Help me convert this PA amp for guitar..

[momo]

Hey all,
Well, score again, this time, I was able to get one of those cool MC-Martin PA amps.

So the 2 mic inputs are split on the individual stages of the 1st 12ax7.
So I would do the Marshall trick to join both inputs to use both stages.
Rob mentioned these things tend to be biased a bit cold, so I might want to change that..
Will change the coupling disc caps, electros seem good with ESR meter.

This will be my first real tube project....
I am now putting together an isolation power supply with variac.

[Rodgre]

I'm not familiar with the McMartin, but I have two vintage Bogen Challenger PA amps (a 22watt and a 50 watt, I think), and a friend has a third (30 or 40 watt?) that I worked on, and these are honestly some of my favorite guitar amps I have ever played. They sound so great with beautiful headroom that takes pedals very well, while still having the harmonic glow of a Fender-style tube amp. They take a bit to overdrive, even the 22 watt versions, but with careful EQing, can distort beautifully, but need to be loud to do it.

Congratulations on this! I love these kinds of amps.

Roger

[amptramp]

R29, the feedback resistor around the 6AV6, is 6.8 megohms, not 6.8 ohms as shown in the schematic.  Confirmed by looking at pin 7 of the 7-pin socket in the underside view.  This looks like a great starting point for a guitar amp.

[momo]

Also, it came with these two bad boys and I have some Valvo 12ax7s to try in there too...

[momo]

Also, I would love to have a tube rectifier in there for some sag, is this complicated to do?
Again, I don't expect anyone to hold my hand all the way with this, but just details from experienced tube builders that I have no clue..!
thanks .

[theehman]

Also, I would love to have a tube rectifier in there for some sag, is this complicated to do?
Again, I don't expect anyone to hold my hand all the way with this, but just details from experienced tube builders that I have no clue..!
thanks .

The power transformer doesn't have a winding (usually 5v) for the rectifier filament.  You could add a 5v transformer for that if you're so inclined.

[momo]

Or use the 6.3v supply and bring it down?....Is this pulling major current?

[Steben]

Nice amp.

- R27 already is de facto a sag resistor but small. You could swap it for a larger one. You do not need a tube rectifier.
- C9/R20 and C10/R22 connections better have grid stopper resistors if you want to drive the output tubes. These help against nasty sounds. I do not know out of the head the best value for 6L6's.
- R4/R6 and R33/R31 could be reviewed with load lines to design the stages to more cold or hot biased. These make for a different character. V2 will do most drive though....
- Both mic circuits are the same. You might make one circuit hotter biased, one colder depending on what you like. You might add a R of 470k in front of the MIC1 and MIC2 pots with one having a 470pf in parallel making for a treble channel.
- You might have a feedback switch and/or add presence control

[momo]

Nice amp.

- R27 already is de facto a sag resistor but small. You could swap it for a larger one. You do not need a tube rectifier.
- C9/R20 and C10/R22 connections better have grid stopper resistors if you want to drive the output tubes. These help against nasty sounds. I do not know out of the head the best value for 6L6's.
- R4/R6 and R33/R31 could be reviewed with load lines to design the stages to more cold or hot biased. These make for a different character. V2 will do most drive though....
- Both mic circuits are the same. You might make one circuit hotter biased, one colder depending on what you like. You might add a R of 470k in front of the MIC1 and MIC2 pots with one having a 470pf in parallel making for a treble channel.
- You might have a feedback switch and/or add presence control

Thanks so much!
That's inspiring and will study it all, thanks

[momo]

At first glance it seemed biased a bit hot, but I missed a few stages.....hehe
I think the 6av6 could be hotter as well as V1 which has 95v at the plate.

[PRR]

The DC is 100 times more than the AC. How hot do you think it should be??

[Clint Eastwood]

At the inputs, there are no grid stopper resistors. And the resistors from input to ground are 100k, they normally are 1meg in a guitar amp.
You could configure the two input triodes like in a marshall 18 watt, so you can choose to connect them in parallel for some more gain.

Nice device!

[GibsonGM]

I second the idea for 'mic 1', 'mic 2' to be switched either cascade or in parallel as Clint alluded to.  For guitar, I'd really want that on a toggle or push-pull pot, and in fact have my 18Watt set up that way.  It's like 2 amps in 1. 

That amp has the potential to be a monster!  Looking at load lines/biasing, bypass caps...will serve you well!

[momo]

The DC is 100 times more than the AC. How hot do you think it should be??

I cannot answer that sir...!....yet
I'm reading and studying all this now.
Reading the 1966 RCA Receiving tube manual, great read.
Maybe I'm wrong in how I interpret the schematic, but it has voltages on there for plates and cathode ,ect. So I compared those numbers with what is typical from the RCA manual.

[momo]

I second the idea for 'mic 1', 'mic 2' to be switched either cascade or in parallel as Clint alluded to.  For guitar, I'd really want that on a toggle or push-pull pot, and in fact have my 18Watt set up that way.  It's like 2 amps in 1. 

That amp has the potential to be a monster!  Looking at load lines/biasing, bypass caps...will serve you well!

Thanks, I noticed the bypass caps were there and thought, maybe the dudes knowingly built this closer to a guitar amp than PA so folks could have a great base to start with.

I am reading up on those load lines and how to chart that.
Would you say this thing is biased idling @ center of the load line or is it a bit hot?

OK to make this simple, I would love this thing to go from nice clean to ACDC crackle to Santana sustain....all on the volume, or maybe diming everything for Santana tone...

[Steben]

At the inputs, there are no grid stopper resistors. And the resistors from input to ground are 100k, they normally are 1meg in a guitar amp.

+1
Right you are.

[Colman]

Also, I would love to have a tube rectifier in there for some sag, is this complicated to do?
Again, I don't expect anyone to hold my hand all the way with this, but just details from experienced tube builders that I have no clue..!
thanks .

The power transformer doesn't have a winding (usually 5v) for the rectifier filament.  You could add a 5v transformer for that if you're so inclined.

I wouldn't bother with a 5V tranny or a tube recto,get a copper cap and instant rectifier with voltage drop and you don't have to worry about the max capacitor value that tube rectifiers have.

Aharon

[momo]

[quote author=theehman link=topic=130590.msg1266542#msg1266542 date=1683558293


I wouldn't bother with a 5V tranny or a tube recto,get a copper cap and instant rectifier with voltage drop and you don't have to worry about the max capacitor value that tube rectifiers have.

Aharon

Can you elaborate a little on how that works?
I am familiar with Hifi copper caps that are used for their "transparency".
Are you saying to replace the 3 caps in a can with a copper cap?

[momo]

I second the idea for 'mic 1', 'mic 2' to be switched either cascade or in parallel as Clint alluded to.  For guitar, I'd really want that on a toggle or push-pull pot, and in fact have my 18Watt set up that way.  It's like 2 amps in 1. 

That amp has the potential to be a monster!  Looking at load lines/biasing, bypass caps...will serve you well!

So something like the front part of this 18w Marshall?

[GibsonGM]

You'll have to search "Marshall 18Watt parallel input" for some examples - it's been more than a decade since I did it, and it was based on the turret board layout and switching I had, so you'll have to reverse engineer the idea and adapt it to your project here!  That should be easy though, as you will pretty much be re-wiring the 2 input channels if you do this.  This is done all the time; there should be countless diagrams to follow of this process.    The pic you posted kind of shows the idea (sort of; both grids get the input signal in actual parallel operation), but that is a tremolo channel so the associated 'stuff' isn't what you'll have in the end.

If you set up for parallel input, you'll have a nice fat but not SUPER driven input stage, which is nice for blues or cleaner stuff where you want a 'creamy' tone (think Little Wing by Hendrix)

When in cascade, both stages feed each other in series, so you'll have a much harder driven tone.   On either setting, a boost will give you more clipping for leads or harder passages.  I prefer a treble cutting boost when using parallel, don't want to make the tone TOO fat / fuzzy.    I think cascade is where this will really shine, in terms of PUNCHING YOU IN THE FACE.

If it's operational, you can see what parallel sounds like by plugging a "Y" cord into both inputs (or use splitter) and balancing the levels.  Your 'build' would be tweaked for tone, of course.