• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

What topology was this old RCA power transformer designed for?

ericj

Member
2008-12-08 10:24 pm
Yeah, I have ADD.

Years ago i bought some pretty potted transformers without really thinking about what specifically i would use them for.

This one has 4.2A of 6.3v and a somewhat complex hv winding

235-90-CT-90-235-130

The markings indicate that the 235v is good for 50mA and the 90v winding is good for 15mA and even clarifies that you can get a total of 65mA off of the high voltage secondary. The 130v winding is rated for 5uA.

So I am imagining four beam power pentodes and maybe a separate supply for the screen grids. Plus a very low current bias voltage.

Feel free to suggest an actual amp design.

Thanks for any suggestions or insight.
 
If the 235-0-235 winding is good for 65mA AC load, then the best you can do on the DC output side is about 93mA with a choke input filter. That's frustratingly close to enough for a small stereo power amp. Since you will load the heater winding far below spec, you might get away with 100mA, and 50mA per class-A output tube isn't too shabby. That doesn't leave anything for powering the driver stages, but creative solutions are possible! Transistors, low-voltage tubes, etc.
 

ericj

Member
2008-12-08 10:24 pm
If the 235-0-235 winding is good for 65mA AC load, then the best you can do on the DC output side is about 93mA with a choke input filter. That's frustratingly close to enough for a small stereo power amp. Since you will load the heater winding far below spec, you might get away with 100mA, and 50mA per class-A output tube isn't too shabby. That doesn't leave anything for powering the driver stages, but creative solutions are possible! Transistors, low-voltage tubes, etc.

Yeah, i have many high quality audio opamps, and it would be trivial to tap the heater supply, FWB, maybe voltage doubler for headroom, and perhaps a darlington follower hanging off the chips.

Or just slap some more iron in there. I appear to have a spare Triad VPS230-110 that can supply 110mA of 230v, which would be more than i'd need for a driver stage. or connect a small 6.3v transformer backwards to the heater secondary, rectify 115 or 230 to power the driver stage.

I have half a dozen or more 6n6p-I with the 900mA heater, and imho they sound pretty great.

I have a few 6n1p, 6n23p, and 6n2p as well. Soviet schematics for 6n2p+6n6p as a push-pull output stage are floating around the net. They describe the output transformer in terms of gauge of wire, bobbin, and number of windings but some research suggests 10k to 12k primary is a good idea for 6n6p at 250-300v.
 
You will have trouble getting 250VDC from 235VAC and a choke input filter. However, older transformers sometimes pump out a bit more than you expect because the primary was designed for less than 120VAC on the line. I reckon you'll get about 2.5W of audio power out with B+ at 220V and OPT primary of 8K.
 

ericj

Member
2008-12-08 10:24 pm
Bump for PSA -- it helps to remember sometimes that archaic transformer markings aren't necessarily what we expect based on current conventions.

I'm getting 365v from each side of the HV secondary.

I think what they meant must have been that lugs 3 and 6 are each 235v from lugs 4 and 5, which are both 90v from ground?

Mains AC here is about 125vac, fwiw.

Glad i tested it with 1n4007 rectifiers and a 400v cap from the junk bin before hooking it up to anything expensive. I suppose I'll be throwing away the 400v cap that i put more than 470v across for a minute or so.
 
Yeah, I have ADD.

Years ago i bought some pretty potted transformers without really thinking about what specifically i would use them for.

This one has 4.2A of 6.3v and a somewhat complex hv winding

235-90-CT-90-235-130

The markings indicate that the 235v is good for 50mA and the 90v winding is good for 15mA and even clarifies that you can get a total of 65mA off of the high voltage secondary. The 130v winding is rated for 5uA.

So I am imagining four beam power pentodes and maybe a separate supply for the screen grids. Plus a very low current bias voltage.

Feel free to suggest an actual amp design.

Thanks for any suggestions or insight.

Is this a big somewhat rectangular type potted transformer? If it is,it no doubt came out of one of the 6L6 or 807 theater amps..A picture would tell us more.
 

ericj

Member
2008-12-08 10:24 pm
Is this a big somewhat rectangular type potted transformer? If it is,it no doubt came out of one of the 6L6 or 807 theater amps..A picture would tell us more.

[IMGDEAD]https://uploads.tapatalk-cdn.com/20160412/1dae2542e2c8783c16ca50e2065eda7a.jpg[/IMGDEAD]


At the moment my target is an old tube theremin (E. J. Schultz design from '49). two 6c5, 6sn7, 6sa7. I'm hoping there's enough b+ current left over for a small instrument amplifier, maybe a couple 12ax7 and a single 6V6.

.:Sent by pneumatic tubes
 
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