Transformer Question - Probably obvious, but I want a second opinion

[mike13]

So here's my question: in my junk acquisition forays I picked up two VOM reel to reels. I have harvested from them power transformers that look perfect for Champ type SE amps. Problem - they have no center tap. Bridge rectified with silicon diodes I get a very B+.

but the weird thing is the original devices used 6x5GT rectifiers and were single ended 6k6. So how did they do that? Were they half-wave rectified?

So my shameful admission is that I've junked the original boxes, have no idea what model I'm looking at and basically I'm dealing with parts.

So what do I do with my center-tap - less 300-300 transformers? Half wave? Voltage dropping scheme? What do the experts recommend?

[Steve Conner]

You can make a "hybrid bridge" with a 6x5 and two silicon diodes. The tube rectifier replaces the two "top" diodes in the bridge: the ones that feed the positive supply.

[mike13]

thanx!

[sgelectric]

A bit off-topic, but I have some single-ended output transformers that came from VOM reel-to-reels that were *really* good when used in a 6V6 guitar amp.The OTs were made by Stancor and are huge for SE 6V6 outputs.The output impedances were 8 and 16 ohms if I remember correctly...........
If your VOM reel-to-reels have them, don't overlook them.
Steven

[Enzo]

You can half wave them, or just use a silicon bridge. 300VAC rectifies and filters to about 425VDC, seems like a reasonable amount. Or include the tube if you need the voltage drop. Besides, what does that 300v drop to under load?

[mike13]

I can't remember but with the silicon bridge it's some ungodly voltage. It's 300 on each secondary. And no center tap. So it came out to something really high. I was scared to try putting through the completed circuit especially with 6v6s. Besides I had other options...

But now I'm down to the VOM transformers. So I'll try using the 6x5 and silicons in a hybrid. HAven't had a chance yet, but that might be more like it.

The OTs from those VOMs are another story. They're weird! I'm sure the original circuit is single ended, but there's 9 wires coming off of those things. Right now, after some experience building other things, it's starting to look to me like a PP transformer - Yellow and black one side, and then Blue, Red, Brown, on the other. But then on the opposite end of the solid color wires there's 2 striped wires on each side coming out in various combinations of white/red, white/green, white/blue, etc. Maybe one of you guys understands this.

Thanks

[Enzo]

Two wires, with 300v between them, right? That's 300VAC. A diode bridge will rectify that to 424vDC when filtered.


Get out your ohm meter, and explore the OT, find out what windings those striped wires are connected to. They could be UL taps, they could be separate feedback windings, they could be CV speaker windings.

[Zipslack]

Transformer testing/identifying leads on power transformers

First off, I love finding old reel-to-reel units at flea markets - great source of iron and vintage tubes for little $$. Just recently picked up another German-made unit for $20 - haven't opened it up, but I'm thinking it's two single-ended (left/right/stereo).

Anyway, here's a technique I like to use for testing/identifying leads on unknown power transformers - it requires a 12VAC source, either a wall-wart or a power supply. Attach the 12VAC to the primary power input (where the 120VAC normally goes). If you did resistance measurements on the other leads beforehand, you should have a decent idea of the secondaries. Measure the voltages now with 12VAC going in and then multiply by 10 to get the actual output you would see. For example, if I put in 12VAC and read .63VAC out, mentally multiply by 10 and I know I have 6.3V heater supply. Even for a sizeable step-up, the output voltage should be well below what a typically voltmeter will handle safely. Sticky labels or even masking tape are good for labeling the leads once you have it figured out so you won't have to do it again in the future. DO NOT DO THIS FOR OUTPUT TRANSFORMERS!! If you connect even 1VAC to the speaker side of an OT, you could get 5000V or more out the other side, depending on turns ratio.

[mike13]

Ah sorry for not being clear. DIY electronics education... Anyway between the two wires of my transformer there is 600v and no center tap -yielding 700+ rectified.

So how did they do it in those VOMs? Must have been half wave...

[Enzo]

Half wave, full wave, voltage would be the same.

Got model numbers? FInd schematics.

[Steve Conner]

If it's 600v there must be a centre tap. Maybe it's connected to the core, or shared with a CT for the heater winding, or similar.

[mike13]

I got these months ago, before i really knew what I was doing, so I threw out all identifying pieces, and now I just have the transformers (tubes have long since disappeared into other fruitful projects). So no idea what models.

They only have 4 wires - heaters and high voltage.

Here's a thought. There's a copper plate on the underside of the transformer - unlike other transformers I've got. Could this serve to ground the center tap to the chassis? That would be cool. I'll let you guys know when I break out the tools in an hr or so...

[Enzo]

Use your ohm meter. From those HV wires, is there continuity to any other wire or TO THE FRAME?


Your copper is more likely a shield.


A tape deck wouldn;t need 700v. Not many things do.

[mike13]

Believe it or not that's what's going on. The copper plate on the bottom is designed to come into contact with the chassis. It IS the center tap!