Help with my 1978 Mark I

[bluebug]

Hi all,

I have just joined the forum, this is my first message. Reading other's stories and posts made me think this was a nice place to talk about our amps and try to understand more things about them. Anyways in 1997 I bought my 1978 (3-78 written on the chassis) Mark I in my first year of college and still have it today. It is serial number A1747, it is a 60w without reverb nor EQ. I talked to the original owner and it originally had a JBL K120, which he replaced with a Celestion Vintage 30 that was there when I bought it. At some point the Celestion started to fail, then I used a spare G12B-150 I had, then a JBL E120 and I now have a EV12L in it which I like best. It was obviously modified to add a tube effect loop with an additional 12AX7, clearly not factory, plastic jacks, tiny pot, and done in a weird floating point to point kind of way.

I have quite a bit of questions about it. First I have to say I do have an electrical background, I studied electrical engineering, but what I actually do is mostly automation and programming and I do not have a very deep knowledge of tube amps compared to some of you guys. So please bear with me while I fail to use the correct terms for the function of the various components in the little Boogie. Reading your discussions of the circuit variations of those amps I feel there is quite a knowledge gap with me, but I would certainly love to learn more, and I enjoy learning complicated stuff so who knows, maybe the new information will sink in better than I expect :lol:

I am having some issues with it so I started searching. It seems in those years it likely originally came with a Fetron. I tried to search for clear indications of that but I am not 100% sure. So I realized maybe I am running it less than optimal since the beginning since I am using two 12AX7 at V1 and V2. I saw some references of how to modify to use regular 12AX7, for instance this thread:

http://www.grailtone.com/forum/viewtopic.php?t=4319

and it pointed to the resistor values and replacing the TCR5302 things on V1 with resistors amongst other things. The fact I have those things below, do you think it means it is still set up to use a fetron?

- TCR5302 on 2 places on V1, I read that those should be replaced by 1500 ohm 1/2 watt resistor parallel with the existing 22uF, 25V cap...
- There are two 82k resistors on V1, I read that should be 100k for a 12AX7...
- There is a big 60uF 350V capacitor next to the first stages board, not every Mark I seems to have that.
- The first tube is identified as a 12AX7/AT7 with permanent marker (seems like the original markings, same as the date and signature). Is this like that when it actually comes with a Fetron? The manual used to say that it could use either Fetron or 12AX7, but I read the result was sub par when using a 12AX7 without modifying the circuit.

Source of the 82k vs 100k and TCR5302 vs 1.5k resistor comment:
https://steelguitarforum.com/Forum11/HTML/000274.html


So, does it looks to you it is still made for fetron? If so I would certainly like to modify it to optimally work with 12AX7, but would need some help to figure this out.

Some maintenance I have done:

- Replaced one cap (don't ask me why I only did one, I replaced only the one that appeared to have swelled, did not know back then that recap was a thing and should have done all)
- Change the tubes a few times
- Corrected bent chassis (around the middle, close to speaked jacks, probably previous owner dropped it with a long jack in there instead of the angled one).
- Replaced cracked 2W resistors on the power tubes (called screen grid resistors I think?)
- Did a hack job of replacing the power on light that was not working anymore, and also the power switch that failed. I say hack job because I did not use Mesa components, and I am not that proud of how did the job back then.

An actual problem I have is the sound tends to cut off pretty much completely (only a very faint and trebly sound) and it would typically come back to life if I put into standby and back on after a few seconds. It seems to do the problem less when I play fairly loud for extended time (for instance for a gig), and more when I play at lower volume, and maybe higher gain. I'm not quite sure since it is quite random. But more recently it started to not return after the standby sequence and I have to do it many times, or shut off the amp completely. I am sure the amp is due for a complete re-cap now anyways, but are the old caps the plausible cause for that kind of issues, or is there likely something else going on?

If you have ideas of measurements to do (I only have a voltmeter, no fancy equipment) while the problem is occurring let me know. I tried to measure voltages at the different pins of the power tubes to compare between working and failed state but found no discrepancy. It does not help that I don't really know the meaning of my measurements

Other questions:

- Why is there no 60uF 350V caps on the Mesa store? Do I need to get rid of that cap anyways for the fetron to 12AX7 mod? If not, where is a good place for caps other than from Mesa directly?
- What do you think of the effect loop mod and how it was done? I use a Lexicon LXP15-2 multi effect in it for my reverb so I don't really want to get rid of it, but I would be curious if you guys think it is garbage or asking for trouble to have it.
- Do I need to change more caps than the big ones, maybe the two silver ones next to the two 220uF 300V ones? (50uF 25V if I recall correctly)

Sorry for the long text, hopefully some of you made it to here I will post some more picture in the next posts, somehow adding pics makes the post appears like spam and I cannot post.

Thank you for any help!

Frederik

 

[bluebug]

Here are some pictures, if you would like to see more or know the values of some of the components in order to help with my questions please let me know:

Close up of the effect loop stuff:

Markings on the chassis:

 

[dalepsmith]

Nice Pics!

What are those extra gray wires and extra jacks and pot on the back? I'm guessing some kind of effects loop?

Was that Mesa installed? Or...?

 

[bluebug]

For some reason the forum was not allowing me to put the pictures in the first post... It would be clearer if I could put the pics close to their description, but yes it appears to be modified to add a tube effect loop with an additional 12AX7, quite clearly not factory because of the plastic jacks, tiny pot, and done in a weird floating point to point kind of way. It does not looks Mesa style at all.

 

[bluebug]

Ok so I went anyway and ordered the capacitors. Also added two 0.1uF 600V for coupler caps just in case, and used the 47uF 100V to replace the factory 50uf 75V bias caps. For now I don't know about the single big 30uF 350V one, is it related to the Fetron or not, and it was not in the Mesa store anyway.


Now going back to the effect loop mod on this one, I saw internals of some more recent Mark series amps and they use similar plastic jacks like on my effect loop. This makes me re-think that maybe the effect loop could be by Mesa Boogie? It would be sometime between 1978 and 1997 the year I got it. The gray cabling certainly looks factory. Still not sure about that smaller pot, and the assembly type with components holding one another in the air like this, but the more I think of it, maybe that's the way to go for this type of mod? What do you guys think?

One cool feature I found about this effect loop, if I just put a direct cable in it, and roll the pot down lower than 1:1 signal, the sound gets progressively brighter (and of course lower) up to a point of sounding like earphones when turned all the way down. Somewhere in between I get a tighter and brighter sound that I just can't normally get since the amp is always quite dark sounding. Yesterday I played quite a bit with that setting, bass to zero, high mid and treble (about 9), presence rolled down about 25%, and I could get a low volume tone that reminded me of Larry Carlton's sound on his first album, or early Santana's sound, which are both quite bright considering they come from a Mark 1 (ok maybe I'm kind of dreaming here and you'd say my tone was not quite there at all, but that tone still felt pretty good )


Still looking for information wether or not this amp looks like it needs to be modded to optimally run a 12AT7 or 12AX7 in V1 vs the fetron that maybe was there from factory? The 12AX7/AT7 marking is confusing me, compared to the components values that seems to match Fetron equipped Mark amps.

Thanks for any help or chat about this! I've had this amp for a long time, but never really had anyone to discuss it, I'm glad to have found this place!

Frederik

 

[bluebug]

Anyone have some hints for me?


Anyways in the meantime, I thought it would be funny to show you the mini-me amp I made sometime around 1998. It actually works, with internals from a basic amplified speakers kit for computers.

It originally had a round paper cone small full range speaker, with chrome dust cap that looked a lot like a Vintage 30 from the back, and the JBL from the front but I blew it so replaced it with some PC computer speakers I had laying around.

 

[mark1406]

Hi all,

I have just joined the forum, this is my first message. Reading other's stories and posts made me think this was a nice place to talk about our amps and try to understand more things about them. Anyways in 1997 I bought my 1978 (3-78 written on the chassis) Mark I in my first year of college and still have it today. It is serial number A1747, it is a 60w without reverb nor EQ. I talked to the original owner and it originally had a JBL K120, which he replaced with a Celestion Vintage 30 that was there when I bought it. At some point the Celestion started to fail, then I used a spare G12B-150 I had, then a JBL E120 and I now have a EV12L in it which I like best. It was obviously modified to add a tube effect loop with an additional 12AX7, clearly not factory, plastic jacks, tiny pot, and done in a weird floating point to point kind of way.

I have quite a bit of questions about it. First I have to say I do have an electrical background, I studied electrical engineering, but what I actually do is mostly automation and programming and I do not have a very deep knowledge of tube amps compared to some of you guys. So please bear with me while I fail to use the correct terms for the function of the various components in the little Boogie. Reading your discussions of the circuit variations of those amps I feel there is quite a knowledge gap with me, but I would certainly love to learn more, and I enjoy learning complicated stuff so who knows, maybe the new information will sink in better than I expect :lol:

I am having some issues with it so I started searching. It seems in those years it likely originally came with a Fetron. I tried to search for clear indications of that but I am not 100% sure. So I realized maybe I am running it less than optimal since the beginning since I am using two 12AX7 at V1 and V2. I saw some references of how to modify to use regular 12AX7, for instance this thread:

http://www.grailtone.com/forum/viewtopic.php?t=4319
and it pointed to the resistor values and replacing the TCR5302 things on V1 with resistors amongst other things. The fact I have those things below, do you think it means it is still set up to use a fetron?

- TCR5302 on 2 places on V1, I read that those should be replaced by 1500 ohm 1/2 watt resistor parallel with the existing 22uF, 25V cap...
- There are two 82k resistors on V1, I read that should be 100k for a 12AX7...
- There is a big 60uF 350V capacitor next to the first stages board, not every Mark I seems to have that.
- The first tube is identified as a 12AX7/AT7 with permanent marker (seems like the original markings, same as the date and signature). Is this like that when it actually comes with a Fetron? The manual used to say that it could use either Fetron or 12AX7, but I read the result was sub par when using a 12AX7 without modifying the circuit.

Source of the 82k vs 100k and TCR5302 vs 1.5k resistor comment:
https://steelguitarforum.com/Forum11/HTML/000274.html

So, does it looks to you it is still made for fetron? If so I would certainly like to modify it to optimally work with 12AX7, but would need some help to figure this out.

Some maintenance I have done:

- Replaced one cap (don't ask me why I only did one, I replaced only the one that appeared to have swelled, did not know back then that recap was a thing and should have done all)
- Change the tubes a few times
- Corrected bent chassis (around the middle, close to speaked jacks, probably previous owner dropped it with a long jack in there instead of the angled one).
- Replaced cracked 2W resistors on the power tubes (called screen grid resistors I think?)
- Did a hack job of replacing the power on light that was not working anymore, and also the power switch that failed. I say hack job because I did not use Mesa components, and I am not that proud of how did the job back then.

An actual problem I have is the sound tends to cut off pretty much completely (only a very faint and trebly sound) and it would typically come back to life if I put into standby and back on after a few seconds. It seems to do the problem less when I play fairly loud for extended time (for instance for a gig), and more when I play at lower volume, and maybe higher gain. I'm not quite sure since it is quite random. But more recently it started to not return after the standby sequence and I have to do it many times, or shut off the amp completely. I am sure the amp is due for a complete re-cap now anyways, but are the old caps the plausible cause for that kind of issues, or is there likely something else going on?

If you have ideas of measurements to do (I only have a voltmeter, no fancy equipment) while the problem is occurring let me know. I tried to measure voltages at the different pins of the power tubes to compare between working and failed state but found no discrepancy. It does not help that I don't really know the meaning of my measurements

Other questions:

- Why is there no 60uF 350V caps on the Mesa store? Do I need to get rid of that cap anyways for the fetron to 12AX7 mod? If not, where is a good place for caps other than from Mesa directly?
- What do you think of the effect loop mod and how it was done? I use a Lexicon LXP15-2 multi effect in it for my reverb so I don't really want to get rid of it, but I would be curious if you guys think it is garbage or asking for trouble to have it.
- Do I need to change more caps than the big ones, maybe the two silver ones next to the two 220uF 300V ones? (50uF 25V if I recall correctly)

Sorry for the long text, hopefully some of you made it to here I will post some more picture in the next posts, somehow adding pics makes the post appears like spam and I cannot post.

Thank you for any help!

Frederik

Hi Frederik

The diodes are there for the fetron, if you still have the fetron in place stick with it as they sound good. If you have a 12AX7 then replace the diodes with 1.5K resistors.

I would LOVE to know the values in the Phase Inverter stage of your amp. I have converted my Boogie SOB to be a Mark 1.

It sound pretty good but it’s not quite there yet.

I would consider changing the filter caps in your Mark 1. I changed the caps in my SOB and it improved the sound of the amp, though I did reduce the the 1st stage filter caps from two 220uF caps to two 100uF caps as I thought the amp sounded too stiff with the 220uF caps there.

The 60uF cap on the first stage is there to stop oscillation, I think it add further decoupling in the power supply.

Regards

Mark

 

[bluebug]

Hi Frederik

The diodes are there for the fetron, if you still have the fetron in place stick with it as they sound good. If you have a 12AX7 then replace the diodes with 1.5K resistors.

I would LOVE to know the values in the Phase Inverter stage of your amp. I have converted my Boogie SOB to be a Mark 1.

It sound pretty good but it’s not quite there yet.

I would consider changing the filter caps in your Mark 1. I changed the caps in my SOB and it improved the sound of the amp, though I did reduce the the 1st stage filter caps from two 220uF caps to two 100uF caps as I thought the amp sounded too stiff with the 220uF caps there.

The 60uF cap on the first stage is there to stop oscillation, I think it add further decoupling in the power supply.

Regards

Mark

Hello Mark, thank you for replying, no I don't have the fetron, it was not there when I got the amp in 1997. I only remember he mentioned it saying he put a 12AX7 instead, but I don't remember talking about modification for it.

For the phase values of the phase inverter stage, can you be more specific what you would like to know? I'll try to get the information for you, but I'm not sure what you need.

I replaced the filter caps already (the 5 big ones) and also replaced the bias caps (the silver ones) with the available Mesa store ones, 47uF 100V to replace the factory 50uf 75V. I Also got two 0.1uF 600V for coupler caps just in case (since I read they sometimes pop when replacing the filter caps) but I have not replaced them yet.

So you are saying all that is required to correctly support a 12AX7 instead of fetron are the diodes for 1.5k resistors? How confident are you about this being the only thing required?

Thanks for your thoughts on the big 60uF cap, do you have an idea why some Mark 1 have it, and some not?

Thanks again!

Fred

 

[mark1406]

With regard to 12AX7 versus the Fetron. The diodes in first and second stage are there only for the Fetron. The diodes clamp the voltage at the cathode at 0.7v. The 1.5K resistor typically has 1 mA going through it, which means you would have 1.5v roughly across the 1.5K resistor.

Regards

Mark

 

[bluebug]

With regard to 12AX7 versus the Fetron. The diodes in first and second stage are there only for the Fetron. The diodes clamp the voltage at the cathode at 0.7v. The 1.5K resistor typically has 1 mA going through it, which means you would have 1.5v roughly across the 1.5K resistor.

Regards

Mark

Thanks Mark. I have found the following maintenance & repair manual for the Mark 1 (I still have the original machine writer document that if I remember correctly had the same comments about the fetron, but I'll have to find it to verify).

https://mesa-boogie.imgix.net/media/User Manuals/Mark I Maint-Repair.pdf
The text below is copied from there, maybe the cutting out is what I was having before the re-cap (sound almost completely out after playing for a good while). What are your thoughts on the recommended 250 pf 1000v cap across the plates of the driver tube, vs your suggestion of the 1.5k resistor instead of the diode? If you think there is a good source of information I should read to try and understand that circuit let me know. I don't want to sound like the guy who wants spoon fed information.

Thanks!

Fred

Use a 12AX7 in place of the Fetron. They are direct replacements for each other and either device can be plugged into the socket and will work perfectly with the surrounding circuitry. Tube performance is not substantially altered by the load resistors selected for the Fetron, but you should select a tube which does not rattle or feed back when an instrument is plugged into INPUT 1. A rubber band stretched tightly around the tube will reduce noise. Replace the shield. Always tap on new pre-amp tubes to check for noise and stability.

If the amp experiences a cutting out when in the heavy distortion, loud driving mode after a 12AX7 has been substituted for the Fetron, then install a 250 pf 1000v disc capacitor across the plates of the driver tube (12AX7) next to the 6L6's. The plates are pins 1 and 6.

 

[bluebug]

For future reference I am copying another link where there is information on converting for 12AX7 instead of Fetron (on an MK II of same date as my MK I) so I don't lose the link

https://boogieforum.com/threads/first-boogie-first-post.46303/
The dates brings another question to my mind. My MK I and the MK II in the link above both have 3-78 on the chassis. Does that mean I have one of the last MK I, or did the production of MK I and MK II amps overlap for some time? (I think I am answering my own question here hahaha, for some reason I thought the Mark amps were more of an evolution than actual models, but this makes me realize they really were models that overlapped production through the years).

Thanks,

Fred

 

[mark1406]

Hi Fred

The Mark 1 and Mark 2 amps are very different amps. The Mark 1 is more or less a Fender amp with mods. The Mark 2 is its own thing.

Regards

Mark

 

[James]

I think there was some overlap. Here's a MK 1 from October 1978, serial A2283. I've never seen any above that.

https://boogieforum.com/threads/mark-series-serial-numbers.46364/post-448976
As for the 12AX7 conversion, remember the IIA was switchable. My MK 1 was originally like yours - it was labeled for 12AX7/AT7, but had the fetron circuitry (which I didn't realize until Mike B offered to remove it)

Also, note how all three speaker jacks share the same connection (green wire) and there's no 8-4-4 label.

 

[bluebug]

Hi Fred

The Mark 1 and Mark 2 amps are very different amps. The Mark 1 is more or less a Fender amp with mods. The Mark 2 is its own thing.

Regards

Mark

Thanks Mark for clearing it up. I love learning more about my amp, in fact after trying to discuss the amp's components with you I realized I needed to learn a bit more about how tube amps work, so I hope eventually I'll be able to discus the circuits a bit more, and understand your suggestions and questions without the need for a screenshot and annotations to understand what you mean hahaha

Here is my read from last night in case it might help someone else to do the same (the Fender Champ circuit helped me a lot to get started). If you have other good links to help get up to speed let me know!

https://robrobinette.com/Reading_Tube_Amp_Schematics.htm
Fred

 

[bluebug]

I think there was some overlap. Here's a MK 1 from October 1978, serial A2283. I've never seen any above that.

https://boogieforum.com/threads/mark-series-serial-numbers.46364/post-448976
As for the 12AX7 conversion, remember the IIA was switchable. My MK 1 was originally like yours - it was labeled for 12AX7/AT7, but had the fetron circuitry (which I didn't realize until Mike B offered to remove it)

Also, note how all three speaker jacks share the same connection (green wire) and there's no 8-4-4 label.

Thank you James for your observations and link! That was another interesting read for me. That A2283 certainly puts a pretty clear spot on the timeline.

Fred

 

[bluebug]

Now regarding the decoupling cap 60uF 350V, do you guys think it also needs to be replaced, or does it have an easy life and should just stay there? If I were to replace it, what impact does different voltage and capacitance values have on tone and operation, should I just keep looking to find the exact match? It seems not all Mk I have it, or maybe it is located elsewhere, and for the few pictures I saw (and they were not all Mk I, some Mk II) I have seen different values for that cap.

Fred

 

[mark1406]

I think there was some overlap. Here's a MK 1 from October 1978, serial A2283. I've never seen any above that.

https://boogieforum.com/threads/mark-series-serial-numbers.46364/post-448976
As for the 12AX7 conversion, remember the IIA was switchable. My MK 1 was originally like yours - it was labeled for 12AX7/AT7, but had the fetron circuitry (which I didn't realize until Mike B offered to remove it)

Also, note how all three speaker jacks share the same connection (green wire) and there's no 8-4-4 label.

Unfortunately, there aren’t circuit diagrams which detail any transitions. That would make it all much easier.

Regards

Mark

 

[James]

Thank you James for your observations and link! That was another interesting read for me. That A2283 certainly puts a pretty clear spot on the timeline.

Fred

No problem - as for the caps, I’d replace all of the big electrolytics and the ones in the bias circuit.

I can’t comment on what difference the 60uf cap makes, but mine had a 40uf in that position originally (now 47uf)

 

[mark1406]

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

 

[bluebug]

No problem - as for the caps, I’d replace all of the big electrolytics and the ones in the bias circuit.

I can’t comment on what difference the 60uf cap makes, but mine had a 40uf in that position originally (now 47uf)

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?