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Hot Power Transformer

10/20/2004 3:36 PM
Hot Power Transformer
I'm working on a '67 Supro/Valco Statesman head. The power transformer gets pretty damn hot just idling for an hour. The complaint was that it loses power/volume after playing for an hour. I recapped it and rebiased. It's cathode biased and was drawing too much current before. The voltages seem correct. Is the PT on the way out? Anything else that can cause it to run so hot? Anything else I can check? Thanks.  
10/20/2004 3:53 PM
What was the current draw before you worked on it?  
What is it now?
10/20/2004 4:46 PM

This amp has two output transformers with a pair of 6L6's for each OT. Both have a load. Each pair shares its own cathode resistor. Before the tubes were drawing about 70 ma each. The plate voltage was 395 volts minus 30 volts across the cathode resistor for an effective B+ of 365 volts. Now the B+ is 400 volts and the tubes are drawing about 55 ma each. I went from a 200Ω cathode resistor to a 270Ω resistor. The amp doesn't seem to lose volume now but it still gets very hot. Thanks!  
10/20/2004 4:02 PM

I am by no means an expert!  
However, I recall a post by a guru in a similar thread where he mentioned that a (inaudible) parasitic oscillation can cause the PT work much harder than necessary. HTH!
10/20/2004 4:25 PM
Paul Drexhage
I'd be interested to hear about this too, as I have an amp with a warm PT after playing for 30 mins, though the voltages coming off the secondaries are a bit high as well. Tubes, components, etc. all check out fine, and the power tubes are all drawing correct current. As I had mentioned in "Vintage Amps", I suspect that I may have some shorted windings in the primary, resulting in some loops that are drawing excess current.  
10/20/2004 7:18 PM
Rob Mercure

Four or five years ago Doc Horner (whatever happened to him?) posted a schematic for a shorted turns tester on this forum which, as best I remember, may be at RG Keene's site. Basically it consisted of a battery, a neon light bulb - and series resistor, and a switch. The battery "charged" one winding and then when the magnetic field collapsed the neon bulb would flash if nothing loaded, such as a shorted turn would, any of the windings - of course all of the windings had to be disconnected from a load. Elegantly simply and quite useful device.  
10/20/2004 11:35 PM
Paul Marossy

Yep, that is over at
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