90s Marshall JCM900 4100 Bias

[ca7922303]

Your bias probe has a way to measure the plate voltage? They should all be fairly close, or at least 2 pairs same. Best would be to measure directly at pin3 of tube sockets. Something in the bias probes is not right or you are not getting a good connection.
You were getting around 495V with around 45mA and now around 505V with around 30mA. That is the correct kind of relationship, lower idle current allows higher plate voltage.

The one that shows 31mA at 490V, I think the 490V is incorrect. That's why I say check those voltages directly at pin3 of socket.
If they are all around 30 to 36mA and all plate voltages around 505V, I think you are good. The caps are 500 Working volts, and can work their whole life at 500V, with some leeway to run higher. 505V is not going to damage them.

Set vr2@-62.4vdc and am getting 500vdc on all pin#3s. Bias probe now reads: 16mA/19mA/22mA/20mA. Which one best to go to?

[g1]

Set for around 30mA and recheck voltages at pin3's. Should go a bit lower than 500V.

[Enzo]

In my humble view, it isn't about the plate voltage, it is about the dissipation. That is plate voltage times current in amps.

This is just a guitar amp. A difference of 5 volts on 500volts is 1%. Such precision in not needed. The difference between 4995v and 505v on a plate is not spacious. Nothing cosmic happens as you cross 500v.

[ca7922303]

Set for around 30mA and recheck voltages at pin3's. Should go a bit lower than 500V.

Your right about the bias probe I believe. I was setting to 30mA when tube white-plated and smoked one of the 5w470 ohm cement resistors. It is O.L now, so will order if I don't have an extra one and will set the tubes with vr2. Won't use bias probe again, it was going up and down from 60mA to 20mA, but I wasn't fast enought to power off.

[ca7922303]

Set for around 30mA and recheck voltages at pin3's. Should go a bit lower than 500V.

Installed good 5w 470 ohm resistor and notice that one tube is red plating. Move it and red plating moved with tube. Replaced all four(4) 6L6s with known good ones and red plating not occuring but tubes at sockets 2 & 3 are getting super hot and 1 &4 are not. Have vr2 set around -40vdc.

[ca7922303]

Installed good 5w 470 ohm resistor and notice that one tube is red plating. Move it and red plating moved with tube. Replaced all four(4) 6L6s with known good ones and red plating not occuring but tubes at sockets 2 & 3 are getting super hot and 1 &4 are not. Have vr2 set around -40vdc.

Tried vr2@-52v and getting 500vdc@pin #3. VR2@-40v was about 475v@ pin#3.

[ca7922303]

Tried vr2@-52v and getting 500vdc@pin #3. VR2@-40v was about 475v@ pin#3.

Tried 30mA, vr2@-55v, pin #3 510vdc.

[g1]

If you replaced the tubes and some are hotter than others, they may be mismatched.
You had 30mA at one tube, or are they all around 30mA ?

Is your line voltage fairly stable? You have a variac so you should always measure the AC line going into the amp and adjust for 120V so you get consistent results. If the line voltage is not consistent the other voltage readings will not be either.

[ca7922303]

If you replaced the tubes and some are hotter than others, they may be mismatched.
You had 30mA at one tube, or are they all around 30mA ?

Is your line voltage fairly stable? You have a variac so you should always measure the AC line going into the amp and adjust for 120V so you get consistent results. If the line voltage is not consistent the other voltage readings will not be either.

So issues with the sockets themselves or components to those two(2) sockets couldn't cause extreme heat difference. All right(8) 6L6s involved got hotter on sockets 2 & 3 only?

[g1]

That's why I asked which socket the 30mA reading was coming from. The sockets that are running hotter must have a much higher reading.

Are 1&4 even running? Check their fuse. Also check that they are getting their proper voltage at pin4.

[ca7922303]

That's why I asked which socket the 30mA reading was coming from. The sockets that are running hotter must have a much higher reading.

Are 1&4 even running? Check their fuse. Also check that they are getting their proper voltage at pin4.

Replaced blown OP fuse 1-4, tubes now equally hot, all four 6L6s are around 30mA, VR2(-52v), 4-pin#3s(485vdc), 4-pin #4s(-51vdc).

[g1]

Sounds good.
The bad tube that red-plated and smoked the 470R screen resistor must have blown the fuse.

[ca7922303]

If you replaced the tubes and some are hotter than others, they may be mismatched.
You had 30mA at one tube, or are they all around 30mA ?

Is your line voltage fairly stable? You have a variac so you should always measure the AC line going into the amp and adjust for 120V so you get consistent results. If the line voltage is not consistent the other voltage readings will not be either.

Bought this power analyzer to check current as suggested many times before while using a Variac. Are these specs up to par to check ac current on guitar amps?

[ca7922303]

Sounds good.
The bad tube that red-plated and smoked the 470R screen resistor must have blown the fuse.

Thanks again, really appreciate the great help.