Phase Inverter Tube voltages?

[Paultergeist]

Greetings!

I am in the final assembly stages of an AB763 (Blackface) build. The amp is derived from a Weber "6A20HP" kit, basically a Deluxe Reverb running a pair of 6L6s.

The amp has a problem with BARELY making any sound, with a harsh distortion edge to what I can get to come out of the speaker. After examining the tube pin voltages, it has been suggested to me that my Phase Inverter (PI) tube voltages do not look correct:

PI tube voltages, DC voltage referenced to ground (heaters omitted):

1. 339V
2. 0V
3. 10V
6. 332V
7. 0V
8. 10V

In short, I now wonder if the PI grids/cathodes should show more significant B+ voltages? I can clearly SEE HOW the plates get their B+, but it is less obvious to me IF/HOW the grids and cathodes on the PI tube would obtain a B+ value? I also cannot find any errors in my wiring thus far.

Can anyone who has built an amp with this style of PI confirm whether or not my PI tube voltages are in the ballpark, or is something truly amiss?

Also, what sort of PI strategy is this?

Thanks very much for any thoughts,

Paul

[NTBluesGuitar]

This amp uses the long-tail PI.

Your lack of volume and harsh distortion could be coming from a bad ground connection. I had done an amp where the 220K resistor junction in the bias-to-PI area wasn't grounded in a cathode bias setup, and it gave me similar symptoms. The cathode bias resistor got really hot, too, but you're in a fixed bias deal.

I'd double-check all the ground connections in the bias and PI areas to start.

[Paultergeist]

Hey NT,

Thanks for the help.

Turns out that the cathode resistor at the PI tube -- called for being 470 Ohm in the schematic -- was installed as a 470K Ohm on the build. I don't know how I did that -- I try to be verty careful -- but I accidentally increased the value of that PI cathode resistor by a factor of 1000!

I swapped out the 470K for a (proper) 470, and the amp seem to now be working correctly. Another lesson learned!

Thanks again,

Paul

[Kap'n]

Hey NT,

Thanks for the help.

Turns out that the cathode resistor at the PI tube -- called for being 470 Ohm in the schematic -- was installed as a 470K Ohm on the build. I don't know how I did that -- I try to be verty careful -- but I accidentally increased the value of that PI cathode resistor by a factor of 1000!

I swapped out the 470K for a (proper) 470, and the amp seem to now be working correctly. Another lesson learned!

That would do it. Good detective work. Half the stuff I know about amps is from mistakes I've made. If I remembered all of them, I'd be a guru.

[NTBluesGuitar]

I actually made that same mistake on a build not too long ago.

As Kap'n says, great detective work.

[Mandopicker]

"Half the stuff I know about amps is from mistakes I've made."

Ditto that!

The problem seems to be that the color coding on new CC resistors are not as good as the old stuff in vintage amps. Often dull in color.

Silly sounding but...a good rule of thumb for me is to just measure any suspect resistor before soldering it in. That litle extra step takes less time than it would take to trouble shoot your way out of a completed amp with strange symptoms.

Anyway, good catch Paultergeist.

[Paultergeist]

Thanks, everyone, as always, for the continued advice and support!