Dropping Line Voltage by 10%

[JammyDodger]

To All,

Is there a way to drop the AC voltage out of the wall by 10% or so? We seem to be running 125-128V here and I would like to try and drop this down a fixed amount, say 10%. I don't need a Variac just a simple, fixed drop.

Cheers, Mike

[Structo]

How high is your heater voltage?

[rsi]

I have done this before with a big power resistor. One of the aluminum cased ones. I can't remember the value I used but it dropped the line voltage feeding into the PT by 7 or 8 volts. This resistor gets hot, so heatsinking helps a lot.

Later, I changed this to installing some zeners in between the standby switch and the first filter cap.

But it does work.

[briane]

you can buy a cheapo 'power condition' for about 50$ us at the local music store.

All it really does is clamp the voltage to 117. I have one (furman), works well.

[stoo]

http://www.mylespaul.com/forums/squawk- ... ac30s.html
About 1/3 down the page
Stew

[selloutrr]

"power conditioners" are not constant voltage transformers.

They still can flutter to some degree most just add a MOV to absorb surges.

Where a constant voltage transformer usually bigger then a fist for 120v absorbs the additional power from the line and produces a consistant and constant 120v

weber makes a step down power transformer for such a need. Im not Adan of weber but it's a cheap easy safe solution.

[Structo]

The Weber V Dump is nothing more than a 25w Zener stud mount rectifier.
You don't need to buy it from Weber, I bought a 25v one for my 6V6, have it on the HT center tap to drop the B+ 25v, works great.
It is a 25w one and I have it mounted through the steel chassis and boy does it get hot.
I have thought about making a heat sink out of some angled aluminum but haven't yet.

Here is the bucking transformer method, that looks pretty good for lowering the wall voltage to a vintage amp.
[img:640:495]http://i135.photobucket.com/albums/q136 ... ormer1.jpg[/img]

[JammyDodger]

Structo,

Heaters are are running 6.8V.

Hey that transformer looks real interesting. Have you tried this?

Cheers, Mike

[Greg Smith]

I did this successfully on a hi-fi amp project, using the unused 5vac rectifier filament winding on the amp's PT as the bucking winding (didn't need to install a second transformer). I wrote a post for another forum about it. See below....

-Greg

"I am in the process of building a couple of cathode biased eiclones and wanted to share a discovery I made wrt reducing power transformer secondary voltages by using the normally unused 5vac rectifier filament winding to "buck" down the primary voltage.

My household mains voltage is typically around 121 vac. Couple this to the typical Hammond PT with primary rated for 115 vac, and I get filament voltages at around 6.7 vac, and plate voltage in the 500 to 505 vdc range. The filament voltage is not too worrisome, but the plate voltage had the output tubes idling at around 28 watts, with plates glowing a dull red in a darkened room.

Remembering something I'd read about bucking transformers, I tried connecting the unused rectifier filament winding in series but out of phase with the PT primary. The resulting filament voltage was reduced to 6.3 vac, and the plate voltage now sits at around 479 to 483 vdc. The outbut tube plates are dissipating 25 watts - still at the max rated plate dissipation, but they don't red-plate anymore.

The simplest way to know the phase of the windings is trial-and-error:

1. Connect the transformer normally and record the 'green' filament (green leads) voltage.

2. Disconnect one of the primary (black) leads from the mains and connect one of the rectifier filament leads (yellow) to the black lead. Connect the other yellow lead to the mains. (you now have the primary and retifier filament winding connected in series across the mains voltage). Measure the 'green' filament voltage again. If the voltage is lower than before, you have the rectifier filament connected out of phase (as desired) and the HV secondary will also be reduced. If the main filament voltage is higher than before, you need to reverse the yellow wires."

[Structo]

6.8 vac is just under 8% over 6.3vac

But if the amp xfmr is made for 110 or 115vac then it may be a good idea to step down the ac supply.

If you have more than one amp that requires the lower voltage then I think I would make a box with the bucking transformer in it.
You could also incorporate a MOV into it to protect the amp from surges and spikes, a power switch and pilot light would be cool as well.