Lowering V+ on an amp in monoblock pair

[legarem]

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

Thanks







Thanks

 

[ubergeeknz]

Why not use one of the taps with the correct voltage?

 

[legarem]

In fact there is other taps for drivers tubes which deliver 300 VAC.

 

[ubergeeknz]

Curious that they're so far different.

Is there any possibility to remove some windings from the secondary on the higher voltage trafo?

 

[legarem]

They are toroid transformers and I don't know if the wires are placed so I could do that or if other voltages windings are over the HV windings.

 

[Alllensoncanon]

Triode Single ended 75 watts amps.

If you do this passively, it will be hot. Let say 400mA * 80V = 35W worth of heat. An alternative is adding some kind of bucking transformer to the secondary. 690V AC will be an issue to be careful with. Either way, there will be a space problem. Best replace or fix the transformer.

 

[kward]

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.

 

[Nectarpuke]

Honestly, I don't think it will make much of a difference whether the voltage drop is before the rectifier or after. Kirchhoff dictates here, and he is rather indifferent when it comes to where in the series the series resistance is in.

 

[Brice]

As a bandage, I would buck that mono amp primary and readjust the Bx+.
As a cure I would replace that PT.

 

[Mark Tillotson]

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.

 

[Diabolical Artificer]

"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.

Andy.

 

[Parafeed813]

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).