Wierd Blues Jr problem-helps

[pintail78]

I was getting ready to throw a basic billmod in a friends Ive done 3 before.

1) I was checking out the amp, and I'm getting 250 volt on pin 7 and 320 volts on pin 7 of the other el84. Reads 2.0 volts P1 to P2, and 3.5 P2 to P3. Swapped new tubes same, swapped phase inverter tube, same.

2) Pulled phase inverter, reads 3.5 volts P1 to P2, and 3.5 volts P2 to P3, replace phase inverter now reads 2.0 volts P1 to P2, and 3.5 P2 to P3,

R47, R35, R36 to spec. Im not sure what to check next????? Im baffled, but i think its something in the Phase inverter circuit. Any ideas would be helpful! John T.

[g1]

Sort out your voltages on pin7 of the output tubes first. They should be very close and the 250V reading sounds like that tube is conducting way too much. Do you have around -10V on pin 2 of both output tubes? What are the pin9 readings?

[pintail78]

yes Im at -10 on pin 2 on both sides, the other pin readings are close as I remember. Ive looked upstream of the EL84s and it seems ok, I am thinking it may be the OT, I have an extra so Im going to throw it in and do the billmod. Im hoping to see something along the way, so far I havent. Any other advise would be appreciated.

[pintail78]

I did the billmods, changed the OT, now Im reading between 315-290 (pin 7) and 290 (pin 9)on V4, V5 is stable at 310 and 290. So its working better but I am still getting this slow fluctuation on pin 7 on V4 between 315 and 290, the amp sounds good, but something is not right on that side, I swapped tubes, same thing, I am thinking its a resistor or capacitor thats changing with heat as its going bad....the question is which one? Either that or its maybe a solder joint maybe in the sockets???? Any other ideas out there?????

[acorkos]

resolder the power tube sockets just to rule them out. and meter the resistors associated w/V4...R17, R18, R20, R21, R25. Diode CR11, too.

[g1]

I remember getting some weird readings on that model due to the position of my hand (inducing hum?). Maybe not the problem but you can rule it out by using clip leads.

[pintail78]

I think I got it fixed, I resoldered the critical areas of the board and power sockets, checked all the resistors and voltages coming up to V4. Checked for continuity on all the billmod stuff. I replaced C15 and C16, as C15 was suspect. I also pushed the ribbon down on v4 and v5, and up on v1, v2, v3, that seemed to help the most. Seems stable and sounds great now, lets see how it does after a few hours of abuse. Thanks everyone.