JCM 900-4100 head Red Plating Issue

[Randall]

No problem going up in voltage on a cap, but why do you want to replace c9 and C10? They are blocking caps, do you think they might be leaking and affecting your bias voltage?

[vintagekiki]

I also want to replace C10 and C9 , they are rated 47nF /400v ... Can I use 47nF /630v? How can affect Using a more voltage version ?

When an amplifier is defective, do not change the components by heart for no reason.
So you will achieve nothing and you will learn nothing. With the help of schematic and a multimeter, do your best to find the CAUSE of the fault.
Random change of components (if you are lucky hands) only eliminates the consequences, the amplifier will work for a while, and since the cause is not eliminated, it will be returned as a complaint again for repair.

[moonmark]

No problem going up in voltage on a cap, but why do you want to replace c9 and C10? They are blocking caps, do you think they might be leaking and affecting your bias voltage?

I wanted to replace them to take advantage my visit to electronic store and bought a few more components to drop new caps into my JCM . Technically I have no reason to change them because I have not yet tested them one by one outside the circuit. It was just a proactive action that I was thinking, But as friend VintageKIKI said, substituting parts without a reason is not going to give me any knowledge.

[moonmark]

When an amplifier is defective, do not change the components by heart for no reason.
So you will achieve nothing and you will learn nothing. With the help of schematic and a multimeter, do your best to find the CAUSE of the fault.
Random change of components (if you are lucky hands) only eliminates the consequences, the amplifier will work for a while, and since the cause is not eliminated, it will be returned as a complaint again for repair.

Yes, You are right Sir, it was just a proactive movement in order to give fresh caps to my amp. This is my amp and probably it always be with me, but I dont want any complaints from myself :hahaha: I prefer play my guitar on this amp ASAP that come back to the electronic bench!

[moonmark]

Hi all.

I have good news for you, the amplifier is working very well, no more Red Plating !!! Thank you Rich from Tonefinder.net, DrSonic from Guitarristas.info, Randall, vintagekiki, g1, Pedro Vecino from this site and Chris in LA, Iron1, anitolli, gunner64, mickeydg5 and Ken0418 from Marshall Forum. I appreciate all your time and effort to make this amp work again!

The damaged components were C12 and D1. C12 was suddenly behaving like a resistor and D1 was shorted.

As advised for you guys, I checked the bias voltage, first I did it without the power tubes and with the speaker connected. I spend 20 minutes, checking the bias voltage stability. Then I installed the 6L6GC again and I managed to configure it at -50volts, I have both tested on pin 5 of the sockets (between -49.79 and -49.96V) in the 4 tubes, and also on the union of r26 / r27,at that point there is 50.26v approximately. Is it normal for it to fluctuate between 49.98 and 50.31? I was manually using the test tip and I was not using any crocodile clip, my shaking hands may be to blame

After that I change c9,c10,c15 and c16 to give fresh caps to my amp, and made again the bias process.

In that point I have a question...... some schematics and sites suggest 60-70% of 30 watts for 6l6gc and this is like 18-21 watts, so it means voltage between -38 and -48V.

What do you think about it? Now I have -50.26 aprox.

I appreciate your guidelines to solve this problem. I have my amp back and ready to play

Any suggestions or recommendations before fully assembling the head? I have taken the opportunity to give it an internal cleaning

[Randall]

"Is it normal for it to fluctuate between 49.98 and 50.31?"

-50v is just a round number, it is the idle watts you really care about. One set of tubes may require -48v of bias supply to make them idle at 60-70% (if that is your method of choice) and another set may require -52v to place them at the same idle dissipation wattage. So the bias voltage is just a means to an end to get your tubes biased where you want them. Plate voltage is another variable in figuring dissipation wattage. There are different schools of thought on this, but I generally will bias a Marshall such as this one at about 60% of max. diss. If it's off a couple of percentage points, I don't lose sleep over it.

[vintagekiki]

In that point I have a question...... some schematics and sites suggest 60-70% of 30 watts for 6l6gc and this is like 18-21 watts, so it means voltage between -38 and -48V.

Manufacturer are tied his amplifiers by the mainly to the tubes of a particular supplier, and for these tubes the required bias voltage for quiescent current is specified.
These tubes generally is recommend as part number, so the replacement tubes reduces to plug and play.

If tubes not specified by the manufacturer are used, bias voltage need to be adjusted.
Bias voltage adjust the quiescent current for a given operating class (A, B, AB), and the specified bias voltage is only an approximate value in relation to which the setting should be started.
For the same anode and bias voltage, power tubes can have different anode currents, and before installation it is require tubes matching.

http://web.archive.org/web/20100923051727/http://diyguitarist.com/GuitarAmps/PT-Biasing.htm

http://web.archive.org/web/20100923053412/http://diyguitarist.com/GuitarAmps/PowerTubeBias.htm

https://music-electronics-forum.com/forum/amplification/guitar-amps/maintenance-troubleshooting-repair/8554-about-biasing-tube-guitar-amplifiers

[Helmholtz]

[QUOTE=moonmark;n913119]
Is it normal for it to fluctuate between 49.98 and 50.31
[QUOTE]

Yes. That's a difference of less than 1%.
Amp supply voltages vary along with mains input voltage. Short term mains voltage fluctuations of up to 3% are quite common (at least in my place).
Good thing is that when mains voltage rises, B+ goes up while bias voltage gets more negative and thus idle dissipation essentially stays the same.