ST35 power transformer running HOT!

Hi All, Im in the process of refurbishing an ST35. It has been completely disassembled...new resistors & coupling caps installed [filter cap is okay]...reassembled...rewired and I'm ready for a smoke test. After quadruple checking the wiring, I fired it up. All voltages match the manual. After running with low level audio for about 3 hours, with the cover off, the power transformer is SUPER HOT to the touch! Burned my fingers touching it! I have measured the B+ current through the circuits and everything seems within spec. 161 mA to the four output tubes, 9 ma to the preamp/phase inverters. Total current drain is 170 mA. Transformer spec is 180 mA. Have not measured the filament current, but voltage is correct and all tubes light okay. Barring a short circuit which would be obvious, I don't think anything is wrong with the filaments. I can't imagine the power transformer should run this HOT! [The output transformers are warm, but are equal and not excessive.] Any thoughts?

Thank,
Hank

The power transformer for this amp runs very hot - mine included. After three hours (like you mention), I can put my fingers on it for maybe three seconds and that's it. Everyone that owns this amp says the same thing basically. I have heard that the equivalent Edcor transformer runs less hot, but I have no experience with it.

I have high voltage at my place and I'm going to knock to down about 5V's. Maybe the transformer will run cooler then.

Good incentive to upgrade to the new iron that has more stacks and reserve. Way I understand it, Dynaco ran these things pretty close to the edge to save production costs ... stands to reason they're gonna get hot.

Wondering why you couldn't add a CPU fan to the back of the PT? Fab a simple mounting bracket using the bell screws and run it off a wall wart ...

I expect even maximizing open air around the PT would make a difference if it's working in close quarters. Take the cover off it it's got one as that can hold in a LOT of heat.

Different animal, but my ST120's PT does get warm after a good workout, but you can still hold a finger to it without coming away blistered ... and that's driving a quad of KT120's at ... substantial volume, let's say ...

The way I understand it this is the nature of tube amplifiers.

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Thanks for all the comments, but the question still remains as to why this transformer runs so hot as to instantly burn one's fingers. I think this is beyond the "normal" hot running Dynakits which survived for years "as designed." I was refurbing this amp for resale, but now the question is whether it is worth investing more $$ in a new power transformer.

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My power transformer is about 116 degrees F after playing for about 90 minutes. This is a freas rebuild with the original transformers, new dynakitparts boards and the Dave Gillespie EFB board. Could be the power supply on Dave's board is more efficient so the transformer isn't working as hard?

(I bought an infrared thermometer a couple years back for homebrewing - comes in handy for other stuff too.)

daveshel wrote:My power transformer is about 116 degrees F after playing for about 90 minutes...........

(I bought an infrared thermometer a couple years back for homebrewing - comes in handy for other stuff too.)

Thanks for the specific temperature. Now I have something concrete to go on. I don't have an IR thermometer, but I have one I can attach to the transformer and see how it compares. Just my gut feeling, but I think mine is going to come back a LOT hotter.

just FYI, the the Dave Gillespie EFB board runs the tubes at about 3/4 the current as the original design, so the transformer and tubes will run cooler and last longer.