Power supply high voltage problems with Melos amp.

[legarem]

I presently have a pair of Melos SE-75monoblocks which are 75 watts single ended amplifiers using four 6GK6 for output tubes. The PP version triode 200 seems to be the same amplifier.

One of these monoblocks has a V+ before output transformer of 650VDC the other has 566VDC. I measured the VAC before the diode bridge and there is 528 VAC on one and the other has 450 VAC. I decided to mesaure the secondary resistance and it gives 9 ohms on one and 10.5 ohms on the other (all with the bridge connected)

As there are many secondaries in these transformers, the other voltages are the same on both amplifiers. It seems that only the high voltage on the secondary is not correct.

Now which one is correct ? The lower one or the higher one ? I suspect that the lower one have more chances to be in trouble.(was it perhaps manufactured like it is ?)

As they are toroidal power transformers, I thought adding more wire turns at the secondary of the lower voltage transformer to get the same on both amps.
Any other suggestions ?

Thanks

 

[kevinkr]

The fact that one is nearly 100V lower than the other makes me suspect that there could be short in the high voltage secondary of that transformer or there could be a malfunction in the amplifier itself, but this seems less likely.

Does the transformer with the lower output voltage and lower dcr (I assume both were measured cold) make any more noise/buzz or run significantly hotter?

I would disconnect the rectifiers (lift one wire) and recheck the winding resistances stone cold. Then measure the unloaded AC voltage - that may prove fairly conclusive, if not measure the primary currents and compare, they should be very close. A transformer with a partial short in any secondary winding will have significantly higher current consumption.

Presumably if these transformers have the same part number and revision level (not always possible to ascertain with boutique manufacturers - myself included back in the day when I made things) they should produce output voltages at a given load current within a couple of % or better.

I wouldn't do anything to that transformer without verifying it is ok. You might consider having someone rewind that transformer, should not be too hard since the primary is probably fine and the other windings will provide a good approximation of the voltage/turns ratio.