HELP!!! with tube please

[wiggs13]

So i have a Peavey classic 30, just need to make sure nothing is wrong of get some advice on it, When I start her up I get that i believe to be a 60 hz hum, along with the hum I notice a tube becomes more red than the others, after 30-40 seconds or so the hum fades and the tube dims. Ive moved them around and it seems to follow the tube, hum does not change with volume or any other knob nor does being plugged in or unplugged, caps look fine visually, not blowing fuses. please any info will help Thankyou!!
Adam

[Jazz P Bass]

Fo sho, replace that one tube that is redplating.

[TomCarlos]

JPB.... and since we are talking about an amp (the Classic 30 in particular) that is Cathode biased and the output tubes are worked hard, you would suggest replacing with a "similar" tube? Maybe we're not talking about "tube matching" but at the least from the same manufacturer if possible?

For Wiggs, this might be a quick check to see if the hum disappears. But longer term, swap out the entire set.

[eschertron]

+1 to the new tube suggestion.

I expected cathode bias too, Tom. But looking to verify the tube set, I pulled up this schem, that shows fixed bias. Not that changes the OP's issue!

Adam, we are talking about the output tubes? Can you verify that they're all the same model, and if the odd tube out is a different brand, or shows evidence of abuse?

edit: added link to schem

http://www.bustedgear.com/images/sch...classic-30.pdf

[wiggs13]

Okay guys, set appears to be just that a set, nothing looks out of the ordinary, and yes we are talking about output tubes.

Since this is a fixed biased amp to rebias I would have to do some soldering, any info on checking the bias so that when I order a set I wont have to adjust anything? Does anyone suggest doing a mod for a bias pot?

And to address the issue once again, Is a tube pretty much the only thing that will create hum during warm up then disappear once its been on a while?

Thanks for the help guys!!

[eschertron]

What raised a flag to me was the 'redplating' comment. If the problem follows the tube and is not evident on any other tube then I'd say the tube needs to be replaced.

How does the amp sound after the hum fades in 3-40 seconds?

On the schem the bias voltage is indicated as -14vdc. Can you verify that? Probe the junction of R45 and R49 to ground.

[wiggs13]

I dont have the best hearing and it is my first tube amp but it seems to sound good Clean and Distorted, one thing I have also noticed it seems to heat up the tube next to it on occasion also, and once again if the one next to it heats up it follows the one that always heats up. I can def check the voltage this afternoon. I have -14.30v (with no tubes) at the jumper after work I will have to remove the board to get to R45 & R49

Thanks Adam

[g1]

The tube is causing your hum. And it is occasionally killing the bias to it's mate. Get rid of it as it could do more damage. Hopefully it has not affected any of the screen resistors.
Some tube sellers will "custom" pick tube sets for your particular model so no bias adjustment will be required. Check around.

[wiggs13]

As measured from the pot I'm pulling -11.30 ,-11.20, -11.57, -11.67 from outside pot towards preamp tubes, if that helps

[52 Bill]

Welcome to the place Adam.

...I will have to remove the board to get to R45 & R49

If there is no reason to remove the board, don't do it. You have already measured the -14 volts at the jumper, the only other thing that you could try and do is to measure for the -14 volts at the tube sockets.

Every time those boards are removed, you risk the breaking of the jumper wires that connect them. Every one that breaks will cause additional damage to the amp.

The real problem is that if the one tube is red plating because of an internal short, that could cause the overall bias supply voltage to drop and then all of the output tubes will be under biased.

For the time being just pull the bad tube out and see if the amp remains stable and will power up without any signs of bias loss, like the hum and the glowing plates.

[wiggs13]

Im not ignoring that the tubes need changed I would like to adress any underlying problems if there are any, but hopefully there is not.

Thanks all

[52 Bill]

I just remembered that the filaments for the power tubes are in series, so the amp won't work with one removed. Forget that idea.

[Jazz P Bass]

The key to the 'underlying problem' is that the redplating followed the tube.

[g1]

Remove the bad tube. Check DC volts at pin2 of the empty socket.
Is it now closer to -14V than what you measured in post #9 ?
And what pot were you referring to in that post?

[DRH1958]

There is no pot to measure from. These are non-adjustable fixed bias. And these are a bitch to work on. The circuit "board" consists of three smaller boards that are folded at 90 degree angles to fit into the top housing connected with jumpers, so great advice from 52 Bill. Just accept the -14v bias and go with that.