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.
(https://i.postimg.cc/VLdSxLb7/Screenshot-2023-05-08-at-8-32-32-AM.png) (https://postimg.cc/LnF6fSDf)
(https://i.postimg.cc/4y6nf65w/IMG-1047.jpg) (https://postimg.cc/n9LnTQNQ)
(https://i.postimg.cc/3wqknJNj/IMG-1046.jpg) (https://postimg.cc/GBk3pRk9)
(https://i.postimg.cc/sf7M4zrS/IMG-1045.jpg) (https://postimg.cc/G933bZ9h)
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
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.
Also, it came with these two bad boys and I have some Valvo 12ax7s to try in there too...
(https://i.postimg.cc/8chfFFkN/IMG-1064.jpg) (https://postimg.cc/FkHH8HJB)
(https://i.postimg.cc/DfL4QQN7/IMG-1054.jpg) (https://postimg.cc/8skCNvDY)
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 .
Quote from: momo on May 08, 2023, 10:41:05 AM
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.
Or use the 6.3v supply and bring it down?....Is this pulling major current?
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
Quote from: Steben on May 08, 2023, 12:12:48 PM
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
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.
The DC is 100 times more than the AC. How hot do you think it should be??
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!
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!
Quote from: PRR on May 08, 2023, 07:50:34 PM
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.
Quote from: GibsonGM on May 09, 2023, 05:10:42 AM
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...
:icon_cool:
Quote from: Clint Eastwood on May 09, 2023, 03:47:23 AM
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.
Quote from: theehman on May 08, 2023, 11:04:53 AM
Quote from: momo on May 08, 2023, 10:41:05 AM
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
Quote from: Colman on May 09, 2023, 06:24:54 PM
[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?
Quote from: GibsonGM on May 09, 2023, 05:10:42 AM
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?
(https://i.postimg.cc/RVjQ8zrL/Screenshot-2023-05-10-at-7-50-07-PM.png) (https://postimages.org/)
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.