Bias problems---

[Valvehead]

But if we are talking about SW4_A there would be only two wires, on pin 8 and pin 9 with pin 7 not connected. Correct?

ok--sorry i was referring to the front panel switch . There are 2 switches . On the back of the amp is the 2 tubes/ 4 tubes toggle ( sw4)...and the front of the amp there is the "half power / full / standby switch..so i was looking at the front one .. SW 2

ill take some more measurements later tonight --the confusion was on my part as usual....

[doombass]

ok--sorry i was referring to the front panel switch . There are 2 switches . On the back of the amp is the 2 tubes/ 4 tubes toggle ( sw4)...and the front of the amp there is the "half power / full / standby switch..so i was looking at the front one .. SW 2

ill take some more measurements later tonight --the confusion was on my part as usual....

Yes, and you will most likely find readings of R84 to come out at the approx 100k it is supposed to be once you have flipped the switch to 2 tubes operation. Then we're back at the strange stray current in the path/junction of C45/R43/R76/R78.

[Valvehead]

Yes, and you will most likely find readings of R84 to come out at the approx 100k it is supposed to be once you have flipped the switch to 2 tubes operation. Then we're back at the strange stray current in the path/junction of C45/R43/R76/R78.

yes R84 is fine.

I took some readings -measuring at pin2 , V6

sw4 2 tube mode / sw2 half power mode = amp starts around 118v and slowwwly...slowly drops down to around 22v......then all of a sudden it jumps up to 90v...then starts slowly dropping again. Im talkng about 1 volt per minute.

sw4- 2 tube / sw2 Full pwr mode = starts at 200v and creeps up to 214v

sw4 - 4 tube mode / sw2 Full pwr = 214v

sw4 4 tube mode / sw2 half pwr = 214v that drops slowly down to 104v...then slowly up to 107v..then back down to 104v

does this sound like a cap creeping up and down / discharging ??

V8 / V9 never changed voltage during this , still around 0v at pin 2

[The Dude]

SW2 lowers B+ by using a different transformer tap.
SW4 adds a high resistance into the cathode of two tubes to basically disable them.

I can see no reason why either of these is causing your problem, which is DC voltage on two of your tube grids. You need to figure out where it's coming from. If it were me, I'd start unsoldering things connected to those tube pins until the voltage went away to try and determine where it's coming from. It may even require unsoldering socket pins and checking the voltage on the trace to see if the socket is somehow "leaking". Look for moisture. Look for solder blobs. Look for debris stuck under the socket, Look for a part lead shorting a trace, etc.

[doombass]

Yes, one way to go could be to lift C45, R43, ,R76 and R78 ends towards their common junction point which in a healthy amp sets it free from everything else. If you still have that high voltage you clearly have a leaking pcb or something else shorting the trace on that junction. Then measure for continuity to other parts. I'd say for example "C" feeding the PI plates would be a logical start. If you find anything there you might initially want to isolate for example the B and C supplies from each other at the PSU end instead of unsoldering the sockets to narrow down which one is the culprit.

[Valvehead]

I did some testing tonight-- the following readings are with C45 and C46 removed from the amp pcb. I have it propped up so i can pull more parts easily now . No tubes in amp .

So , at V5a plate 340v ---- V5b plate 340v

the joint of C45 + R73 is 196v
the joint of C46 + R74 is 0v

r73/74 are going to ground like they are supposed to on other end.

joint of R71 and V5a plate is 340v
joint of R72 and V5b plate is 340v

point C is 344v

the switches are in full power and 4 tube mode

im not seeing any solder blobs or anything ..

[doombass]

I did some testing tonight-- the following readings are with C45 and C46 removed from the amp pcb. I have it propped up so i can pull more parts easily now . No tubes in amp .

So , at V5a plate 340v ---- V5b plate 340v

the joint of C45 + R73 is 196v
the joint of C46 + R74 is 0v

r73/74 are going to ground like they are supposed to on other end.

joint of R71 and V5a plate is 340v
joint of R72 and V5b plate is 340v

point C is 344v

the switches are in full power and 4 tube mode

im not seeing any solder blobs or anything ..

This confirms your previous measurements. That high voltage is still present. Now you need to begin checking why you have that voltage (post #49 and #50).

[Valvehead]

Made some progress !! Got rid of that high v at the grids. It must have been a trace or the pcb ... I have some parts " flying " off the pcb now = C45 / R73 / 76 / 78 --going directly to the pin at v6.

Now im measuring -2mv to +3 mv at all grids ..and the meter is just jumping up and down constantly

the pcb is grounded where it needs to be and no tubes are in yet

so without tubes im needing around 20mv or so yes?

[doombass]

Nice! I may be wrong but without tubes and healthy C45 and C46 I would'nt expect you to see much of any DC voltage at all on the power tube grids. Possibly your meter might detect a small amount of AC ripple from the DC supply depending on the efficiency of the filtering?

[Valvehead]

Ok, here is where Im at. Amp up and running--no red plating,,passes signal fine .

All tubes are in , but my grids arent settling down . So its in full pwr and 4 tube mode , at V6- V9 grids my meter is jumping around from 5mv up to 16mv...its all over the place on all tubes , but it settles on 12mv for about 2 seconds then moves up and down , fairly quickly . I re-checked my ground and tried a different meter .
Any ideas ?

When it cools down ill try a diff set of el84s i have

Also that bad trace i bypassed runs directly under C3 (radial cap 33u/450v )...i wonder if that is going bad or leaking...

[The Dude]

You aren't running signal through the amp when you check grid voltage are you?

[Valvehead]

You aren't running signal through the amp when you check grid voltage are you?

nothing plugged in... controls at 0

[The Dude]

IMO, if it's just mV, it's not anything to worry about anyway, unless you hear noise.

[TimmyP1955]

When the power tubes are in, make sure the amp is connected to a load - without one it might oscillate.

[SoulFetish]

Ok, here is where Im at. Amp up and running--no red plating,,passes signal fine .

All tubes are in , but my grids arent settling down . So its in full pwr and 4 tube mode , at V6- V9 grids my meter is jumping around from 5mv up to 16mv...its all over the place on all tubes , but it settles on 12mv for about 2 seconds then moves up and down , fairly quickly . I re-checked my ground and tried a different meter .
Any ideas ?

When it cools down ill try a diff set of el84s i have

Also that bad trace i bypassed runs directly under C3 (radial cap 33u/450v )...i wonder if that is going bad or leaking...

leaking electrolyte can absolutely cause instability/conduct voltages and could have been (could be) the cause of HV leaking on the grids. Often this can be tough to diagnose with radial caps. I've seen what looks like a complete breakdown on the underside of the caps with no apparent bulging or visual indication when looking over the top of the PCB. It can be easily obscured by silicone or a densely populated board