Identifying the use of a PT?

[LeftyStrat]

I have this PT I want to use, for something, not quite sure what. But what intrigues me is what its original use was. There are very many model type markings that I see, "R8" and "7T".

Specs:
375-0-375
5V @ 3A
2.5V @ 10A
6.3V @ 4A

And it looks like another set of taps for 2.5V without a current rating.

I was thinking about building something with a pair of EL34's using a Dynaco A-470 (4.3k) for the OT. Perhaps I could use the 5V tap for DC heaters in the preamp.

First pic is the PT next to the OT, and an ECL84 hiding between them.

[mmmoser]

While I can't ID the original use of this transformer, I would think it wise to test the real voltage output of it with today's higher line voltages- I sometimes find they put out quite a bit higher than originally rated because of that.

[LeftyStrat]

Yeah, I'm sure it is old enough that those voltages are for 110V primary.

I'm just curious as to what needed 2.5V at 10A.

[Phil_S]

It's a UTC R-8, here:
http://www.ebay.com/itm/NOS-NIB-UTC-R-8 ... 19d47b880d

[LeftyStrat]

It's a UTC R-8, here:
http://www.ebay.com/itm/NOS-NIB-UTC-R-8 ... 19d47b880d

Master!

[Phil_S]

The UTC R series were apparently meant for "replacement" duty. I think this one must be fairly old, as I was unable to turn up anything for R transformers numbered in the single digits. I am willing to suggest that the HT winding is good for at least 250mA. I am basing this guess on the amperage of filament windings and the HT secondary voltage. I think you'd have a pretty hefty amp and beefy output tubes if you were demanding that much filament current. The way to know for sure is to load the PT and see how it performs.

[Prairie Dawg]

The UTC R series were apparently meant for "replacement" duty. I think this one must be fairly old, as I was unable to turn up anything for R transformers numbered in the single digits. I am willing to suggest that the HT winding is good for at least 250mA. I am basing this guess on the amperage of filament windings and the HT secondary voltage. I think you'd have a pretty hefty amp and beefy output tubes if you were demanding that much filament current. The way to know for sure is to load the PT and see how it performs.

The high voltage winding on the UTC R8 is only rated for 100 ma which puts this transformer in Deluxe country.

The 2.5v winding would be used by 2.5v filament audio tubes from the early 1930s like the 45.

Here's a 1949 catalog with the info in it.

http://www.bunkerofdoom.com/xfm/UTC_1949/UTC49.pdf

[LeftyStrat]

Wow, thanks for the info.

100 MA is disappointing. Well, it was practically free.

[Phil_S]

Dawg, thanks for setting the record straight. I didn't know where to find a catalog that old! I forgot about the BOD.

Lefty, if you don't want that old thing, you can send it to me. I can do lots with a 100mA and 6.3@4A! Sorry for getting your hopes up. I was wrong to venture a guess.

[LeftyStrat]

Dawg, thanks for setting the record straight. I didn't know where to find a catalog that old! I forgot about the BOD.

Lefty, if you don't want that old thing, you can send it to me. I can do lots with a 100mA and 6.3@4A! Sorry for getting your hopes up. I was wrong to venture a guess.

I want to use it for something. What are your suggestions?

[Phil_S]

Possibilities are good. With 4A on tap for filament supply, you will get opportunities. Often filament supply is a limiting factor. In this case, it will be the high B+. Since there is no 5V supply, the rectifier will have to be solid state. I think 6CA4, 6X4, 6AX5, and 6X5 may be a bit below the 375-0-375 HT winding. Remember, you are likely to get 9% or more above 375 due to the assumed 110VAC input rating with today's wall voltage at 120V or more. With SS rectifier and input voltage difference, you may be looking at B+ of 575 or so. It is difficult to know for sure. I figure 375 * 1.1 (input differential) * 1.4 = 578VDC, allowing 4% loss puts it at about 555VDC. I'd say upper 500's is the B+ range.

It depends on how much you want to go off the beaten path. My guess is that your PT is suitable for one octal power tube that will take this high B+. The preamp can you your choice from what's out there, to you taste. I'm thinking one of the KT-?? variety of tubes or a 6650. I think we are talking about a pretty loud amp here.

[ToneMerc]

Possibilities are good. With 4A on tap for filament supply, you will get opportunities. Often filament supply is a limiting factor. In this case, it will be the high B+. Since there is no 5V supply, the rectifier will have to be solid state. I think 6CA4, 6X4, 6AX5, and 6X5 may be a bit below the 375-0-375 HT winding. Remember, you are likely to get 9% or more above 375 due to the assumed 110VAC input rating with today's wall voltage at 120V or more. With SS rectifier and input voltage difference, you may be looking at B+ of 575 or so. It is difficult to know for sure.

I concur, modern voltages around 122V will put this PT around 416-0-416, easily higher 500's unloaded.

TM

[Phil_S]

I took a break and looked at tube data sheets for 6L6, 6CA7 (no EL34 in the RCA book), KT66, KT77, KT88, and KT90.

A few of these tubes are not rated for Va over 500V so they are out. Most of them will take the plate voltage. Generally, as plate voltage rises, current draw is reduced. I don't know exactly how much and some of these tubes are pigs for mA in SE mode. I am thinking maybe the KT66 will perform OK as a SE tube and not use up the 80mA or so you will have available to feed the plate. I would drop the screen voltage as much as I could. I think maybe a choke between the plate and screen is a good idea. Then, you'll need to drop voltage again for the preamp tubes.

There must be something you can build with this PT!

[LeftyStrat]

I took a break and looked at tube data sheets for 6L6, 6CA7 (no EL34 in the RCA book), KT66, KT77, KT88, and KT90.

A few of these tubes are not rated for Va over 500V so they are out. Most of them will take the plate voltage. Generally, as plate voltage rises, current draw is reduced. I don't know exactly how much and some of these tubes are pigs for mA in SE mode. I am thinking maybe the KT66 will perform OK as a SE tube and not use up the 80mA or so you will have available to feed the plate. I would drop the screen voltage as much as I could. I think maybe a choke between the plate and screen is a good idea. Then, you'll need to drop voltage again for the preamp tubes.

There must be something you can build with this PT!

That's interesting, I'd love to do a high power SE. It does have a 5v 3a filament, so I could drop the voltage perhaps with a tube rectifier.

Most of the application notes have these tubes running at much lower voltage in class A. Can't seem to find too many examples of a 500 volt SE amp.

[LeftyStrat]

An 811A can take 1500V VAMax, but needs 4 amps for the heaters