Frankensteining a Peavey Triumph w/ JCM800 front end

[JustMike]

 The issue I'm having is crazy high B+ voltages..for a typical Marshall.
 And what doesn't help is the layout of the Peavey. I've left the Output board and the PSU board intact. Because of the crazy design, they've got filter caps all over the place.
 In the attached schematic you'll see I have circled in red where the PSU board ends. In order to get 465 VDC here, I had to add 47K in series with R79. C57 is NOT in place as it was located on the preamp board.
 I DO NOT KNOW IF C57 IS NECESSARY, but it is not in place at this point.
 All of my voltages downstream are considerably higher than those shown on a Marshall 2203 schematic.
 I've attached a 2203 layout and I've circled in red what I've built as the front end. The .022 coupling caps are what I changed on the Peavey output board as they were .047 and I thought not appropriate for the Marshall sound.
 So, generally I think nothing is wired wrong, it's just that the Peavey PT puts out more voltage than the Marshall iron. Can I just adjust the B+ voltages by increasing the dropping resistors? I want a JCM-800 sound and feel.
 My voltages are;
 V1-1-313v
 V1-6-410v
 V2-1-160v
 V2-6-426v
 V3-1-461v
 V3-6-460v
 I know this only tells part of the story, but I think these voltages are about right for the Peavey, but my understanding is this is way too stiff for the Marshall. 
 What are the smart guys' thoughts?..especially to the questions in red.
BTW, Merry Christmas!

[AlNewman]

It doesn't look like your PI is conducting. 

Does the amp pass signal?

Is this the amp you started with?

https://el34world.com/charts/Schematics/files/Peavey/Peavey_triumph_60_120_sch.pdf

If so, if you look at the Peavey voltages your pins on the PI should be around 340V.  I think C57 is important, as it creates a separate node between the PI and everything downstream.

You can change dropping resistors to get desired voltages, and if you can get your PI voltages to Marshall specs, the rest of the amp should follow suit, assuming it's wired the same as a Marshall.

You should create an as built schematic, and a new voltage chart with voltages at all tube pins, and B+ voltages.

[JustMike]

Yes, that's the amp. It works and it sounds like a Marshall. Except the presence control doesn't seem to do anything. I'm concerned about the high voltages. When I look at an old 2203 JCM-800 "Master volume" voltage chart, my voltages are way high. Am I worried about nothing? I hear that high plate voltages can sound stiff, and that's what I'm hearing. I've played a lot of Marshalls over the years.
 What's throwing me about C57 is I can't find it's equivalent on the Marshall layout or schematic.

[SEL49]

What's throwing me about C57 is I can't find it's equivalent on the Marshall layout or schematic.

The equivalent on the Ceriatone layout is that 2x50 cap can in the lower left corner of the drawing. It feeds node "E" which is the PI B+ node.

[AlNewman]

Yes, that's the amp. It works and it sounds like a Marshall. Except the presence control doesn't seem to do anything. I'm concerned about the high voltages. When I look at an old 2203 JCM-800 "Master volume" voltage chart, my voltages are way high. Am I worried about nothing? I hear that high plate voltages can sound stiff, and that's what I'm hearing. I've played a lot of Marshalls over the years.
 What's throwing me about C57 is I can't find it's equivalent on the Marshall layout or schematic.

Yeah, like I mentioned before, I would take voltage readings at all pins of all tubes, and also install C57 to decouple AC voltage, and go from there.

[JustMike]

 After putting in the Marshall value (50+50µ) filter cap and increasing the dropping R from 2k to 42k, here's my voltage chart;
B+ 517
V3a-259
V3b-256
V2a-130
V2b-232
V1a-169
V1b-201
 Still don't think the presence ckt is working, but I'm at a loss. I'm using the 2203 values-25k-L pot, 4.7k R, .1µ...I confirmed the pot is good.
 Also, gotta trace the buzz. No CT on the heaters yet. That may be it.

[AlNewman]

From what I see, your voltages still aren't adding up.

Pretty much impossible to see what's happening without a complete voltage chart and schematic.
 

[JustMike]

I adjusted the dropping resistors and now my readings are;
V1a-176
V1b-210
V2a-136
V2b- 238
V3a- 265
V3b- 261


 The amp sounds good, but I'm still getting no presence control functionality.
 

[AlNewman]

Compare your voltages with Marshall voltages.

https://el34world.com/charts/Schematics/files/Marshall/Marshall_jcm800_2204_lead_series.pdf

Just measuring plate voltages only tells about 1/3 of the story.

[JustMike]

Thanks for hanging with me Al. Your diagram is of a 2204 50w. My amp is built on a 120w Peavey PSU so the B+ voltage and the droppers are different. I've attached a 2203 schematic with my voltage readings. I've gone back and used the 10k inter stage droppers.
 The Peavey doesn't have a choke, they used a 400Ω ceramic resistor in it's place followed by a 2k 5w cement dropper. I replaced this with a 8.2k 5w to get the voltages I have.


 And still, the presence control doesn't work. Every JCM-800 layout I see has the cap and the 4k7 paralleled to ground, but the schematics show the cap in series with one side of the variable resistor. Am I looking at this wrong?

[SEL49]

And still, the presence control doesn't work. Every JCM-800 layout I see has the cap and the 4k7 paralleled to ground, but the schematics show the cap in series with one side of the variable resistor. Am I looking at this wrong?

You are looking at this wrong. The cap is ALWAYS in series with the pot. The layout you posted in the original post clearly shows this. If you connect the cap in parallel with the 4.7K then the pot can't possibly do anything.

The layout you posted shows the cap connected to ground, but the schematic shows the pot connected to ground. This does not matter. In both cases the cap is still in series with the pot and the operation is the same. Look closely at the Ceriatone layout. Notice that the cap and the resistor both connect to ground. Also notice that the cap connects to the wiper of the pot but the resistor connects to the right lug of the pot. This means the cap and resistor are NOT connected in parallel.

If you wire your circuit exactly like the layout the presence will work. Or, if you wire your circuit exactly like the schematic the presence will also work.

[AlNewman]

Thanks for hanging with me Al. Your diagram is of a 2204 50w. My amp is built on a 120w Peavey PSU so the B+ voltage and the droppers are different. I've attached a 2203 schematic with my voltage readings. I've gone back and used the 10k inter stage droppers.
 The Peavey doesn't have a choke, they used a 400Ω ceramic resistor in it's place followed by a 2k 5w cement dropper. I replaced this with a 8.2k 5w to get the voltages I have.


 And still, the presence control doesn't work. Every JCM-800 layout I see has the cap and the 4k7 paralleled to ground, but the schematics show the cap in series with one side of the variable resistor. Am I looking at this wrong?

Except your pre amp is based on the 2204/2203 schematic.  It shouldn't matter what your voltages are at the power tubes, if you can create the B+ voltage at the PI node, and you wired the  pre amp and power rail correctly to follow that schematic, your voltages should be close to the same across the board. 

With your last attachment, at least we can see the PI is drawing some current, but it looks to be drawing less than half of what it should, even in comparison to the Peavey schematic.  Anyways, if you're happy with it then it works.

Marshall does draw the NFB from the 4R tap, compared to Peavey which draws from the 8R, which would change the output some if you are still attached to the 8, but it sounds like your presence issue may be a bad connection.

[JustMike]

 Happy New Year Al & All!
 Now to figure out why the PI current draw is what it is... this might explain why the amp, in my opinion has a lower output than I would expect from a 120w power section. I'm playing it thru a closed back 2-12 cabinet and it should be tearing my head off, but with the master at about 10 o'clock, it's not.
 I've got the 100k NFB resistor tied to the Peavey 4Ω tap. I started checking other voltages around the PI and according to the attached Marshall Schematic and voltage chart, I should be seeing 37.5v at the cathodes. I only see 5v. At the junction of the 1M grid leaks and the 470 Cathode R, I only have 1v. Of course, this all ties in with the NFB...

[SEL49]

Check resistance between the cathodes (pins 3 and 8) and chassis. Theoretically the resistance will be 470 + 10,000 + 4,700 = 15,170Ω. If not check those three resistors and replace the one that's wrong value.

[JustMike]

Thanks SEL. That was it. The 470Ω cathode resistor was a 4.7k. Those 5 band resistor values get me every once in a while. Once I replaced that, the presence control works, the amp is LOUD and the amp SQUEAL(ed) until I swapped the output leads.
 An interesting observation- It seemed to have more preamp gain (distortion) before we "fixed" it. I'm guessing this has something to do with the negative feedback circuit, but I just don't know enough...
 The next thing I'm going to try is putting in an adjustable bias ckt.

[AlNewman]

Not to beat a dead horse... but...  What are your voltages now that the PI is performing?