Mesa DR hot PT

[tubby]

Hello all,

i noticed recently that the PT of my early 2-chan dual rec (serial ~900)
gets so hot, you can't put your hand on it for longer than a sec.
the former owner had it stored somwhere in a basement so there is some rust
on the laminates.

now, i think i've read both somewhere ():
1. rusty laminates can become conductive and therefore
there's current induced into the laminates, which heats them up.

2. rust is non-conductive and is not a problem besides the visual aspect.

so the question is: even if 1. is true, does that explain the high operating
temp. even if don't nearly crank it? and if not, what else could be the cause
for such a hot PT?

i tried different sets of pre- and powertubes so i think i can rule out excessive current draw from one or more of the tubes.

[6267]

Make sure it's biased properly in addition to just not glowing red. Otherwise, I'd check for shorts...I've seen rusty PTs get hot, and I've seen them not get that hot too. Does it get hot with the tubes pulled? If you had a short somewhere, say in the filament circuit, there would be tons of current draw. Also, make sure you are using the original tube types, not "substitutions". Pull all the tubes and measure the resistance from a filament pin to ground with the amp unplugged. It should read high or infinite. I don't know if you have a hum balance control, but that would read as about 100 ohms or so to ground. A short *should* look lower than that though. Otherwise it should be open or infinity between the heaters and ground. Check your filter caps for leakage...they should not be reading as though they were a resistor. If that's not it, try disconnecting each set of taps until the PT doesn't get hot. That'll be the short.

[tubby]

hey 6267,

thanks for those tips, i almost gave up this thread.
i'll check the amp as soon as i can get to it.

[txstrat]

I agree, the bias could be too hot.
Did the amp run at lower temperatures before, or did you just recently recognize it running very hot? Tube amps usually run very hot.
BF Fender Super amps PTs tend to get so hot you could fry eggs on 'em.

to evaluate the PT temperature, keep in mind that if you can keep your hands on it you' re under 50 degC, if you can keep them on it for a sec or so you' re around 60 degC ( still an acceptable value ) but if you' re higher than that something is probably wrong.

Thanks Bob

If the amp plays OK, the tubes don't redplate and nothing smells burned, I would keep an eye on the amp but wouldn't do anything right now.