Need help with decisions for PA to Guitar amp convo

[MelodianChew]

Hey fellow TUTians,

This is my first post ever here, and thank you for taking the time to peek at my question.

MY QUALIFICATIONS/SKILLS (you can skip this part): I am basically a very enthusiastic and adventurous amateur electronics repair and build enthusiast. I have tinkered with fixing and morphing electronic gear for myself and bandmates throughout my years. I have a small working set of test gear and a solder station. I have mostly been educating myself through reading various books, manuals, websites and forums. I'm getting much better at reading through schematics. I feel like I am ready to up raise the difficulty of my project levels.

I have a little 20w McMartin PA head that is wired point to point that has 2x 6L6, 2x 12ax7, and 1x 6av6 tubes. This is my first attempt at trying to convert a PA into a Guitar amplifier. This attempt is an intention to both sharpen my electronic skills and hopefully end up with a killer little tube amp in the process. The project is a little bit above my pay grade but not drastically so, and besides, there is no greater teacher for an ambitious mind than 450v and an idiot that just feels lucky. (That's a joke)

What I could really use some help with here though is possibly some coaching through this process. (you don't know how difficult it is to find mentors once you reach the age that you start to look like a mentor.)

Some questions I have right at the outset mostly concern with trying to figure out the best direction to steer this carcass into, and which direction makes the most sense as per guitar amplifier designs that I could use for inspiration to try to model:

Here's the schematic:


  McMartin MA-20 Schematic GOOD.pdf (Size: 43.17 KB / Downloads: 5)
and some pictures:

           

1) what does an expert notice right off the bat that this amplifier's design is telling you when you see the physical layout and read through the schematic?

2) We have to start somewhere, right? And there is no better first step than arriving at establishing a goal or endgame for our project. So, looking at the schematic and considering the transformer voltages and tube compliments, etc. what type of known good guitar amplifier design could be most easily cloned or used as a model for this build? 

3) There are three (extra) tube sockets on the chassis for mic and line transformers. Does having those sockets already built in and ready for use change or expand the range of tube amp models we might like to try to emulate with this build?

4) I understand that this amp, being intended for PA use, will have been designed with the intention of avoiding the kind of distortion characteristics that are most pleasing for guitar amplifiers, so what types of changes would need to be addressed in order to make the conversion?

5) What logical steps should we take in assessing the schematic, and then planning our approach to hashing out the methods for accomplishing this mod?

6) Is it a best practice here to piece through this mod by breaking up the schematic, referentially, into separate systems that we should work on solving, one at a time, in order to keep our goals and tasks focused and with clear endpoints for each of those?

7) Should I break this down into DC Signal path, Input section, Gain stage 1, stage 2, power tube amplification, output section. AC Power section, etc.?
 
Well, I was going to ask a ton more questions, but I think I should probably take my time and try to get the full benefit of having expert advice at my disposal, and would be best served by letting more knowledgeable eyes guide me that can lead me through a project like this so that I can somehow through osmosis or some other such wizardry, absorb some of your wisdom and logical methodologies in trying to adopt them for my own.

Well, OK, but there are just a couple of questions right off the bat that I know would be further down the road but that I find irresistible in inquiring about right now:

1) the sections of the amp that were intended for plugging in transformers for the microphones in the input sections: what will most likely become of those extra tube sockets? If I were planning on just eliminating those sockets from the whole build, would I just remove most of those connections to those sockets and then run through the schematic one section at a time trying to figure out where each connection should go to in following my new plan?

2) could those now empty and not needed mic trans tube sockets be used as potential opportunities to build in other new gain stages, like for reverb or tremolo?

Thank you to any that could offer any help to me for this kind of thing, I really appreciate it.

Scott

[K O'Connor]

Hi Scott

The first thing to do is make sure the stock amplifier works and that the OT and PT are functioning properly, BUT...

The "really first thing" is to build a Power Limting Safety Socket for your test bench, unless you already have one. This uses incandescent lamps to restrict power to safe levels appropriate for each step of a test procedure and/or to accommodate different types of equipment to be tested.

Yes, a PA the circuit is designed to be used within its clean output range. We assume this is a 20W amp based on its model name? In any case, you need a load resistor that can briefly withstand the full output of the amp, but first, you do a test with an open load. 

There are posts about how to do power tests in the Test and Measurement section of this forum, so I will not repeat the procedures here.

Assuming that the expensive parts of the amp are confirmed "good', the conversion to a guitar amp can begin.

The power amp is similar to many tweed-era PAs and will have a warm basic tone. As a guitar amp, you must install a 1k-5W screen-stop for each tube. If you want the tubes to survive continuous clipping and mismatched loads, this is mandatory as TUTs explain. The resistor should be elevated above the socket and tied to pin-4 and pin-6 with the wire from Vs tied to pin-6. You should add grid-stops, as well, right at the tube socket. These should be 1k5 to 10k 500mW hanging right off pin-5.

The next mod is to add cathode-current sense resistors and meter jacks for safe idle measurements. We offer this as BMK2 Bias Mod Kit for two tubes, or you can buy the parts from any broadline electronics distributor. TUT3 shows how this is wired.

To compliment the easy bias monitoring, you need easy bias adjustment. The stock bias supply has no adjustments, so you have to add them by replacing R38 (5k6) with a pair of parallel pots in series with a range resistor to ground, and also reduce the value of R21 (3k3). We typically use 25kB bias pots (panel pots mounted on the tube plane of the chassis) and a 4k7 or higher range resistor. R21 should be 1k or less.

Break the connection between the grid-stops R20,22 (100k) and feed each resistor from the wiper of its own bias pot.

The entirety of the above arrangement allows you to et the first tube  by meter for safe operation and the second tube by ear for best hum-balance.

The plate filter cap is woefully undersized and should be 100uF minimum. It is the first point of filtering to reduce ripple and hum throughout the amplifier. To assure the lowest-noise and best note articulation, you should have TUT3 handy (The Ultimate Tone vol.3). This is the best guide to wiring and laying out amplifiers and every hobbyist, tech and amp builder should have it - most do and it has changed the nature of MI for the good

The preamp has three triodes available and this is enough gain to create any useful amount of distortion you may desire. However, this is the "Scott" amp and you are looking for your own tone. You need to define that goal and that will determine how the preamp is reconfigured. As it is now, there is only one gain stage between the input and the PA, and the PA has a low closed-loop gain.

You can open up the tone of the PA by increasing the values of R14 (1k) and R23 (5k6). The gain set by these is 6.6 overall, which is lower than even what fender uses. You could simply change R23 to some higher value, say 22k and have much more gain and an more dynamic tone from the PA. You could also simply lift one end of R23 to hear the open-loop sound and gain from the PA. If you like that sound, you could add a switch to open/close the loop, or add a pot to dial between 5k6 and nearly open-loop. The pot can be any value with higher values approaching the open-loop gain - anything over 100k will assure that.

If you change R15, you must also change R14 (100k) and these are the cathode-bias and plate load resistors, respectively.

These PA gain changes might let you assess how much gain you need from the preamp and are otherwise nice to explore.

The mic transformer sockets could be used as preamp tube sockets if you want to try out octal-based types. generally, they sound a bit mellower than the equivalent-mu subminiatures. As TUT suggested, you can achieve a similar sound using parallel triodes without gain loss, or use lower-mu tubes with the attendant gain reduction. If you plan to add tubes, you should add an auxiliary PT to support the extra tube heaters. You cannot count on the stock PT to support any more heater load than the stock tube set represents. If you ad this PT, you can bump its rating to accommodate the use of different power tubes, as well. Using alternate tube types will not change the power output but will change the tone and you can mix tube types.

Have fun

[MelodianChew]

Hi, K,

Thanks for the reply. I really appreciate it. Have just a couple questions in reply:

The "really first thing" is to build a Power Limting Safety Socket for your test bench

I actually have built one of those just for this build. I will also be purchasing a variac to be able to ease into the older components of unknown condition.

In any case, you need a load resistor that can briefly withstand the full output of the amp, but first, you do a test with an open load. 

I plan on building a heavy duty high watt load resistor at 8 Ohms and wired to a 1/4" phono plug for ease of connection to amps I build.

You mention doing a test with an open load. Is this for the PT and OT tests specifically? Also, Aren't we supposed to NEVER run a tube amp without a load? What is running an open load test going to tell me, briefly?

There are posts about how to do power tests in the Test and Measurement section of this forum,

Thank you for this, I shall see.

The power amp is similar to many tweed-era PAs and will have a warm basic tone.

Which Tweed model or models is my little amp most similar to, or which specifically did you have in mind when you made this statement, because it would be nice to download a fender schematic that I could sit next to mine to be able to compare and to use as a reference for mod and inspiration and to better help me see the differences between the PA and the guitar of the circuit.

The next mod is to add cathode-current sense resistors and meter jacks for safe idle measurements. We offer this as BMK2 Bias Mod Kit for two tubes, or you can buy the parts from any broadline electronics distributor. TUT3 shows how this is wired.

Considering all of the help I am getting from the use of this site, it only makes sense for me to reciprocate with checking out a few of your wares, especially the TUT3 and bias mod kits.

The plate filter cap is woefully undersized and should be 100uF minimum.

Which cap type should I use for this replacement. Do you have any suggestions about which type of composition should I use for the caps and resistors you mentioned for the mods you proposed?

The preamp has three triodes available and this is enough gain to create any useful amount of distortion you may desire. However, this is the "Scott" amp and you are looking for your own tone. You need to define that goal and that will determine how the preamp is reconfigured. As it is now, there is only one gain stage between the input and the PA, and the PA has a low closed-loop gain.

Well, I have the most unfortunate and unoriginal obsession with the clarity, articulation and singing dynamic gain that is heard in Hendrix's live recordings, the Winterland Concerts specifically. Its just sublime and I can't help myself. And I also play single coils almost exclusively. So maybe that helps.

You could also simply lift one end of R23 to hear the open-loop sound and gain from the PA. If you like that sound, you could add a switch to open/close the loop, or add a pot to dial between 5k6 and nearly open-loop.

Wow. Great idea, thanks.

The mic transformer sockets could be used as preamp tube sockets if you want to try out octal-based types. generally, they sound a bit mellower than the equivalent-mu subminiatures. As TUT suggested, you can achieve a similar sound using parallel triodes without gain loss, or use lower-mu tubes with the attendant gain reduction.

OK. bypass themBut suppose if I were to want to just keep it simple and not use the trans sockets at all, how would I go about wiring their elimination? As it stands, I would have to  to move the inputs and bypass them obviously, right? So I should lift the wires that go from the input jacks to the trans sockets and move them to V1 and V2, respectively, with observation of proper impedence matching, correct?

If you plan to add tubes, you should add an auxiliary PT to support the extra tube heaters. You cannot count on the stock PT to support any more heater load than the stock tube set represents. If you ad this PT, you can bump its rating to accommodate the use of different power tubes, as well. Using alternate tube types will not change the power output but will change the tone and you can mix tube types.

Hmm, makes sense. What voltage and current PT should I look for? Center tap? Any ideas for model suggestions? Of course you may not be able to answer this besause we arent sure which tubes, right? When you say octal tubes you mean like the 6QN7 (i think that is the number) type or similar?  Or do you have any expert saggestions? I think I actually have some NOS GE's somewhere. Wow this is interesting.

Using alternate tube types will not change the power output but will change the tone and you can mix tube types.

This is because they will be part of the preamp and not the power amp section right?

Could one of those trans sockets work as a gain stage for reverb? I have a cool old tank reverb..


Thanks for all of the help.

Scott

[K O'Connor]

[quote pid="1708" dateline="1746460309"]
You mention doing a test with an open load. Is this for the PT and OT tests specifically? Also, Aren't we supposed to NEVER run a tube amp without a load? What is running an open load test going to tell me, briefly?

There are posts about how to do power tests in the Test and Measurement section of this forum,

Thank you for this, I shall see.

Reply:
The open-load test demonstrates that the circuitry is properly connected and that the output is symmetrical up to clipping. A damaged OT may pass this test, but more likely not. adding the load and sweeping the level quickly up to clip then back down is the acid test for the OT.

The power amp is similar to many tweed-era PAs and will have a warm basic tone.

Which Tweed model or models is my little amp most similar to, or which specifically did you have in mind when you made this statement, because it would be nice to download a fender schematic that I could sit next to mine to be able to compare and to use as a reference for mod and inspiration and to better help me see the differences between the PA and the guitar of the circuit.

Reply:
Be inspired by the circuit you actually have.

There are two basic PA schemes used in MI, this one you have is the Tweed vesrion with separate gain and splitting functions (splitting via concertina splitter), the second uses the Schmitt splitter front-end. The tweed-era circuit is more toneful and warmer.

The plate filter cap is woefully undersized and should be 100uF minimum.

Which cap type should I use for this replacement. Do you have any suggestions about which type of composition should I use for the caps and resistors you mentioned for the mods you proposed?

Reply:
Stay away from voodoo brands unless you enjoy spending $$$ and just stay with common capacitor brands except for Illinois Capacitor -their plastic caps are okay but their electrolytics are very poor.

The preamp has three triodes available and this is enough gain to create any useful amount of distortion you may desire. However, this is the "Scott" amp and you are looking for your own tone. You need to define that goal and that will determine how the preamp is reconfigured. As it is now, there is only one gain stage between the input and the PA, and the PA has a low closed-loop gain.

Well, I have the most unfortunate and unoriginal obsession with the clarity, articulation and singing dynamic gain that is heard in Hendrix's live recordings, the Winterland Concerts specifically. Its just sublime and I can't help myself. And I also play single coils almost exclusively. So maybe that helps.

Reply:
Those amps would be plexi-style circuits voiced to be bright - almost unusably bright when played below clipping. You can achieve that sound with any amp.


OK. bypass themBut suppose if I were to want to just keep it simple and not use the trans sockets at all, how would I go about wiring their elimination? As it stands, I would have to  to move the inputs and bypass them obviously, right? So I should lift the wires that go from the input jacks to the trans sockets and move them to V1 and V2, respectively, with observation of proper impedence matching, correct?

Reply:
There are no impedance matching issues here. Use a 1meg grid-leak for the input stage and any guitar pickup will be happy.

If you plan to add tubes, you should add an auxiliary PT to support the extra tube heaters. You cannot count on the stock PT to support any more heater load than the stock tube set represents. If you ad this PT, you can bump its rating to accommodate the use of different power tubes, as well. Using alternate tube types will not change the power output but will change the tone and you can mix tube types.

Hmm, makes sense. What voltage and current PT should I look for? Center tap? Any ideas for model suggestions? Of course you may not be able to answer this besause we arent sure which tubes, right? When you say octal tubes you mean like the 6QN7 (i think that is the number) type or similar?  Or do you have any expert saggestions? I think I actually have some NOS GE's somewhere. Wow this is interesting.

Reply:
The guidance for this is in the quoted statement.

Expanding on it simply means you have to look up the heater current requirement for the tubes that are in the amp now. Add that up and this is the current you can safely expect to pull from the stock PT.

Now look up the heater current for prospective tubes you might wish to use. See how much higher some of these are. TUT3 explains this process and has a list of common tubes used in MI. In general, the octal-based preamp tubes draw more heater current than the subminiatures do. Octal power tubes that are pin-compatible with what you have draw from 450mA (6V6) up to 1,800mA (KT-88).

The auxiliary PT does not need a CT. A faux-CT using resistors works better and in any case, you should use a DC-stand-off for the heaters to reduce noise, as TUT3 shows in every project.

You can simplify things by deciding to power the power tubes from the aux PT, say, in which case a 4A unit would handle any tube pair you ever want to try. This leaves ample capacity in the stock PT to support a lot more preamp tubes or even the small power tube for a reverb driver, below.

Using alternate tube types will not change the power output but will change the tone and you can mix tube types.

This is because they will be part of the preamp and not the power amp section right?

Reply:
No, this is with respect to power tube swaps.


Could one of those trans sockets work as a gain stage for reverb? I have a cool old tank reverb.

Reply:
You can use a power tube as a reverb driver just as Fender did early on to save $$$. Their first reverbs used a 6V6 and the same OT from the Champ. Later units used a 12AT7 and a custom OT. A reverb tank only requires 150mW or less.

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