Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: hmontgomery on January 12, 2026, 06:32:07 pm

Title: Ab763 V3 issue
Post by: hmontgomery on January 12, 2026, 06:32:07 pm

Building an AB763 two channel, using the Hoffman turret board.  6L6s with a MM Vibroverb PT and a Classic Tone 40 Watt OT.  My issue is I am showing 8-12 volts depending on the 12AT7) on Pin 2 of V3.  Tried multiple tubes.  Have checked wiring.  Replaced tube socket.  This started by trying to track down a nasty distortion when volume was above 8 or nine.  At lower volume, amp sounds great.  Another post on the distortion issue pointed me to the reverb circuit.  Pin voltages for V3 are:


Pin 1, 6:  427V
Pin 2, 7:  11.7V
Pin 3, 8: 7.68V


Any thoughts would be appreciated.




Thank you

Title: Re: Ab763 V3 issue
Post by: AlNewman on January 12, 2026, 06:58:50 pm

Yeah, you shouldn't have much if any measurable DC voltage on the grids of the reverb driver.  My guess is is that something is wired wrong.

Maybe the 500Pf cap is leaky.

Title: Re: Ab763 V3 issue
Post by: hmontgomery on January 12, 2026, 09:35:39 pm

Was wondering about the 500.  I have some on order but won't be here for a couple of days.


Thanks

Title: Re: Ab763 V3 issue
Post by: SEL49 on January 12, 2026, 10:05:48 pm

It's highly unlikely that 500pF is the issue because there is no dc voltage on the other side to be leaking through. Look at the schematic.

Title: Re: Ab763 V3 issue
Post by: AlNewman on January 12, 2026, 10:29:40 pm

Yes, Sluckey's right.  I was looking at the 763 schematic where the signal for the reverb passes through the B+ on the drawing, but there's no connection there.
So that means there's an issue with the parallel 12AT7.  There must be something crosswired between the grid and cathodes between the 2 sides of the tube.

Title: Re: Ab763 V3 issue
Post by: pdf64 on January 13, 2026, 02:15:15 am

If the grid voltage isn't referenced to 0V common, it may be due to the grid leak resistor not being connected.
What's the resistance between V3 pin2/7 and the chassis?

Title: Re: Ab763 V3 issue
Post by: hmontgomery on January 13, 2026, 09:43:08 am

Resistance from V3 pin 2/7 to ground is 1.001M.  And to confirm, 1 is connected to 6 and connected to blue transformer lead.  2 and 7 connected to 500pf lead and 1M resister to ground, and 3 and 8 connected to 2.2k resister and 25uf cap to ground

Title: Re: Ab763 V3 issue
Post by: stratomaster on January 13, 2026, 10:02:59 am

Do you have any DC at the junction of the 0.022μF and 500pF caps? There absolutely shouldn't be any here.

Title: Re: Ab763 V3 issue
Post by: punkykatt on January 13, 2026, 10:10:39 am

If you pull the tube do you still have voltage on pins 2 & 7?