JCM-900 SL-X DC filament not so typical problem

[booj]

typical dead filament voltage on V1 - 3. Br3 had an open leg. I replaced that with (4) 6 amp individual diodes. Connections seem ok. I'm getting 7+ volts AC in. With the 10,000uf cap in I was getting .56 volts DC out. I took out the cap and the tubes. Now I get 7+ volts in and +92 volts out??? Not 9.2 volts, but 92 volts. It's late. Just hoping someone has had this problem before and can give me a clue.

[soundguruman]

It's always possible that some other circuit is bleeding into the heater supply, but test again.
It's not impossible for plate voltage to bleed into heater voltage on a fiberglass board, and I have seen it occasionally.

[g1]

BR3 seems to be connected to BR1, that is the only clue I can give, I'm waiting for someone to explain BR1.
I am currently finding the drawing very offensive, but a lot of that is quitting smoking cold turkey after 30+ years.

[soundguruman]

BR3 seems to be connected to BR1, that is the only clue I can give, I'm waiting for someone to explain BR1.
I am currently finding the drawing very offensive, but a lot of that is quitting smoking cold turkey after 30+ years.

No problem, I quit every night before I go to bed.
Just light up a smoke, it's required for fixing amplifiers.
It is not impossible for preamp plate voltage to bleed into
the heater circuit, on a fiberglass board. I have seen it happen
sometimes.
Do you see any burned spots on the circuit board?

[g1]

He was not getting any DC when C8 was in circuit so I don't think it's bleed from the plate voltage.
BR1 appears to be some clipping circuit attached to V1 & V2 cathodes. However, it appears to share a common with BR3. I'm wondering if some interaction is giving the high reading when C8 and V1,2,&3 are removed. Or it could be a meter issue.
Suggest installing at least one preamp tube or any cap. in place of C8.

[booj]

Thankyou soundguruman and g-one! Yes the board is dark where the rectifier resided. Yes the scats are really bad. I'll clean up the board and dis-connect what is downstream and begin isolating the problem. I really appreciate your responses! I thought I was losing my mind and I don't smoke!

[J M Fahey]

I see it from a very different point of view.
1) *How* are you measuring your .56 DC volts at C6 (10000uF)?
If to ground, I think that circuit may be "floating" and thus measuring to ground can give any random wild voltage which means nothing.
Please measure AC voltage across the AC bridge legs, and then DC voltage across the DC ones and across C6, as a confirmation.
2) following with my "floating" suspicion, that filament may be elevated respect to ground, and the mysterious BR1 may be supplying such a positive voltage.
Of course with no indication as to what transformer winding feeds Br1, I have to guess more than usual.
3) and of course that "schematic" is crap.
An absolute lack of respect to technicians.

[booj]

Agreed J M! I got it fixed. I re-flowed the solder on my new "discreet bridge". The 92 volts was measured to ground. I figured it was floating and put the cap back in.
HOWEVER, now I can't bias either the matched quad of Mullards the customer supplied or a set of matched JJ EL34's. They run 300 - 180 - 180 - 300 degrees F. they are all running cold and the output power is about 70 watts max. into 16 ohms. (but only with the impedance selector set for 8 ohms. When set for 16 ohms I get about 50 watts)
I may have to start a new thread.

[g1]

OP fuses good? Screen resistors (R31 thru R34)? Hi/Lo power switch?

[booj]

Thanks for your reply g-one! What's an OP fuse? The H.T. fuses are good. I'll check the values of the screen resistors. There is voltage at pin 4 of each tube. That Hi-Lo power switch will get looked at.
Is this the Marshall model that has problems with ghost voltages and weird biasing problems?

[J M Fahey]

now I can't bias either the matched quad of Mullards the customer supplied or a set of matched JJ EL34's. They run 300 - 180 - 180 - 300 degrees F. they are all running cold

What does this mean?
In numbers, please.

There is voltage at pin 4 of each tube

Idem.

Hi/Lo power switch?

Where is it set? What +B voltage you have?

What's an OP fuse?

The Output Tubes individual fuses.
We suspect a couple tubes are not connected, but you must check, of course.

[booj]

sorry, by "voltage" I meant "normal voltage" or about the same as the plate voltage of about 525+ volts (with no tubes in the sockets)

What does Idem. mean? the high/Lo power switch connects the outputs to triode mode in the low setting.

The H.T. (actually cathode) fuses are the only ones I see. Those are good. They also cover either the right half or the left half power tubes. The problem is the two inner tubes are cold while the two outer tubes are hot.

I'll just give it a close point to point inspection. I do suspect that two of the tubes are not operating. It's probably something dirt simple I'm not seeing.

Thanks for your reply J M!

[J M Fahey]

Idem means "the same". Just to avoid rewriting the exact same answer.
Put those tubes in those sockets and post all 12 voltages, meaning each plate, screen and cathode voltage, measured at each socket pin.
If possible, try to touch the actual tube leg from below, we might have a bad contact socket.
We might find a surprise there.

The problem is the two inner tubes are cold while the two outer tubes are hot.

EDIT: repeat tests in both switch positions, we might also have a surprise there.

[g1]

The H.T. (actually cathode) fuses are the only ones I see. Those are good. They also cover either the right half or the left half power tubes. The problem is the two inner tubes are cold while the two outer tubes are hot.

H.T. Refers to "high-tension" (high voltage), so when they speak of HT fuses they are in the DC B+ line. These are cathode fuses, I think they are labelled as "OP valve fail" or some such thing on the actual amp.
The schematic shows one fuse going to V4&V8, and one going to V5&V6. So I think you are wrong about the right & left half power tubes. One fuse should be the inner pair, one fuse the outer pair. This is why I'm suspicious of the F2 fuse or the connection to it.

[booj]

Thanks guys! I'll check the amp with the tubes in it It's fused to V6-V7 and V5-V8. Nonetheless yes, the fuses protect the inner two and the outer two tubes. big clue there. the hunts on!