Help with diabolical PI voltages

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Big_Bottler
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Joined: Sun Feb 14, 2021 12:03 pm
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Help with diabolical PI voltages

Post by Big_Bottler » Sun Feb 14, 2021 2:39 pm

Hi guys and amp building gurus!

I'm an old forum member returning (no idea what mail or nick i was using :palm: ) with some experience behind me building tube amps: 18w Marshall, Deluxe reverb, 5e3, JTM50, several champs.. and i do get fair amount of theory behind the amps, having read Merlin Blencowe's book on tube amps.. and so far i've been lucky with all the builds and troubleshooting.
But to be honest nothing has prepared me to the nightmare i'm having with the current build.
It's based on a Bassman/JTM45 with certain tweaks and options (bright cap 250/500pf switch, V1 cathode 0.68uf/25uf, V2a cathode bypass options off/0.68/25uf, Bass/Lead tone stack switch, 39k/56k NFB switch, Fixed/Cathode bias, VVR/no VVR - and i plan to have it enclosed into a small 1x12 so it's giggable.
For the discussion here i've eliminated all the options/switches, so it's a regular JTM45 with 0.47uf instead of 1uf coupling caps.
now, the issue:
I've triple-checked all the connections for cold joints, continuity, ground, etc, and all measured up well.
Then i inserted GZ34, and all is well, all the heaters, spot on! 4.9V for rectifier, 6.28V for preamp and power tubes.
Then go in the preamp tubes:
V1 - voltages check out just fine (pins1&6: 156V, pins2&7: 0V, pins3&8: 1.48V)
V2 - just as well (pin1: 146V, pin2:0.1V, pin3: 0.2V, pin6: 295V, pin7: 146V, pin8: 145V)
and by the time i got to V3 i was hoping for smooth sailing, but then:
V3: pin1: 107V, pin2: 157V, pins3&8: 102V, pin6 343V, pin7: 85V
https://imgur.com/jmeeAv6
I've tried literally everything. Swapping different preamp tubes, complete new PI wiring with a different valve socket and new components, removing NFB, removing Presence wiring, going cathode bias... even suspected something wrong with cathode follower tone stack (like a bad coupling cap), so i went and plugged volume control via grid 470k resistor to grid of PI to make sure that i don't allow for any DC on PI grid. Things are still super weird with voltages on PI, now in a different way (suppose change of phase of input?)
V3: pin1: 410V, pin2: 0V, pins3&8: 28.5V, pin6 217V, pin7: 14V
https://imgur.com/SvE6QhN

What on earth am i missing here?!
Running out of patience, but fear that even by ripping out everything and starting from scratch i'd end up with the same issue :shrug: :scratch: Stuck as i can be i guess...
From my experience this is not how Long Tail PI should behave, it should be fairly balanced with voltages of both triodes, regardless of what you plug into its grid.

Any help much appreciated, so please just shout out freely whatever springs to mind, maybe i'm missing something easy to fix...

Many thanks!!!
Best
Luka

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