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Old 12th February 2016, 04:57 AM   #1
ericj is online now ericj  United States
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Default What topology was this old RCA power transformer designed for?

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.
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Old 12th February 2016, 05:54 AM   #2
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Give the low current capacity, you will be lucky to even get a pair of small power pentodes to work, so it is likely made for preamplifier, small radio or some industrial applications.
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Old 12th February 2016, 06:09 AM   #3
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Yeah, I suppose you're right. All that heater current is probably for full wave rectifiers.
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Old 12th February 2016, 09:50 PM   #4
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Maybe a 6AQ5 PP, fixed bias, with a simple preamp or phono or radio on the front?

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Old 12th February 2016, 09:55 PM   #5
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Could be. Right now i am thinking 6n6p pp - mostly because i already have a bunch

.:Sent by pneumatic tubes
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Old 12th February 2016, 09:59 PM   #6
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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.
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Old 12th February 2016, 10:15 PM   #7
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6N6P in PP class AB1 biased around 15mA per triode could work. That puts anode dissipation at a conservative 3.3W each if B+ is 220V, and leaves 30~40mA for the drivers.
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Old 12th February 2016, 10:23 PM   #8
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Quote:
Originally Posted by BinaryMike View Post
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.
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Old 12th February 2016, 11:05 PM   #9
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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.
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