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    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Lowering V+ on an amp in monoblock pair

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I have a pair of Melos triode Single ended 75 watts amps. One has a V+ of 690VDC and the other has a V+ of 610VDC. Even If I take out the output tubes the difference is still there so this is not caused by load on the power supply. The two transformers don't deliver the same VAC unloaded on the amps. Problem is only on the V+ for output tubes. There are other taps which have correct voltage. The two amps worked well but the one with higher voltage burn tubes more often than the other one. As I will never find a transformer for this amp, I want to lower the V+ of the amp which has 690 VDC to 610 VDC. Which method should be the best so it should not harm the sound.

1- Placing a resistor directly after the diode bridge (before the caps) to lower the DC voltage

2- Placing resistors on Ac feeding the diode bridge


Since its a toroid, you could take one of the high voltage leads and wrap it around the core several turns in the opposite direction as the high voltage winding. Loosen the mounting bolt so you can wrap the wire around the core, then tighten it back up. That will equivalently remove some windings or provide some bucking action. You need to remove approximately 60 AC volts. One concern is to use well insulated wire since that wire will be smashed up against the chassis on one side. That's a poor man's method anyway.

The other two approaches you mention will also work but those resistors will get hot. In the case of placing a resisitor in between the rectifiers and reservoir cap, you need to size it for the quiescent current plus ripple current--sizing for both resistance and wattage.
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"If the transformers are the same part number, this could be due to an internal fault in the lower-voltage one, I'd check they run at the same temperature and primary current." Good point, sound to me that there's a fault somewhereand if the amps were mine I would want to find out why HT/B+ is so different. I'd measure AC secondary and as Mark suggests monitor temperature and listen to see if you can hear a difference; mains tfmr's are louder when under strain.

Could be as simple as a mislabeling or wrong connection on the power transformer: I had one with the labels of the HT center tap and bias tap reversed.
So instead of grounding the '0'of the 300-0-25-300, the '25' went to ground: 325-25-0-275.
I don't remember the exact voltages anymore, but the resulting HT was higher than expected (and the rectifier didn't like it after a while).
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