Could a filament transformer be used as a grid choke? - diyAudio
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Old 8th September 2003, 04:21 PM   #1
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Default Could a filament transformer be used as a grid choke?

I know it's a pretty random question, but I have an unused pair of filament transformers, and I was wondering if they could be used as grid chokes. These are Hammonds, 110V primary / 2.5V 2.5A CT secondary. I could measure the DCR of either winding, but I don't have anything that can measure the inductance of the windings. Does anyone know what the inductance typically is on these things? Would this be worth it? The power tube is a 2A3, and I've read a few people say that it likes to see a grid choke, because it is "prone to grid current" or something like that.

My hookup options would be:

* Use the primary winding
* Use the full secondary winding
* Use paralleled halves of the secondary winding
* Use half the secondary winding (don't see why this would work, one of the previous two should be better)

Thanks,
Saurav
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Old 8th September 2003, 04:35 PM   #2
SY is offline SY  United States
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Easy way to find out- measure its impedance at the range of frequencies of interest. If it's sufficiently high, it'll work.
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Old 8th September 2003, 04:47 PM   #3
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I.e., no more excuses to put off buying that function generator Or I could use my test tones CD, actually.

What would be considered sufficiently high inductance?
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Old 8th September 2003, 04:56 PM   #4
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Quote:
Originally posted by Saurav
I.e., no more excuses to put off buying that function generator
I'm with you there. I just found one reasonably priced

here.
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Old 8th September 2003, 04:59 PM   #5
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Wow, that's a good find. Thanks.
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Old 8th September 2003, 05:07 PM   #6
dhaen is offline dhaen  Europe
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Please do try them, but I'm pretty sure that the inductance will be orders of magnitude too low.

Cheers,
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Old 8th September 2003, 05:10 PM   #7
SY is offline SY  United States
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The nice thing is that you don't even need to determine the inductance- or any of the other parameters like leakage capacitance. Just measure the impedance directly. How much is enough? It depends on what you want as far as frequency response and what the plate load of the driver stage is, or the value of the grid resistor you're replacing. If the driver plate load is, say, 20K, then if the impedance is more than, say, 100K at frequencies of interest, it will work well.

My guess is that the inductance will be too low so that you'll see a severe impedance loss at low frequencies. But it's worth going through the exercise just in case- and for educational purposes.
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Old 8th September 2003, 05:41 PM   #8
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Well, right now the driver is an SRPP 6SL7. It will eventually be a CCS'd ECC99, but that's a bit of a ways away (still waiting for some parts, for starters).

I'll try it out, and if I can understand what's going on, I'll report back with my findings
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Old 8th September 2003, 06:10 PM   #9
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If you want any help, you know where to find me
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Old 8th September 2003, 06:16 PM   #10
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Thanks a lot, I really appreciate your offer.
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