Trace Elliot Velocette Redplate

[buffalono]

I had originally replyed to a post about a Blues Jr. with a redplating problem but figured I would start a new thread.

I noticed the EL 84 output tubes redplating in my Velocette so I first tried installing new tubes but they also redplated. The plates don't fully glow it is more of a small streak of red. A tech friend of mine checked it out for me and said I had 0V at the grid and to check for cold solder joints which I did and touched them up. I then tried another pair of new JJs and they also redplated. I have an old pair of JJs that rattle but run without redplating but all the other EL 84s including some NOS Sylvannias g0 red.

Enzo suggested checking the plate current dissapation and I came up with 18.6 watts. Please bear with me while I show the math to make sure I did it correctly. The cathode resistor measures 120 ohms and 12.5 V at the cathode = .104 / 2 = 52 millamps. I measured 370 volts at the plates which 20 v higher than the schematic I figured the anode voltage by subtracting the cathode from the plate and got 357.5 V. Then I multiplied the estimated anode voltage by the cathode current and got 18.59 Watts.

I'm guessing the 20 extra volts at the plate would only account for 1/2 a watt or less so does the value of the cathode resistor need to be increased? Or did I do the math wrong?

Thanks in advance for any help.

[tubeswell]

Yep you did calculate it right (maybe you can subtract 2mA from each side for the screen current - but that still leaves it way too hot for EL84s. And they don't like very high plate voltage either. They should be about 350V at idle).

The schematic says 120R, with 12.5V at the cathode. So...

1) Possibly the screen voltage is too high?? Maybe you should try lowering the screen voltage a bit (rather than increasing the cathode resistor). That should bring the plate voltage down. For cathode bias you want to get them idling at about 12W. Have you got 100R screen grid resistors? What is your screen node voltage? (should be about 340) Maybe you might have to increase the screen supply resistor???

2) Also check that your OT is hooked up properly and at you haven't got the speaker going into the wrong secondary.

[buffalono]

Ok, I finally found some time to open up the amp and check out Tubeswells advice.

The screen grid resistors both measured 100 ohms.

Checked the screen voltage at pin 8 and found 368 - 370V.

The Power supply voltage to the screens also measured high at 369V.

Is the screen supply resistor r 32? a 1K 4w resistor located between the CT supply and the screen supply?

Checked the link for the OT and it was set correctly.

It looks like I need to figure out a higher value for the screen supply resistor to bring the voltage down to 344V per the schematic or should I go lower?

[buffalono]

I would like to follow tubeswell's advice about lowering the screen voltage. I need to know how to calculate what size resistor to add in order to lower the screen supply voltage. Do I just measure the voltage and current before the resistor and plug the figures into a ohms law calc?

Presently the screen supply resistor is a 1K 4w. I'm getting 370V after the resistor and I would like to bring it down to 344 Volts.