Help needed - Strange PI voltages on JTM45-like build

[bulatovic]

Hi all,
new guy here, but quite some time building amps for a hobby. I've done a few 18watt Marshalls playing with VVR, a 5e3 with reverb, Deluxe Reverb, restored a nice 1966 Dual Showman, changed the guts of a 1978 Superbass to 1967 Spec with turretboard and mustards... so i've got some reasonable theoretical knowledge how those things work, also having read a few books, getting inspiration and details from Merlin Blencowe's guide on how to build guitar tube amps...
Now nothing prepared me to the madness i'm experiencing with my current build. It's a JTM45/Bassman based build that should suit a gigging guitarist in a nice 1x12 combo (yes it does sound stupid vs. 4x12, but hey.) with a few tweaks after it's proved to be working (different cathode bypass caps, NFB resistors, tone stack options, possible VVR or passive attenuator enclosed...)

I've completed all the wiring, doublechecking all the connections for any loose wiring, bad solder joints, different reading resistors and caps, bad ground... all checked out well.
Then the usual voltage checks:
Insert the GZ34 rectifier - all is well, all the voltages check-out just fine, 4.9V for heater, 6.28V for filaments of preamp and power tubes, great!
In go the preamp tubes:
V1 (triodes in parallel) - pin1&6: 156V, pin2&7: 0, pin3&8: 1.48V - all is well
V2 - pin1: 146V, pin2: 0.1V, pin3: 0.2V, pin6: 295V, pin7:147V, pin8:145V - all clear
then we come to the Phase Inverter:
V3: pin1: 107V, pin2: 157V, pin3&8: 102V, pin6: 343V, pin7:85V, - Whhhaaaat?
Not only that the triodes are significantly disbalanced, but grid being at higher voltage than plate?!
see the schematic and voltages:
https://imgur.com/jmeeAv6

So the troubleshooting started, checked all the wiring again, all the components values check out, even rewired to another socket using new resistors and caps. Nothing. Tried removing NFB, removing presence circuit, still nothing. Suspected a leaking capacitor and a bad cathode follower tone stack and wired output of 1st preamp stage (mixer resistor after volume pot) to the grid of Phase inverter, hoping to eliminate DC current out of the grid input - now slightly different situation with voltages happened, but still way unbalanced (i suspect different phase of the input?)
V3: pin1: 410V, pin2: 0V, pin3&8: 28.5V, pin6: 217V, pin7:14V,
schematic with voltages:
https://imgur.com/SvE6QhN

To be honest i'm out of ideas, and fear that ripping it all out and starting from scratch will not resolve the issue since i pretty much did cover all of that so far...

Would highly appreciate hearing anything from you guys that springs to mind on what may be the culprit of such stupid behavior of the PI.. i might be missing something super straight forward...

Many thanks in advance!
Best regards
Luka

[2deaf]

The second set of voltages looks right to me.  Amp won't work right, however.

[pdf64]

So all this is without the power valves in place?

What are the V3 voltages with just the rectifier and V3 valves in place?

[2deaf]

Pin 2 on the PI has a DC path to ground.  Put a blocking capacitor there. 

Pin 1 is at or near the HT voltage because that stage is in cut-off.

Pin 7 is half pin 8 because of your meter.

[glass54]

2deaf you beat me to it.
Pin 2, PI has an alternate DC path to the 1M resistor (via Vol Ctl)
Could be your problem? Try 22nf as 2deaf suggested
Regards
Mirek

[PRR]

> Pin 2, PI has an alternate DC path to the 1M resistor (via Vol Ctl)

That can't work. Who drew this?

[glass54]

bulatovic
Just for clarity I'm attaching sections of your drawings (around PI) and a Marshall JTK45 original Type1961.
What is your final schematic, so that our colleagues can assist?
Regards
Mirek

[DummyLoad]

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