Cheapo transformer substitution for Zen - diyAudio
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Old 20th December 2001, 09:23 AM   #1
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Default Cheapo transformer substitution for Zen

I apologise in advance if this is a silly question....

So I'm about to put together one channel of the Zen amp. But, being a jobless student, rather than buying a nice toroidal transformer, i've been scrounging around my house for a free replacement.

I found a pretty big 375V hammond (115 on the primary, and 375V on the secondary plus some misc. filament windings), which, to me, is better than no transformer at all. Is it legit to use the secondary winding of this transformer as the primary? (i.e.: put 115v on the secondary, and get out ~35 on the primary)?

35V is close enough to the ~25V Pass accounts for with the Zen. Is this workable/safe?

Cheers,
Chris McGraw
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Old 20th December 2001, 10:57 AM   #2
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more importantly, the VA ratings.
with voltages so high, the current rating of the wires are gonna be low. unless its a large transformer, with a large current rating per rail.
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Old 20th December 2001, 12:41 PM   #3
paulb is offline paulb  Canada
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Yes, calculate the amperage rating of the windings (VA rating / V) - that will be the maximum amperage.
What model of Hammond?
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Old 20th December 2001, 01:52 PM   #4
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Shooting from the hip, I wouldn't think it would work. It'd be a pity to roast a nice Hammond transformer that way. Build a tube preamp (or amp, depending on the VA rating) instead.

Grey
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Old 20th December 2001, 02:38 PM   #5
paulb is offline paulb  Canada
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Ham radio folks do stuff like that all the time. Transformers are inherently reversible. As long as you derate the VA according to the amperage rating of the windings.
Transformers are bloody expensive, I try to use surplus ones too.
Grey probably just wants to see more vacuumtubeaholics around here.
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Old 20th December 2001, 07:22 PM   #6
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I believe it would work OK.
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Old 20th December 2001, 07:26 PM   #7
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I'm not speaking from the standpoint of running a transformer backwards--got two of them backwards in my system even as we speak--just trying to guess at the VA aspect. Lacking knowledge as to which transformer it is (i.e. the ratings) we're condemned to guessing...

Grey
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Old 20th December 2001, 08:48 PM   #8
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Default Hammond specs

The transformer I have is a hammond 274X, a supply transformer intended for tube use, cause it has filament windings. the rating is 138VA (which i'm not sure i understand--what's the relation between VA and Watts?)

While we're talking about cheaping out on the transformer, how bad would it be if I just used any old 25v (suitably rated) transformer for this project (i.e. not a toroidal one)?

Thanks for the help,
Chris
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Old 20th December 2001, 09:58 PM   #9
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Standard 'square' transformers are commonly called EI (after the shape of the individual pieces of metal that make up the core). They will work just fine. There are advantages to both kinds of transformers, but one that will appeal to you is that EI transformers are generally cheaper than equivalent toroids. Some people go so far as to claim they sound better. I have no opinion, myself, having never tried swapping transformers.

Grey
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Old 20th December 2001, 10:32 PM   #10
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Okay, somebody check me on this, as they're replacing the phone system here at work and there are about 10 million people running around in circles being loud and distracting.
Went to the Hammond site (www.hammondmfg.com) and found that the secondary on the 274X is 375-0-375 V @110 mA, which works out to 82.5 VA on the secondary (yes, the transformer is rated at 138 VA, but a fair hunk of that is on the filament windings & can't be used).
Now, the turns ratio is going to be 6.25, so if the transformer is run backwards, we're going to have 19.2VAC on the primary (which now becomes the secondary). If you use a cap input filter after the rectifier, and subtract a couple of volts for rectifier losses, you'll end up with something like 25V coming out of the power supply. That's about 9V lower than the original rail, but the amp will still work fine, albeit at reduced power.
Now, the limiting factor, current-wise, is going to be the secondary (which is now the primary). The original primary (now the secondary) is good for the full 138 VA. It then becomes a question of how much current we will pull at 25V.
Assuming that the circuit is built as published, the revised Zen drew 3A (I think...somebody will let me know if I'm wrong, I'm sure). Yes, it will still pull 3A even at the reduced rail, because the current draw is set by the sense resistor in the current source, not by the rail. That would lead to about a 75 VA load. Offhand, I'd say that would run a bit warmish, as that's a full 3A all the time.
The original Zen, however, only drew 2A (again, quoting from memory)...which would lead to about 50VA. That sounds better. More elbow room.
Okay, so here's my take--if you're going to do it, run the amp at 2A. Mind you, you can still build the newer version, but you'll need to substitute the older sense resistor (.33 ohms, yes?) for the newer value, which is, I think, .22 ohms. Or perhaps a .27 ohm resistor which would give about 2.4A (resulting in about 60VA...fairly heavy load, but it might work out).
Somebody run through that and double-check me, but it sounds as though it's just barely doable.

Grey
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