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Which Power Transformer?
Which Power Transformer?
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Old 21st February 2019, 01:02 PM   #1
Troy Madden is offline Troy Madden  United States
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Default Which Power Transformer?

I have a couple pairs of output transformers suitable for 4X 6550/KT88, so I have decided to build a pair of monoblocks from scratch. Never done that before, but I have built , modified and repaired many tube amps. Trying to narrow down which Hammond power transformer to use. I want a bias winding for fixed bias so that narrows it down to the 300 series. Would like to run around 500 volt B+, so I'm thinking the 370 MX transformer? Hammond Mfg. - Universal Primary - "Classic" Power Transformers
http://www.hammondmfg.com/pdf/EDB370MX.pdf
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Old 21st February 2019, 04:01 PM   #2
GoatGuy is offline GoatGuy  United States
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Looks good.

You might want to use a full-wave-bridge on the 550 V red-red secondary instead of the usual center tap, and then go for a so-called "choke loaded" filter section. This way you'll get closer (or at) your 475-to–525 volt B+ ideal.

There's plenty of horsepower in that transformer. Remember tho: it'll get HOT if you are running it close to limit-specification. Make sure the chassis has ventilation holes around the transformer to allow some airflow upward/convection style.

Also… you might want to use series B+ regulation (either with, or in lieu-of, the choke-loaded filtering). It is hugely effective, and you will give up very little in order to secure a SOLID (line voltage independent) B+. Indeed, using MOSFET series B+ regulation and a stack of Zeners (and some cute high-Ω resistors and mid-value capacitors), you can quite easily also simulate a slow-turn-on vacuum-tube rectification, whilst still using silicon rectifiers for all their volt-saving (and maintenance) glory.

Just recall, always, that the design goal of a HiFi power supply is to produce solid, low impedance, very, very low ripple DC output. The method (vacuum or sand) is actually pretty darn agnostic. Get 'er done, and get 'er well filtered.

Nuff said.
GoatGuy
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Old 21st February 2019, 04:12 PM   #3
Troy Madden is offline Troy Madden  United States
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Thank you! I was planning on silicon bridge rectification. Still contemplating filtering, thanks for your advice. Will definitely consider regulated B+.
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Old 21st February 2019, 06:00 PM   #4
kodabmx is offline kodabmx  Canada
Affordable triode connected sweep tube amps.
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Which Power Transformer?
378CX is what I would use. B+ of 560V or so.
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Old 21st February 2019, 07:17 PM   #5
GoatGuy is offline GoatGuy  United States
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Quote:
Originally Posted by kodabmx View Post
378CX is what I would use. B+ of 560V or so.
... to be honest, I kind of thought you'd be using a hat full of SMPS switching supplies! ... Just joking, LOL. But... you might! GoatGuy
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Old 21st February 2019, 10:58 PM   #6
kodabmx is offline kodabmx  Canada
Affordable triode connected sweep tube amps.
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Which Power Transformer?
I use whatever is more convenient/cheaper if I'm honest
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