Is it possible to unwind a power trans to lower voltage ?

[chopstuck]

I have a large collection of beefy iron and most of my voltages are pretty high in the 340-340 to 410-410 range for big octal tubes of old PA amps and old wall voltage. I would like to reuse these beasts but other than fitting these amps with Dana's VVR or a ridiculously expensive zener, could I unwind the HV secondary on these old transformers ? I don't know if that winding tends to be the first or last to go on but has anyone done it ? Is it practical or another weekend lost ? I found nothing in my searches so it must either be obvious or I'm the first one to think of it. Probably the former.

[Jana]

I have been wondering the same thing for a long time. I am guessing that if the high voltage winding is the last one on the bobbin, then it might be possible if it is a single winding meant for a bridge rectifier. Otherwise you would have to unwind to the center tap, take a bit off, then wind back on the other half to match the first half.

[chopstuck]

I see, good point about the rectifier and center tap. I guess that the sec. would have a monster amount of turns to unwind to get to the CT, even if it was the last winding.
Thanks.

[drz400]

You can always add a series transformer with the secondary in phase or out of phase to boost or to buck, unwinding is a pain since you need to undo all the laminations

[paddy]

Hi there,

here is a link to a circuit that will allow you use inexpensive parts
to achieve voltage reduction.
members.optusnet.com.au/eyes.only/vj_mods_3/Zener2%20Lower%20B+.pdf

cheers
paddy

[doctord02]

You could also build a choke input power supply to drop your voltages down, tho it will require a hefty choke that can handle your original high voltage. Perhaps combine that with a tube rectifier with a big voltage drop to drop it some too.

What kind of voltages are you looking for? Your 340-0-340 PT will yield about 480 DC when rectified and a 5U4GT rectifier will drop about 44 volts per side, so that would give you about 392 VDC... about perfect for a Wreck... and you can drop that further with a choke input filter supply...

[brewdude]

Could you use an internally adjustable VVR circuit to tune the voltage to any desired value?

[chopstuck]

That's the direction I'm going. Thanks.

[guitronics]

Just keep it simple and use a wirewound power resistor in series with the secondary.

[Merlinb]

I would like to reuse these beasts but other than fitting these amps with Dana's VVR or a ridiculously expensive zener,

You can use a cheap zener and add a powerful transistor to pass the wanted current. Transistors are cheap.

[Stanz]

I would just increase the first dropping resistor to bring them all down.