V2A gain problem (?) on JTM45 build

[El Caballo]

I'm having a strange problem with my JTM45/Plexi build (I used the split cathode for V1 and the 470K resistor mixer from the Plexi/SL circuit). Everything is up and working and sound is coming out, but the volume is literally a whisper.

Using the scope and function generator, I believe I've traced the problem to the V2A gain stage.

Using a 1Vp-p input on the bright channel, V1 puts out ~25Vp-p, as you'd expect. With the bright volume dimed, this gets knocked down to about 8Vp-p at pin 2 of V2 (the input to V2A, after the resistor mixer).

Here's what I measure at V2. AC measurements:
pin 2: 8Vp-p
pin 1: 1Vp-p
pin 8: 1Vp-p
pin 3: 5Vp-p

The V2B cathode follower has a gain of 1, as I'd expect. However, V2A has a gain of about 0.125 (8Vp-p to 1Vp-p). This seems seriously wrong, and would certainly account for the lack of output. I've swapped tubes from V1 and V2 to no effect.

DC measurements:
1: 294.5VDC
2: -0.1VDC
3: 2.1VDC
6: 292.5VDC
7: 294.5VDC (tied to pin 1)
8: 291.8VDC

Heater voltage is correct, heaters are happily glowing orange, transformer outputs are good, B+ is 455, 453, 370, 292 going down the choke and then the resistor divider, so that's all looking plausible.

I've checked all the resistor values on the board. Disconnecting NFB increases output slightly but doesn't change the behavior at V2: neither does disconnecting the tone stack.

Can anyone point me at some other places to check? I'm pretty much stuck.

[Enzo]

I note the pin 1 voltage is actually a couple higher than the B+ at that point, so i will write that off to where the meter was grounded or something. But it looks like almost no voltage drop is occuring in pin 1 plate load resistor. But a couple volts on that cathode means current flows.

So is the plate load really a 100k or whatever? Make sure you didn't install a 100 ohm or something that looks similar.

And if that is OK, then I would look at pins 6,7, and 8 to see if any are shorted together. Power down, pull the tube, and check with a meter. Or alternatively, pull V2 and see what voltages are on the pins without the tube.

[El Caballo]

I found the problem.

I connected B+ to pin 7 of V2, not pin 6. The resulting circuit topology is left as an exercise for the reader. Hence the loss of signal.

I am embarrassed. Fortunately this mistake didn't toast anything, so I have a working amplifier now!

Unfortunately it's really late so I can't crank it up and hit that power chord I've been looking forward to for a week. That problem will be solved in the morning.

[Enzo]

Well, that is close enough to pins 6,7,8 being shorted together.