Dropping Voltage on Old PT's

[boots]

What would be the preferred method for scrubbing off some voltage on old equipment that was designed to run on 110 VAC?

I see guys doing refurbs on old gear without any concern for the higher voltages that will result from running the old transformers on modern wall voltage.

I know you can lower B+ by putting a Zener in the CT of the HV winding. And I know you can lower filament voltage by putting series resistors in the filament supply. But - is there a good way to drop the voltage to the primary of the transformer, so that we can lower both the HV and filament voltages in one fell swoop??? (short of using a variac)

Is it feasible to use a rheostat in the primary? That seems like it would be impractical. Can anyone point me toward an old thread where this is discussed? I've done some searches but mostly come up with threads about dropping only the B+.

[sluckey]

Look at page 2 of this pdf...

http://sluckeyamps.com/misc/Amp_Scrapbook.pdf

[Phil_S]

Somehow it seems like it shouldn't be so simple
Good scrap book!

[boots]

Thanks! I've heard of the bucking transformer, and it might just be the best solution. It would be nice to be able to mount it inside an existing amp as a permanent mod, rather than having an extra box outside the chassis. I think that should be feasible.

[sluckey]

You can build it any way you like. I prefer the external box because I have several old original amps that I don't want to modify. I can easily move this small box to another amp if I want.

[R.G.]

See http://www.geofex.com/article_folders/v ... ntvolt.htm for the "Vintage Voltage Adapter" - which is exactly the same thing, a bucking transformer to wipe a few volts off the incoming AC power line.

[boots]

Thanks guys, that was the kind of info I was looking for! I knew I had seen an article about this once, but couldn't seem to find it.

[Stevem]

You should take advantage of the overvoltage condition you have and use more dropping nodes of filtering to end up with far less AC ripple voltage on the first gain stage.

[R.G.]

You can build it any way you like. I prefer the external box because I have several old original amps that I don't want to modify. I can easily move this small box to another amp if I want.

I built one of these for a friend who put it into a vintage Concert. The thing was a metal Bud box that (barely) contained the bucking transformer, an IEC line input socket, an AC outlet socket, and an internal fuse. It was about 2.5x3x4 as I remember. I have some pictures somewhere.

He velcro-ed it onto the inside/side of the cabinet, plugged the amp's cord into it, and just took an IEC cable out of the voltage adapter box It was moderately difficult to notice that it was being used. And of course, it was portable from amp to amp.