Knob on top of power transformer

[GBguy]

Hi,
This is my first time on a forum, and I am the first to say I know little about electronics. So please go easy on me. Thanks.
I just bought a vintage audio tube amp on EBay, probably from the 1950s or 60s. It has 2 6L6 power tubes, a 5Y3 rectifier tube, and 3 pre-amp tubes. It doesn't have a brand name on it, just homemade labels for the volume, bass and treble controls. It has a hefty power transformer that is 3.75" x 3" x 2.5 high" for the steel, plus about 2" above that which contains a cover and an adjustment knob. The only number I can find on the transformer is C-94230Z. The knob has several positions numbered 1 - 8.
What does this knob control? Voltage? Does it select different windings? How do I know which position to use?
Will it damage the transformer or amp if I have it in the wrong position? Thanks.

[Enzo]

Do you know the difference between power transformer and output transformer? Are you sure which one this is? A tube amp will have both. Can you post some photos?

As a rule I would not expect switches on transformers, but potentially an output transformer could have impedance selector switch. Power transformer could have a mains voltage selector.

Was this amp intended for something other than guitar? Does it look like an old "PA" amplifier?

[GBguy]

Transformer with knob on top - here is a photo



Hi,
Thanks for responding. Here are photos of the transformer and of the bottom of the amp.
Take care!

[Enzo]

OK, you were right, that is the power transformer. SInce there is a power switch on the panel, I'd have to think that was a mains voltage selector, I can't imagine what else it would be.

I love the totally mis-matched 6L6s - one coke bottle and one metal it looks like. Keep them for show and tell, and get fresh ones.

Looks like 6 screw terminals on the output. Do most of those have wires running back to the output transformer? (The square black metal thing on the corner.) It looks like an old PA amp to me.

I'd say those old wax caps need replacing. Well, I'd say pretty much ALL the caps need replacing. The cardboard sleeved ones are electrolytics. New ones of same rating will be MUCH smaller physically today.

If I am right about the switch, we could find out what it does by turning it. Pull the tubes and note which went where. I'd have a light bulb limiter connected - do a search for that around here. Turn the thing on and measure the AC voltage between pins 2 and 7 of the 6L6 sockets. If you get about 6 and a half volts, then it is right. There should also be about 5VAC between pins 2 and 8 on ther 5Y3 socket.

The switch could be 110-220 or something. (how many positions does it have?) In which case you'd have either 6vAC or 3vAC on the 6L6 sockets.

Or it could be one of those 105-110-115-120-125 types. In which case our 6v would vary from like 5 to 8 or something.

This is ALL just my opinion.

[wilbur]

What are all of the tubes designations for that amp?

[Enzo]

Hi Wilbur, welcome to the forum.

Looks like GBGuy only ever made those two posts 4 years ago and never came back.