Supro black magick reverb - Red Plating power tubes 6973 Class A

[spunko]

The owner said that the amp does some kind of distortion or feedback at some notes or frequencies, and has been working like this since he bought it new some years ago. I tested it and the owner is correct.

I turned the lights off and I noticed one 6973 tube red plating like hell (V6), and the other one (V7) also red plating but just a little (original tubes). Swaped positions and the red plate follwed them, so the hotest one was now on V7.
I checked the power sockets and there were some cracked joints, so I re-soldered all pins from the power sockets and installed new tubes that the client gave me, and they also started red plating, but just a little, barely noticeable that I almost didn't see it.... but once you see it, you can't unsee it 
The PCB is darker around the power sockets from all the years of use, but it has never blow a fuse or a tube.

I'm not familiar with this tube, nor the amp, and I haven't found the schematic.

What could be the cause of red plating in this amp?

These are the readings I got from the old and new tubes. Also checked all the resistors and they are in spec.


Thank you very much.

[Latole]

Tube red plating in class A amp.

 It can only be due to the plate bias  ; the value is too low ( often see  on many amps ) or one of the tubes is defective.

What it the value of this resistor  ?
What is the bias current on each tube  ?

[Latole]

All tubes specs ;

https://www.tubesandmore.com/sites/default/files/associated_files/6973.pdf

[spunko]

I appreciate your reply

I used the shunt method:

V6: 42.2 mA
V7: 40.4 mA

I'm not sure about the resistor, how can I identify it?. There are two below the PCB. One is 470 ohm 5w, the other one is 200 ohm 15w. 
The 200 ohm resistor is soldered in a trace marked as "CAT".

Should I step up that resistor to 270 ohm to match the data sheet specs?

[spunko]

The board have seen some heat. There is the 200 ohm 15 watt resistor.

[astronomicum]

I appreciate your reply

I used the shunt method:

V6: 42.2 mA
V7: 40.4 mA

I'm not sure about the resistor, how can I identify it?. There are two below the PCB. One is 470 ohm 5w, the other one is 200 ohm 15w. 
The 200 ohm resistor is soldered in a trace marked as "CAT".

Using Ohm's Law, 0.042 x 470 = 19.7V while 0.042 x 200 = 8.4V. Your cathode voltage from your chart will give you the answer which resistor it is.

[spunko]

Using Ohm's Law, 0.042 x 470 = 19.7V while 0.042 x 200 = 8.4V. Your cathode voltage from your chart will give you the answer which resistor it is.

Thanks for the explanation.

If I got this right, in order to cool down the power tubes I have to rise the bias voltage, and step up that resistor?
55V / 0.042 = 1.3 K   

[SEL49]

The simplest way to determine which resistor is the cathode resistor is to measure resistance between pin 7 and the resistor leads. The lead that measures 0Ω is connected to the cathode. I'll bet the 200Ω/15W is the cathode resistor.

Increasing that resistor to 470Ω will lower the current through the power tubes and will likely totally eliminate any red plating. Here's an example that dealt with the same issue...

     https://sluckeyamps.com/supro/supro.pdf

[spunko]

The simplest way to determine which resistor is the cathode resistor is to measure resistance between pin 7 and the resistor leads. The lead that measures 0Ω is connected to the cathode. I'll bet the 200Ω/15W is the cathode resistor.

Increasing that resistor to 470Ω will lower the current through the power tubes and will likely totally eliminate any red plating. Here's an example that dealt with the same issue...

     https://sluckeyamps.com/supro/supro.pdf

Thanks!!! Checking the link right now. I’m going to try it and come back with the results. 

[tubeswell]

Always check the bias voltage before you pop new output tubes in.


If it’s cathode biased, check for DC continuity between cathode pins and chassis before switching amp on. If you aren’t measuring the expected resistance, then find out which connections are broken before popping the output tubes in.

[Platefire]

Have you checked the coupling caps on the power tubes side for any DC leakage? That can also cause red plating!