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Old 23rd March 2010, 06:09 PM   #1
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Default How suitable is this tranformer for a Tube Amp Project?

I came across a local classified ad offering a few of these transformers for dirt cheap.

I have a solid state amp on my resume. I'd like to build a tube amp next. I haven't selected a design yet.

Can anyone comment on how suitable this power transformer would be and possibly suggest a compatible design (Simple SE or other)?
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Old 23rd March 2010, 06:16 PM   #2
TheGimp is offline TheGimp  United States
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No good, send them all to me.

Just joking, it looks like you could get roughly 530V B+ at 0.185 A for the plate supply ( less tube loss of tube rectification, etc).

That is sufficient for roughly 90W total power. You can get about 45W class AB(PP), or 22 W class A (SE).

Build monoblocks, or use two transformers, one for each channel if you want that much per channel.

I've been concentrating on low voltage amps (B+ = 265V) so I don't have any recomendations for the amp. I'm sure someone will chime in. Maybe a 6V6 20W/Channel design.

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Old 23rd March 2010, 06:57 PM   #3
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If they are cheap, pick up a couple. They are well suited for PP monoblocks, or stereo SE, maybe even lower power PP stereo.

A 530V B+ may be a little optimistic; I would guess 475-480V B+ using SS rectification. 730/2*1.41=515V, then subtract volts for R or L in CRC, CLC, etc.
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Old 23rd March 2010, 07:22 PM   #4
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I can see several homes for those. Well suited for a kt88 or similar.
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Old 23rd March 2010, 08:46 PM   #5
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If you happen to have a 250 mA choke, you could also make a choke-input power supply for a push-pull stereo 6V6 or 6BQ5 amp. With cap input, I think it would be great for the Simple SE (El34 or KT88).
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Old 23rd March 2010, 09:59 PM   #6
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Thanks for your help. I'll scoop up a couple of them.

The seller tells me that they came out of some "military tube audio / rf equipment". I'd expect that means decent quality.
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Old 24th March 2010, 12:05 AM   #7
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Quote:
Can anyone comment on how suitable this power transformer would be and possibly suggest a compatible design (Simple SE or other)?
I see no reason why it wouldn't run a Simple SE. I have used Mil surplus transformers like these with good results.
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Old 24th March 2010, 02:06 AM   #8
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I like Tom Bavis' choke I/P filter idea. Used that way, the trafo being discussed would provide everything needed to power an "El Cheapo". Phase up the 5 V. and 30 V. windings and use the "35" V. composite to energize the B- hybrid bridge.

Not much in the way of a "fudge factor" cap. would be needed to bring the B+ rail up to a "perfect" 355 V. Perhaps a Hammond 193M would do as the 1st inductor in the cLCLC filter.
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Old 24th March 2010, 10:19 PM   #9
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Originally Posted by tubelab.com View Post
I see no reason why it wouldn't run a Simple SE.
Would this transformer "plug-and-play" into the Simple SE design, or would other modifications be required?
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Old 24th March 2010, 10:53 PM   #10
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Quote:
Originally Posted by seabass23 View Post
Would this transformer "plug-and-play" into the Simple SE design, or would other modifications be required?
I believe that it should be 'plug and play'.

This transformer has a 12.6 volt 4 Amp centre tapped secondary for the heaters. The Simple SE uses 6.3 volts for the heaters, but does not need a centre tap as it has an elevated reference circuit on the board. You should be able to use the centre tap and one 12.6 volt winding of the transformer, that will give you the 6.3 volts required. The output tubes needing the most heater current would be the KT88 with 1.6 Amps each. The ECC81 (12AT7) needs 0.3 Amp. This comes in at 3.5 Amps heater current total.

The Volts x Amps (VA) rating of the 12.6 volt winding is 50.4VA. With 6.3 volts, that works out to 8 Amps available. Since you will be using half of the winding to get the lower voltage, I am not sure if this halves the current back to 4 Amps, but in any case you are still safe. Someone more knowledgeable than me on transformers might like to speak up here to clarify.

They look like good transformers. Nice case, useful voltages and currents, and multiple primary taps to account for mains voltage variations. If they are cheap, I would pick up a few. Once you build your first amplifier, you will be hooked and want to build more!

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Chris
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