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Old 14th January 2005, 10:53 PM   #1
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Default How to use one tube instead of another in this headphone amp?

I have built a headphone amp with 3xECC88. Today, when I was listening, half of the first tube flashed a few times (taking one side of my headphones with it). Now it's silent in that channel, and output capacitor of the other channel is giving strange smell... I don't have spare ECC88 (6N23P) so I inserted 6N2P in it's socket (V1). The amp is working, but has very little power (my headphones are 32ohm). I think it would be possible to modify the amp to give more power with 6N2P as V1. The closest non-russian tube to 6N2P is 12AX7. How I can modify the amp to give as much power as it gave with 6N23P as V1? Schematic of this amp: http://headwize.com/projects/showfil...=cmoy5_prj.htm (the modified one). Datasheet of 6N2P: http://www.russiantubes.com/prop.php?t=12&p=195
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Old 15th January 2005, 01:03 AM   #2
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Default Re: How to use one tube instead of another in this headphone amp?

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Originally posted by Pentium100
and output capacitor of the other channel is giving strange smell...
I hope you replaced both capacitors before even thinking about swapping tubes.

As for 6N2P, if it's closest to a 12AX7, you can never get as much power because the tube is not designed to allow a tenth of the current through. And into 32 ohms, almost all your power comes from that current.

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Old 15th January 2005, 11:28 AM   #3
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I replaced them.

I thint V1 does not have to give much current, V2 is for that. Because the grid of upper parts of V2 is conncected directly to the plate of V1 and because V1 has lower current, I think I should change R2 to a higher value, so the voltage on the grid of V2 is bigger than it should be. I think I should change R2 to a higher value, so it's the same voltage on the plata of V1 as it should be.
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Old 15th January 2005, 03:57 PM   #4
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Hmm.

*Looks at schematic*

Ah, yes. V1 doesn't need to make any current...indeed. Changing the plate or cathode resistor to get the voltages back within range would help a lot as you surmise. R3 could be a little higher and R2 in the 100 to 330k range.

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Old 16th January 2005, 07:21 PM   #5
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Thanks. How could I discharge output capacitors when I turn off the amp. For power supply capacitors it's simple, I just put 200k resistor between + and - of the capacitor. If I put a resistor here too, then my headphones would get DC.
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Old 17th January 2005, 03:00 AM   #6
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Put a resistor to ground on both sides. R7 takes care of the 'phones side so if you had them unplugged, you don't get a nasty surge in your ears (POP!) when you plug in. You can easily put a similar resistor on the live (tube) side, though maybe 47k or 100k given the higher voltage.

Residual charge shouldn't be a problem unless you're concerned about it while working... in which case you could insulate a 100 ohm resistor to discharge it by hand.

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