JCM800 "faint" sound at full 10

[sonofmickel]

Solved, thanks Xtian!
Solution for very weak volume JCM800:
Check 470 ohm PI tail resistor, you just may have put in a 470k!

Original thread...
New JCM800 build using old parts.
Voltage looks good all around, 20v high everywhere with 125v at the wall.

Master at noon and preamp at noon:
The tone coming out of the speaker cab is "good", sounds like a nice Marshall JCM tone, but is VERY quiet.
Sorta of sounds like listening to a cranked amp from outside the studio or like a "direct" to the board kind of tone.

So I believe the inputs are good, the input pots and tone controls all "act" like they should.

Could it be a bad Output Transformer?
It's a Classictone 50 watt Marshall style ot.

[xtian]

Could it be a bad Output Transformer?

Extremely unlikely. I see you were asking about a "listening" device, so you've asked yourself the question, "Where is the signal vanishing?" A scope is perfect for this.

A common error is swapping the 470R PI tail resistor with 470K (as an example).

[sonofmickel]

Thanks, I'll check for a mis-specced component

The amp "sounds" exactly what I think...JCM800 just really quiet. The bite is there and the karrang, and the tone is very mids, nothing out of proportion... tone pots work correctly as well.

B+ 430v, biased at 40v tube 4/ 37.8v tube 5, slightly out of balance KT77s

Tube Voltages with a looper pedal playing constant clean riff
V1:
1. 299
3. 2.9
6. 255
8. 2

V2:
1. 193
3. 1.2
6. 342
7. 192
8. 192

V3:
1. 383
2. 189
3. 6.39
6. 383
7. 189
8. 6.38

V4:
1. 40
3. 430
4. 428
5. -35
6. 430
8. 37

V5:
1. 37
3. 431
4. 428
5. -35
6. 430
8. 37

[sonofmickel]

Xtian, thank you!

It was the blasted 470 ohm vs 470k tail resistor!
I had the 470k in there.

Loud and proud.

Now, what mods....

[xtian]

Haw! Awesome.

I like to add a switchable cathode bypass cap on V1b, so you can get a little crunch out of the Low channel.

[Reeltarded]

"Not all 470s" The bumper sticker.