Will driving 8ohm spkrs off a 4ohm tap cause hot PT?

[cubby01]

Does this increase current draw? It's a small console amp (magnavox 8601) that if I use 4ohm speakers PT gets warm - can easily keep hand on it, but if I use 8ohm speakers it gets a good deal warmer - can keep hand on it about 8 to 10 seconds. OPT are also warm but I think it's heat radiating from the PT as the one closest to the PT is warmer than the one further away. (I haven't replaced any components yet. I did remove a 4700 cap for eq per recommendation somewhere in the threads.)

 

[Jon_Logan]

....I don't think that should make the transformer run warmer.

 

[dshoaf]

The prudent thing to do......

I'd shut down the unit until you're clear on why the power transformer is getting too hot. Specifically, the larger metal can caps should _not_ be getting warm at all. If they are, then you have what I consider to be a safety issue on your hands.

Overheating transformers are due to higher than rated current flowing through them. This is not good. There's something downstream of the PT that's pulling too much current. Not good.

I strongly recommend you plan on rebuilding the power supply first. Then, and only when you're in the room, operate it a bit but watch out for Cherry Red glow on the output tubes. If so, then you'll need to swap the coupling caps. There's plenty of discussion here about all this.

Cheers,

David

 

[M Jarve]

That's good advice, but also keep in mind that a fairly warm (if not hot) power transformer is par for the course with a tube amp. I think it's been mentioned before that 130-150F is about normal. Hotter than that is getting worrisome.

However, that it changes with load impedance is peculiar, and I would have to defer to those with more experience.

 

[dspear99ca]

Tube amps develop more power into higher impedance loads, the exact reverse of solid state amps, that's why you can't run them with no load (infinite impedance draws max power and will smoke the amp). I had an RCA console amp for years that the power tranny got too hot to touch. I completely rebuilt the amp, there was nothing drawing over-current, the engineers just designed the PT a tad small to keep cool... or, given that it had run just fine for over 50 years, exactly the right size. Realize that consoles were a very competetive business in the 50's and 60's and, like today, everybody was in it to make a buck which meant designs that minimized costs by using the cheapest, lowest-spec parts which would suffice and this includes power transformers.

 

[cubby01]

Thanks all. The multi-cap canister is not getting hot. I've been thinking about rebuilding as an RH84 circuit that I've read about here lately so I'm a bit shy about putting much time/money into the existing PS if i've going to rebuild it anyway with different components.

 

[dspear99ca]

Caps for the power supply are cheap even by po folks standards, and the ones in those cans are ancient... the question is not IF, but WHEN will they fail. The ones you REALLY want to replace though are the interstage coupling caps and cathode bypass caps. When they fail the **** hits the fan and they are every bit as ancient as the power supply caps.