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porkchop61 24th August 2004 05:28 PM

McMartin MA-20?
Hello all-
New to this forum, but not to the tube world.
My only real project has been building the "November" amp on the AX84 web site. I think it came out pretty good considering it was my first attempt, and built from scratch!

Aside from that, I fool mostly with tube radios.
I'm a novice in this field as I'm a mechanical designer by trade, so this electrical stuff is relatively new to me.

I recently aquired a McMartin MA-20 PA amplifier, and was wondering if any one knew anything about these amps?
My intention is to modify this amp for use with guitar. Looked to me like a good candidate since it has 2 6L6 output tubes.
I have a schematic coming in the mail, but I'm trying to gather more information before it get's here.
I'm sure I'll have a lot of questions as I'm going through this process, so bear with me.

Any information on these amps would be greatly appreciated!

porkchop61 27th August 2004 12:16 PM

Bias type for this amp?
2 Attachment(s)
OK, I now have the schematic for this amp.
Can someone tell me what type of bias is used in this design?
I would like to change it to fixed bias for future ease of biasing the tubes. I plan on changing to KT66's.

whitfiel 15th September 2004 02:26 AM

McMartin Schematic for MA-50
Thanks for sharing the MA-20 schematics. I am restoring a McMartin MA-50 that has 7027A output tubes. Where did you get the schematic for your MA-20? I'd like to get a schematic for my MA-50.

porkchop61 21st September 2004 11:22 PM

Sorry for the late reply.
I got the schematic from Charles Goodrich at
He was really fast with the schematic, and only charged me $10!

This amp is coming along nicely, and I'm very pleased with the sound. All those gain stages (3) with a master volume is awesome!
I may throw away some of my overdrive pedals!
Good luck.

whitfiel 22nd September 2004 01:43 AM

Thanks for the link to the Goodrich site with McMartin schematics. My MA-50 is running pretty well now that I added 47uF to the output of the voltage regulator tube.

My favored hi-fi amps are a pair of Ampex 2012 (6V6GT outputs) pulled out of a 2-track Ampex reel-reel tape recorder. But that's getting off topic... <g> I was originally hoping the McMartin would be on par with the Ampex 2012, but being a PA amp instead of a hi-fi amp, it may not have a full frequency response. This sage advise came from mjd_tech. This weekend I'll complete my A-B tests, but the McMartin MA-50 will probably be best for guitar. But every time I putter with a tube amp, it's a great learning experience.

Best regards,

Enzo 15th December 2004 09:06 AM

Very hard to see the drawing. But it appears to be a conventional fixed bias arrangement. Nothing esoteric in little PA amplifiers. The bias supply lookes like it has its own tap off the HV winding. it is a negative supply, half wave. The bias the tubes sees is determined by the voltage divider - that ratio of the two resistors in the supply. From the first filter cap there is a series resistor, and then another resistor to ground. The bias is taken at their junction, and fed to the output tube grids by the pair of resistors up there. They would be high value like 220k or something similar.

The two resistors in the supply form a voltage divider. The voltage out would be proportional to the resistors. Add them both, then use the grounded resistor to figure its percentage of the total. The bias is that percentage of the raw supply.

That is how it works, if the details vary, it is because I c an't see the picture.

To make the thing adjustable for differnt tube types, make the grounded resistor variable. The bias setup uses very little current, so you can use a range of values.

If you plan to use larger tubes, keep in mind they often use far more heater current than a 6L6 and the power transformer may or may not be up to it.

whitfiel 16th December 2004 02:39 AM

I'm not clear on what schematic you are talking about, nor what prompted the bias discussion and/or tube substitutions. Sorry if I seem dense today... must be the snow. Can you clarify which schematic you are talking about, and what prompted the bias discussion. It sounds interesting to a newbie like me, but I just need some more exposition.


PS: my re-hab of my McMartin MA-50 went really well. My teenage son is now using it for his "big" guitar amp, driving a Peavey PA speaker that has a Black Widow driver for the woofer. It's working well and sounds awesome. My initial A-B tests against my Ampex 2012's didn't go well, but it turns out I had the bias set wrong. I need to do that again. Yes, I know PA amps are often not well suited for high fidelity, but most of the fun is learning that with your own two ears.

Enzo 16th December 2004 04:14 AM

Art, go up and read post #2. Glenn posted his schematic, asked what sort of bias arrangement it used adn how he might make it adjustable for future biasing and suggested he might want to try KT66s.

Hmm. I was thinkin KT88 when I wrote the extra current response.

If you like fishing around in guitar amps and things like them, go visit Plenty there. The AIken AMps and London Power links there are full of useful stuff too.

thee8track 10th June 2005 12:15 AM

mcmartin model TR-35 reciever
i have this mcmartin reciever its a one knob unit.
no tuner knob.
its a mystery?
it gets one fm station.104.0 fm.
the solid state circuit board number is 550179.
it says its a 5 watt amp/unit,
if you have any where online you can direct me to an answer or info on this unit .
thanks David

peobus 30th July 2006 03:10 PM

I, too, am a beginner. Just picked up a McMartin MA-20 and would like to use it as a guitar amp. Any hints?

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