might allow more noise through though.

Regards

Mark

 

[bluebug]

No problem, you are welcome. I read somewhere when there are only two digits before a K it means the value is exactly in pF. But caps markings “conventions” are quite eclectic, if not cahotic lol

 

[Joey1970]

The original picture does not look like the "Post FX" loop mod that was done to my 1978 model, at MESA. It worked very well at controlling the volume, and a Boss OD-1 in the front of the signal chain.