Super reverb PI weird voltage

vacuumdust

Member
Messages
24
Hello! Just finished a scratch build AB763 build. After my usual routine of first time power up I was enjoying the tone of the amp at low volume. Happy to have little to no hum or buzz (even on the reverb!). When I dug in to the strings however I got a nasty jump scare/loud distortion. Checked voltages and everything is right on except the plate of the PI (12ax7) at pin 1. Pin 6 (other plate) is 335vdc from node C across 100k res. Pin 1 however is 449vdc! That 82k res is only dropping 4 volts? Tried with no PI installed, swapped 3 different 12ax7 and replaced 82 k res. All same results. Kinda stumped here...I'm just assuming at this point that the loud distortion is a symptom of this anomaly. Schematic shows 320vdc at both pins 1 and 6 of the PI.
 

vacuumdust

Member
Messages
24
Didn't mention I installed a master volume pre PI (I was asked to). So even with the master cranked there's always essentially a resister to ground off of pin 2 of PI. Apparently that's not cool :/. I lifted the ground lug on master vol pot and the voltage in question went to normal. Unfortunately the amp also started squealing like a pig so I guess the OT leads to plate need to be swapped.
 
Messages
826
I think the PI here is an LTP and there should be high DC on the input grid of the PI, so you need a coupling cap (say 0.1uF) between the MV and the input to the PI.
 

Steppin' Wolfe

Silver Supporting Member
Messages
5,059
I will note that even the lower voltage is too high. What are the voltages at the cathodes?
Imho, the first thing that should be done..before playing....is to work up a full voltage chart.
 
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