barn find power amp 4x EL34 + 1x EL84

[Bieworm]

Hi guys!

I recently got my hands on an old power amp that allegedly was used to power a Revox of some type. I regretfully haven't got any more background on it...

After first inspections I noticed the huge transformers on this thing. On the inside of the chassis there is a smaller transformer of which I initially assumed it was a choke, but the single EL84 on the chassis soon made me realize it's some sort of SE EL84 option?
Anyway, since it's 4x EL34 I figured the amp must have about 100W output and given the size of the iron it would make it a suitable donor for a plexi or bass amp project. So I am possibly giving it a shot to build a 100W version of the AA864 and have kinda both in a way.

PT specs:
220V primary
2x245V HT secondary (800mA)
0-50V secondary (10mA) is that even sufficient for negative bias tap? But maybe a zero has faded in the marking and it actually is 100mA...
0-6.3V secondary (7A)

Onto the OT: On the primary side it looks like it is an UL OT. it has 6 wires, where the center 2 wires are tied together at the filter cap. The 2 outer wires are taped off and the inner 2 next to the center ones ware tied to the plates of the EL34 tubes.
On the secondary side there are only 2 wires, so 1 winding. And there is a black wire there too, that the lugged end is bolted to the chassis of the OT.
Now here's the odd thing (to me).. the markings were very unclear, due to the faded waxed paper covering the windings, but I could see there was some writing underneath. When I removed the wax paper I saw that on the primary side there is the following marking: "475 - 425 - 0 - 0 - 425 - 475" (UL?) and on the secondary there is only written "220". Is this something clear for the expert or is there some more investigation required?  does this point to something in the turns ratio? I assume the primary is about 4K, given the 4x EL34 tube layout.
In fact, I need to know the secondary impedance of it to know what speaker load would be ideal. Is this to be determined with the given writings on it, as mentioned above? Or do I absolutely need to check the ratio with my variac?
The 220 marking on the secondary is confusing. It would lead me to believe it's a power transformer. But it's used as an OT, and there are no current specs at the secondary markings. Also, the secondary wires are not soldered to an output jack, but go to a tag strip. The subsequent wiring is removed, but I guess it went to a dedicated terminal which the previous owner needed for something else?

Also, do the depicted transformers look familiar to anyone? Brand? the small transformer inside the chassis

Sorry for my ignorant questions...

[Latole]

Transformers look European made ; Great Britain, Germany ......
Not north america

[danhei]

On the inside of the chassis there is a smaller transformer of which I initially assumed it was a choke, but the single EL84 on the chassis soon made me realize it's some sort of SE EL84 option?

EL84 is very likely driving the small transformer as an interstage transformer phase inverter.


See the 6V6GT in the Fender 300 PS for an example: https://el34world.com/charts/Schematics/files/Fender/Fender_ps300.pdf


Suggest you try to draw a schematic if possible, and try to reuse the power supply, power amp, and phase inverter as-isle, with a preamp of your choice.

[sluckey]

Show us some more pics. We need to see views from all sides of the chassis. Need to see all input/output connections. Also a hi-rez gut shot of the inside of the chassis.

Is that white cord actually a shielded coax cable? Power cord? What is it connected to?

The odd markings on the OT make me wonder if that may possibly be a modulator chassis for a radio transmitter.

[Bieworm]

On the inside of the chassis there is a smaller transformer of which I initially assumed it was a choke, but the single EL84 on the chassis soon made me realize it's some sort of SE EL84 option?

EL84 is very likely driving the small transformer as an interstage transformer phase inverter.


See the 6V6GT in the Fender 300 PS for an example: https://el34world.com/charts/Schematics/files/Fender/Fender_ps300.pdf


Suggest you try to draw a schematic if possible, and try to reuse the power supply, power amp, and phase inverter as-isle, with a preamp of your choice.

Thanks for the suggestion, but I’m just gonna use the transformers to build something I can use. I just need to know the secondary value of the OT, that’s all. If I were to restore the current amp I’d still need to figure that one out.

[sluckey]

I’m just gonna use the transformers to build something I can use. I just need to know the secondary value of the OT, that’s all.

You need to determine the impedance ratio. Apply an AC voltage to the secondary and measure the voltage across the plate leads of the primary. (No need to disconnect plate leads from the tube sockets.) I like to put 1V on the secondary just to make the math easy. The voltage ratio is equal to the turns ratio. Now square the turns ratio to get the impedance ratio.

[Bieworm]

hi Steve

The white cable is a non grounded 2 conductor wire, not shielded.
I would be surprised that this isn't a straight up power amp given the size of the transformers tho..

[Bieworm]

more pics

[Bieworm]

more pics

[sluckey]

I would be surprised that this isn't a straight up power amp given the size of the transformers tho..

I'll be surprised if that "OT" has an impedance ratio that will match with a typical speaker load.

[Bieworm]

I would be surprised that this isn't a straight up power amp given the size of the transformers tho..

I'll be surprised if that "OT" has an impedance ratio that will match with a typical speaker load.

Lol! Me too. At least the power transformer will be of some use. And there are some nice Tronal EL34 tubes too

[kagliostro]

May be It can be used as transformer of the modulator in a tube transmitter

Franco

[Bieworm]

May be It can be used as transformer of the modulator in a tube transmitter

Francop

What do you mean? The power transformer? I could easily use it for a B15N cathode biased project I would think

[sluckey]

I'm surprised you have not checked the output transformer impedance ratio yet. Git'r dun!

That chassis does not look like any consumer electronics. Looks more industrial quality. I'm leaning strongly toward thinking it's a modulator chassis for an AM radio station. If that's the case, you can certainly use the PT in a guitar amp, but the OT would make an interesting doorstop. 

Checking that OT will reveal a lot. Just do it.

[shooter]

I don't see enough inductors for broadcast, so I'm putting my beans on an Audio amp for a big-ol-place like a 70's mall, the OT being an impedance matching tranny to drive all those speakers

[sluckey]

I don't see a need for inductors in the audio modulator. Save those for the RF sections. I also can't see a 100 Watt amp pushing all the speakers in a '70s mall.

[Bieworm]

Checking that OT will reveal a lot. Just do it.

I will have to look into that some more. I never did that check, but I'm willing to learn. Is there a way that works better/easier than the other?
I have a scope, DMM, 1000 watt variac

I found this: https://www.radioremembered.org/outimp.htm#:~:text=All%20we%20need%20to%20do,ac%20is%20all%20we%20need.

[sluckey]

That radioremembered.org link is perfect. I've attached a simple diagram showing your transformer...

[Bieworm]

Ok, did the test. V prim = 3.66V  when V sec = 1V.
I conclude the turns ratio is 3.66/1
So squaring that makes 13.4
At 4 ohm load the prim impedance = 53.6 ohm. At load 8 ohm it’s 107 ohm and at load 16 ohm it’s 214 ohm. All way too low and the OT will end up at the back door to let through the summer breezes. Well in time! Imagine I checked this transformer in the fall!!! I’d have to wait for a long time to use it for that purpose. 😎

[shooter]

53.6 ohm

close enough to transmit