SVT-VR bias issue

[lowell]

R24 in the bias circuit keeps frying and the output tubes are redplating. I cannot turn the amp on because the tubes are flashing and R24 fries. R24 fries even if the tubes are pulled out. I don't really get this bias arrangement... Any guidance much appreciated.

working on getting schem...

[lowell]

here's schem...

[DirtyGeorge]

R24 is the cathode resistor for J5:B the CF driver for the output tube grids. J5:B could be shorted or C10 could be passing DC. Check the bias voltage. The cathode of J5:B is supposed to be around -45V so the grid of J5:B should be more negative than that (maybe -55V). With J5:B removed what is the voltage at R25, R28, C10 junction?

[lowell]

thanks for your help w/ this... I have J5:B removed and R24 still fries... not sure if I can make a good measurement w/ that being the case.. or actually that tells us that it's not J5:B that's causing the problem right? The socket for that tube looks fine too... no arcing. I'll pull end of C10 at R28 and see if any leakage.

[DirtyGeorge]

With J5 removed R24 should have -150V on one side and open on the other (assuming the output tubes are also pulled). Even with the outputs in, there should be no path for current flow thru R24. I would look for a short somewhere between R24/J5 junction and the output tube grids.

[lowell]

that's what I thought... I will check it.

[lowell]

disconnected power tube board and R24 does not fry... think I'm onto something... at this point I cannot see the components on that board till I remove it.. however I can't see what could be shorted in there... all that's in there are the grid stopper resistors and the tube sockets. The only thing I can guess it to be is a shorted grid pin on the socket. I will see and report back.

[lowell]

ok I see all grid resistors on top half of schem are fried. It seems they fried because a screen resistor fried and flashed off the protective layer on one of the grid traces, shorting the 3 top tubes' grids to the screen supply. I'll clean the carbon off and will be replacing all grid and screen resistors as well as R24 again. Will report back w/ results.

[lowell]

Ok the amp shorted again in same spot... the same plate resistor fried and shorted to the grid trace in the same spot. Wow there must be a lot of electrical pressure/attraction at that spot. I replaced all again and this time put that plate resistor on the opposite side of the PCB as to insulate it more from the arced trace.

I'm now biasing and have adjust the 2 green LEDs to light. It says on the balance to adjust to light LED w/ 25v 40hertz signal. I'm using a low E string on a bass and measuring voltage at 25AC w/ my meter into a dummy load... I cannot get the balance LED to light and stay lit. It will flicker if I move it around. Anyone know why this might be?

[oc disorder]

I'm curious re this adjustment..I had one a while back feeding it a sine wave from an ocsillator and it seemed unstable as you describe.
As I had the luxury of an oscilliscope observed the balance control altered the amount of signal between the +'ve and -'ve halves of the sine wave at the output.
I just adjusted it to make the waveform as symmetrical as possible.
Hard to hear any difference at low volume.
Also perhaps the bass guitar signal is not consistant enough
(full of harmonics and decays rapidly) for the test.
Maybe a 40Hz flute from a synth may be a better option !

[Enzo]

Is a socket arcing? Once they arc, they should be replaced.

[lowell]

no arcing sockets... at least not visually. Ever had an arced socket that you could not see?

[Enzo]

Can it arc below the surface? Yes. Turn the room lights out and watch close, the little blue light from an arc is not always bright.

[lowell]

Ok thanks Enzo.. I'll try that.

[lowell]

Ok this amp has come back to me w/ same problem and worse... The power transformer is fried and smokes after 1 minute of turn-on.

Another resistor fried and shorted to the trace. The first time this amp came in R18 had shorted to grid trace. I replaced all plate resistors w/ 5ohm 5w cement types. This time it is on the other side of the OT : R3.

The customer said the amp shut down in the same song as it did the first time. He said when he steps on his distortion pedal and octave pedal to get this CRAZY sound that that's when the amp blew both times. I've never experienced an instance of a pedal overloading an amp. Does this makes sense to anyone? Also, how might the power transformer fried from this?? Too much current was being drawn w/ the plate supply shorted to the grid? And yes the fuse was blown when it came in.