Marshall 9005 bias supply?

[Bubbastain]

Been a while since I've been here but you guys were super helpful before. I picked up a Marshall 9005 power amp. Channel A works fine. Channel B had a pretty good hum when I took it off standby and then the two power tubes started to red plate. I hooked up a bias probe to each tube and turned the trim pot all the way to the cold side. Still it started to climb too high. Popped in a old but still good pair of el34's and again it started to climb too high. I shut it down once it started climbing past 25.
So what should I change out. I can safely work on an amp and solder, but my skill peaks at paint by numbers. If I'm told what to change I can do that. I will post a gut shot as soon as I get home. Any help would be appreciated.

[sluckey]

Before changing anything, pull the EL34s and measure voltage on pin 5 of each EL34 socket. Turn bias pot from one end to the other. What is the maximum and minimum voltage reading for each tube?

[Bubbastain]

Before changing anything, pull the EL34s and measure voltage on pin 5 of each EL34 socket. Turn bias pot from one end to the other. What is the maximum and minimum voltage reading for each tube?

I measured 483v on pin 5 on all power tubes on both channel A (good side) and channel B. Turning the bias trim pot (PR2) from one end to the other made no difference.

[sluckey]

I measured 483v on pin 5 on all power tubes on both channel A (good side) and channel B. Turning the bias trim pot (PR2) from one end to the other made no difference.

Impossible! Please redo. Count pins carefully.

[Bubbastain]

I measured 483v on pin 5 on all power tubes on both channel A (good side) and channel B. Turning the bias trim pot (PR2) from one end to the other made no difference.

Impossible! Please redo. Count pins carefully.

Okay. Wrong pin. Pin 5 is 0. Turning the trim pot made no difference. On channel A adjusting the trim pot brought it from 28 to 38.

[thetragichero]

okay so you're not getting bias voltage on the output tube grids. makes sense why the tubes are redplating

here's a schematic i've found:

time to work backwards. the phase inverter is confusing me

[sluckey]

How much voltage do you measure at the negative lead of C13?

How much voltage do you measure at the negative lead of C8?

[Bubbastain]

0v on C8 and C13 in standby or out with no power tubes installed.

On channel A I get 54 on C13 and 45 on C8 in play mode and 72 on C13 and 66 on C8 in standby

[sluckey]

Measure resistance across C13 leads. What have you?

[Bubbastain]

Measure resistance across C13 leads. What have you?

Cheap DMM set to ohms 200k it's goes to 84. Amp completely off right?

[sluckey]

Does your meter have a diode check function? If so, unsolder one end of D1 and check it.

[Bubbastain]

Does your meter have a diode check function? If so, unsolder one end of D1 and check it.

I measured 593 on D1.

[PRR]

I measured 593 on D1.

Volts? Ohms? Rods?

Swap the leads. Now what?

[Bubbastain]

I measured 593 on D1.

Volts? Ohms? Rods?

Swap the leads. Now what?

Not sure. I unsoldered one leg, set my mm to the diode test setting and put the negative probe on the side with the silver band and the positive probe on the other end and 593 came up. If I reversed the probes no number showed up.

[sluckey]

Not sure. I unsoldered one leg, set my mm to the diode test setting and put the negative probe on the side with the silver band and the positive probe on the other end and 593 came up. If I reversed the probes no number showed up.

That means the diode is probably good. Was there a decimal point in front of the 593, IOW, was it really .593?

[sluckey]

Verify you have several hundred AC VOLTS between the white and blue wires. (One probe on white wire, other probe on blue wire.)

If voltage is OK then problem is in the schematic snip. Most like suspects are C16, D1 (already checked), and C13.

You have two identical circuits. Compare checks between the two. Should be a fairly simple fix.

[Bubbastain]

I measured 334 vac on the white & blue wires. Same on channel A.
So should I go ahead and order and swap out C16 and C13? Or is there anything else I should check? What else should I replace while I'm at it?

Thanks for the help so far.

[shooter]

just for prudence's sake;
with power off and no PA tube installed, ohm the 4 resistors connected to the 2 power tubes
the schematic shows 2 10ohm and 2 2k ohm

[sluckey]

I measured 334 vac on the white & blue wires. Same on channel A.
So should I go ahead and order and swap out C16 and C13? Or is there anything else I should check? What else should I replace while I'm at it?

Replace C8 and D1 while you're at it. Please note... C16 is a special Class X cap. Replace only with another 47nF Class X cap.