Extremely low output volume in brand new AB763 build?

[joulupukki]

Rather than bog down an already lengthy build thread (here) I figured I would ask specifically for some help in a new thread to help me diagnose what might be going on.

I am getting some sound out of my new build but it is really really low volume and I’m not sure what to look for/test next. Here are some things I’ve tried:

1. Tried the tubes in a different amp. All good.
2. Tried the speaker with a different amp. Good.
3. Power tubes are biased at 55%
4. This amp has a power amp input and so I’ve tried injecting sound from another amp’s SEND (preamp) and the same exact problem happens so I think I can safely say that the issue is isolated to the Phase Inverter and beyond, right?
5. Removed the NFB temporarily just to see that’s not causing any issue
6. Speaker output ground
7. Verified the capacitor and resistor values of the phase inverter and output section

Here are the voltage readings I’ve taken so far (and all look great as far as I can tell):

B+: 410V
B+2: 407V
B+3: 375V
B+4: 370V

Rectifier Heater: 5VAC
All other heaters: 6.3VAC
(heaters are right on the money)

Bias supply: 55VAC
Rectified bias: -47.1V
Bias after bias pot: -39.8V

V6 OT Plate to B+ Resistance: 33.3 Ohms
V7 OT Plate to B+ Resistance: 40.8 Ohms

	Pin 1	Pin 2	Pin 3	Pin 4	Pin 5	Pin 6	Pin 7	Pin 8
V1 7025 Preamp	254		1.7			256		1.7
V5 12AT7 Phase Inverter	391	0.449	10.53			369.3	0.45	10.53
V6 6V6 Power Tube			409	408	-39.74	408		18.3mV
V7 6V6 Power Tube			409	408	-39.82	408		18.3mV
V8 GZ34 Rectifier	410

Attached is my schematic and layout diagram.

Any idea on what sounds suspect or what I should look into next?

 

[King Fan]

Bummer for sure. Do I recall you recently build an audio circuit tracer? Not sure... there were a few audio trace threads here in the last couple months....

 

[Ten Over]

Something wrong with V5. Seems to have very little current on both triodes. Is the 470 Ohm resistor (R30) really 470 Ohms?

 

[joulupukki]

Bummer for sure. Do I recall you recently build an audio circuit tracer? Not sure... there were a few audio trace threads here in the last couple months....

I have stuff to make an audio tracer, but I don't know how much that would help since the audio is definitely making it in all the right places. Maybe an oscilloscope. But...I don't think that's needed (read on).

Something wrong with V5. Seems to have very little current on both triodes. Is the 470 Ohm resistor (R30) really 470 Ohms?

You, my friend, are a genius. I wasn't methodical enough and what I installed is actually a 470K resistor instead of 470R! So...let me get that switched out and I'll report back. This makes sense as to why the PI is not amplifying hardly anything!

 

[King Fan]

Congratulations, sir, you are our 100,000th TDPRI member to install a 470K for a 470R. You've won our top prize (big pause).... "A working amplifier." I hope...

 

[schmee]

It's always some little thing on new builds.

 

[joulupukki]

Good and bad news…

Good news is that switching the 470K out for a 470R in the LTP brought the PI and amp to life! Thank you @Ten Over !!!

I’m heading back to my main build thread for the bad news (TLDR: amp isn’t functioning like it should).

 

[2L man]

How other PI anode voltage can be higher than B+3? Are anode resistors correct? I think PI cathodes should come higher because/if there is 22k tail resistor? Is bias resistor correct?

 

[joulupukki]

How other PI anode voltage can be higher than B+3? Are anode resistors correct? I think PI cathodes should come higher because/if there is 22k tail resistor? Is bias resistor correct?

Sorry 2L, after fixing that particular problem, that brought the PI voltages back around where they ought to be. I posted an updated voltage chart here: https://www.tdpri.com/threads/single-channel-ab763-build-s.1120924/page-6#post-11767685