Abut 10 years ego I brought fully working large tube Tesla radio from Czech republic home to Australia. Before I've plugged it in, I've changed the Hertz from 50 to Australia's 60hz. Than I've noticed the power trafo started getting really hot and showing high load on my "bulb tester". I've stored the radio since. Now I'm about to restore it but wonder what could cause the sudden heat problem. Not sure if the frequency change could have cause it but I wonder if perhaps the change of frequency could have affected the all ready old main power cap and it started to short. I've not looked it yet but thought I'd ask for an opinion before I start working on it. Thanks in advance for any suggestions.
I must apologize for a stupid mistake, not sure what I was thinking of at the time. I've changed the trafo voltage from 220V to 245V (AUS), not the frequency.
Took it to my workshop today, plugged it in to my light bulb tester. The radio doesn't play anymore, the light bulb looks to be at about 80%, which is more more than I remember. Tubes still get lukewarm so at least I know that power is still going in. It does seem like a short somewhere. I'm gonna take the chassis out which might be bit of a pain, it is Tesla Festival 721A. Than I'll disconnect the main caps and see if it still draw so much power.
Again ,sorry about the confusion.
Took it to my workshop today, plugged it in to my light bulb tester. The radio doesn't play anymore, the light bulb looks to be at about 80%, which is more more than I remember. Tubes still get lukewarm so at least I know that power is still going in. It does seem like a short somewhere. I'm gonna take the chassis out which might be bit of a pain, it is Tesla Festival 721A. Than I'll disconnect the main caps and see if it still draw so much power.
Again ,sorry about the confusion.
Schematic here: https://www.radiomuseum.org/r/tesla_festival_721a.html
Just remove the rectifier - AZ11- if the transformer still overheats, it's shorted. It it's OK, replace the rectifier and pull out the output tube - 6L31. Gassy tube or leaky grid coupling cap will make it draw excessive current.
Just remove the rectifier - AZ11- if the transformer still overheats, it's shorted. It it's OK, replace the rectifier and pull out the output tube - 6L31. Gassy tube or leaky grid coupling cap will make it draw excessive current.
a temperature rise of about 25*C is already hot, if your ambient is 30 deg. C that means the temperature is 55 deg. C....not really a big deal...you may never know "hot" unless you can measure the surface temps..
the thing to watch out for is the insulation material, and the bobbins, they should not melt at these temperatures...
i often hear these "hot" complaints, but until you can come up with numbers, then we can never can tell with some semblance of accuracy..
btw, a defective or faulty ptx will blow a fuse, replace it and it blows the fuse again...even if you unplug the rectifier tube..
the thing to watch out for is the insulation material, and the bobbins, they should not melt at these temperatures...
i often hear these "hot" complaints, but until you can come up with numbers, then we can never can tell with some semblance of accuracy..
btw, a defective or faulty ptx will blow a fuse, replace it and it blows the fuse again...even if you unplug the rectifier tube..
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Thanks. I took the rectifier out., (much more sensible than disconnecting the cap). The lighbulb still glows the same. It doesn't look to good for the trafo so far. I don't want to plug it straight to main yet. But the badge on the radio says it takes 65W which is quiet a bit.......I guess 'll have to plug it to mains to really test it. Wish me luck 
Well I've plugged it in to the mains. Radio works loud and strong. But there's 60Hz hum which would have to be cap and trafo gets hot fast to the point that I can smell it within 2 minutes. Next I'll remove the rectifier and plug it in and see if it gets how. Is it possible for power trafo to be partially shorted but still supply normal current?
What was the wattage of the bulb you use for the light bulb limiter? If the wattage is too low a disproportionate amount of the current will flow through it, so less power to the radio and a brighter bulb. It should be similar in power to the radio.
Now you are using the radio without the limiter if you have coupling capacitor issues, then the output tubes could be consuming far more power than if they were correctly biased and that can cause excessive current to be pulled from the PT, causing the overheating.
Safest is to assume the coupling capacitors are leaky and should be replaced, and probably the power supply smoothing electrolytic capacitors are also ready for replacement too.
One option is to get everything ready under the chassis to take readings of all voltages on the valve pins, then grab some values in a quick test session. Start with the power tubes, and check the cathode to ground voltage. That should help to determine the dissipation of the tube, which could point to a simple fix.
Now you are using the radio without the limiter if you have coupling capacitor issues, then the output tubes could be consuming far more power than if they were correctly biased and that can cause excessive current to be pulled from the PT, causing the overheating.
Safest is to assume the coupling capacitors are leaky and should be replaced, and probably the power supply smoothing electrolytic capacitors are also ready for replacement too.
One option is to get everything ready under the chassis to take readings of all voltages on the valve pins, then grab some values in a quick test session. Start with the power tubes, and check the cathode to ground voltage. That should help to determine the dissipation of the tube, which could point to a simple fix.
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For safety sake count on all the power supply caps and coupling caps being bad. Just change them out all together. Those old caps have lots of ways they go bad just by sitting there! Pull the rectifier tube and see if the bulb tester changes and if the tran still heats up. If it gets hot with no rectifier tube then it has a problem. Most likely it is ok. Don’t try using it until the caps are all changed, it’s asking for trouble and damage.
I've now disconnected the trafo from everything, it runs on it's own and still gets hot, It only takes about two minutes. I've measured the secondaries and get 300V on 1st one, and 250V on the 2th one. Seems to me there is a problem with the winding on the 2th one. I suppose there is nothing I can do about it since I can't actually look inside.
The lacquer is burning out... and shorting the windings. It happens with valve amps... it happened to me once. Hard to believe, but possible in particular if working with the transformers that have been wound in the workshop. It was a 6S33S amp from Poland. Brilliant design...great sound... let down by poorly wound mains transformer.
The main design culprit for shorted transformers IMHO is crossed turns / random winding and heating + cooling cycles. Crossed turns bring high pressure points between turns. On top of that if the transformer is to be used with capacitor input filter, that causes vibration between windings, friction points and dielectric wearing.
Long life transformers need careful layer winding with no crossed turns.
Power transformers by textbook look like very simple devices, but in practice you get a huge headroom to screw them up.
Long life transformers need careful layer winding with no crossed turns.
Power transformers by textbook look like very simple devices, but in practice you get a huge headroom to screw them up.