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Old 27th December 2004, 12:31 PM   #11
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Found it, pentode mu=19, triode mu=19.4
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Steve
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Old 27th December 2004, 12:38 PM   #12
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Cheap red LEDs have a lower impedance than the UV LEDs, even taking into account the need to series more red LEDs.

With a mu of (roughly) 20, you've got a feedback factor of 20 to get to unity gain. So the input resistor's value is multiplied by 20 to get the input impedance. A 100K resistor will make sure that grid current isn't a problem, and it will appear to the outside world like a 2M resistor. Use that value to calculate f3 from the input cap.
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Old 27th December 2004, 12:49 PM   #13
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That sounds nice and simple Thanks.

Is there any way of calculating the maximum possible resistor accross the output (if this is desirable?) like some kind of time constant related thing to the output cap?. I guess the value of the output cap will be determined by the lowest impedence headphones likely to be attatched (probably around 36ohms).

I think the main thing to sort out now will be how to drive this. I have a pair of 6922 and 6n1p's on hand, though people say lots of good things about ecc99's which shouldn't be too hard to get hold of. Any thoughts?

Thanks for your time!
Steve
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Old 27th December 2004, 12:56 PM   #14
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Can't help you there, I'm afraid. I don't even own any headphones.:-(

But the ECC88 is a classic as a driver tube. And cheap, too.
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Old 27th December 2004, 12:57 PM   #15
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Well, thanks very much for your help so far!! and the original idea
Steve
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Old 27th December 2004, 02:07 PM   #16
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Hello,
Does a grid leak of 220k (giving an input resistance of approx. 4.4Mohm) and a coupling cap of 22nF sound ok? This should mean an f-3=1.6Hz. What are peoples preferences on coupling caps around this value? Silver Mica (7.50) or PP or something else?
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Steve
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Old 27th December 2004, 02:12 PM   #17
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Oh, yeah, that's a way lower f3 than you'll ever need. 22nF will work fine. So would 10nF, for that matter. The limitation will certainly be in the output cap.

In the input spot, a nice foil/pp cap would do very well. Nothing fancy, a Wima FKP or something like that is as good as it gets in this usage. Output cap, I have no experience, but a motor run polyprop would be the first thing I'd be inclined to try.
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Old 27th December 2004, 02:25 PM   #18
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Does it need a 200V output cap or would 100V be ok?
Sorry for all the silly questions
Steve
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Old 27th December 2004, 04:31 PM   #19
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Yeh I found a thread or two about some 250v 330uF motor run caps that would be good, having trouble sourcing them at the moment, I think I'll just have to leave space for them in case some turn up.

Well here's what I have so far then, now onto the driver stage
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Old 27th December 2004, 04:48 PM   #20
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Dug out these tubes too now,
1 x Mullard GZ34
2 x Sovtek EL84M
2 x Svetlana 6N1P

Cheers,
Steve
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