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Old 1st January 2013, 12:34 AM   #1
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Default 300V Tube Power Supply From 120VAC Without Rectifier?

I stumbled across this interesting tube amp power supply in my research for my amp (832A as a Single Ended Tetrode Amp?), and I was wondering if someone could take a bit of time to explain the principles of it, and it's appropriateness in a tube amp. The creator uses a 230/120VAC multi-input transformer, with the first tap on the primary side being the input, and the second tap on the primary side as the output. Usually these two are bridged or paralleled, but here they're used almost like an isolation transformer, I think:

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Old 1st January 2013, 01:16 AM   #2
TheGimp is offline TheGimp  United States
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Voltage doubler. Used by lots of older designs.
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Old 1st January 2013, 01:19 AM   #3
roline is offline roline  United States
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You can use the primary as an output, but you will have less current capability rather than the same size core using both primaries and having another set of matching outputs.
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Old 1st January 2013, 03:16 AM   #4
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Quote:
Voltage doubler. Used by lots of older designs.
I too use the Greinacher (AKA "full wave") doubler, as it is a cost effective way of obtaining a truly high performance PSU. "Idiots", like Frank McIntosh, Saul Marantz, and Stu Hegeman, made very good use of the topology in some of the most revered "vintage" amps.

Gimpy is correct about TANSTAAFL applying and only 1/4 of the RMS current capability of the rectified winding is available as DC. When the setup is done right, copper losses can be small. The rectifier winding in a H/K Cit. 2 is rated for several amperes.
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Old 1st January 2013, 03:14 PM   #5
kevinkr is offline kevinkr  United States
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Using one half of a dual primary is potentially hazardous due the potential lack of isolation between the windings. In addition the transformer must be derated 50% or more for safe (?) operation if this is done.

With the ready supply of OTS and inexpensive custom power transformers from outfits like Edcor this approach should not be encouraged.
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Old 1st January 2013, 03:15 PM   #6
wa2ise is offline wa2ise  United States
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Be aware that the quality of the insulation between the two 120VAC windings is likely not "HiPot", ie, being able to insulate 1.4KVDC. The powerline sees a lot of transient spikes, from washing machine motors turning on and off, lightning bolts, and whatnot. Requiring 1.4KV insulation seems to avoid these issues. You don't want these causing a flashover from the line into your amp.
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Old 1st January 2013, 03:37 PM   #7
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Thanks for the tips guys. I'm only going to use this for low-current applications (one or two tubes).

Wa2ise, would plugging the transformer into a surge protector help protect my amp from the potential transient spike hazard?
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Old 1st January 2013, 06:16 PM   #8
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For 1 or 2 tubes, buy a Triad N-68X isolation trafo. It has the "stones" to power a SE stereoblock built around 2X 6BM8s. Lesser rated units actually cost more. Given its dual primaries, the N-68X is useful to DIYers in both "120" and "240" VAC mains zones.
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Old 1st January 2013, 07:52 PM   #9
kevinkr is offline kevinkr  United States
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Eli's suggestion is a good one - only use the secondary not the primaries for this purpose. (Primary to mains, secondaries used as you see fit.)

In your proposed approach interwinding leakage currents between primary windings can be significant, posing a shock hazard particularly if the project in question is not properly grounded for some reason. (Like the receptacle miswired or 2 conductor house wiring) This just is not a very good idea at all, I hope you are receiving this loud and clear.
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Old 1st January 2013, 10:10 PM   #10
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10-4

I have an isolation transformer I used when rebuilding an old 5 tube radio from the war that puts 120V across its heaters...good stuff.

Would you guys be able to weigh in on this design? 832A as a Single Ended Tetrode Amp?
I'm looking for....positive feedback

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