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Old 8th September 2011, 04:36 AM   #191
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Yes, Ill test the CF first., which BTW Im not very fond of those with my short experience with other tubes. Have you tested output impedance for this configuration? Or have an estimate?
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Old 8th September 2011, 04:42 AM   #192
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Nice schematic Salas, thanks
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Old 8th September 2011, 05:51 AM   #193
Salas is online now Salas  Greece
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Quote:
Originally Posted by apassgear View Post
Yes, Ill test the CF first., which BTW Im not very fond of those with my short experience with other tubes. Have you tested output impedance for this configuration? Or have an estimate?
~200 Ohm. Teflon 0.1uF/200V input capacitor is highly recommended. Even 0.022uF will work due to the extra high input impedance of the bootstrap (post#144).
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Old 8th September 2011, 06:34 AM   #194
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The bootstrapped CF offers one other benefit.
There is a DC bias across the input coupling cap which means that the dielectric stress is never reversed (with audio signal) and therefore there is no hystersis to worry about. That means that the performance of any good polypropylene cap will equal that of a Teflon or (sweeping generalization) any other piece of audio jewellery cap.
Cheers,
Ian
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Old 8th September 2011, 06:38 AM   #195
Salas is online now Salas  Greece
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True for the cap foils are ''biased''. Still the Teflon was superior for clarity in this one after I tested some caps. Don't imagine anything extravagant money wise. CCCP NOS FT-1/2/3 are perfectly adequate.
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Old 13th September 2011, 09:41 AM   #196
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Hello Salas.

Do you think that your preamp will work properly with these tubes:
6P14P, 6P14P-V, 6P14P-EV
These are russian equivalents of el84 which is very similar to 6v6 (like I heard).
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Old 13th September 2011, 10:12 AM   #197
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Don't be sure before you test. The rough guideline would be they bias at 22-25mA in these circuits here with ~200V developed across them, that you will see 0.0xxx THD for 2.82V PP out, and microphonics will be tolerable in line pre duty.
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Old 17th September 2011, 09:03 PM   #198
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I guess this is much better than my buffer. I also need some gain... I must build one 6v6 preamp
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Old 27th September 2011, 08:13 PM   #199
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Was looking thru the thread and didn't see this mentioned...

Has anyone tried a string of LED's to get cathode bias voltage? (Like SY's Red Light District amp.) Five white LED's all in a row should get 15V or very close. Only a few ohms effective resistance. That would get the cathode bypass cap out of the circuit.

Or maybe a zener string? (High wattage zeners needed.) I calculate less than a half of a watt dissipated across cathode load (15V * 0.025A = 0.375W).

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Old 27th September 2011, 08:22 PM   #200
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Did try on different preamps (12B4 & 6SN7). But I preferred the classic R//C tone, so I did not try again in this one. An easy and maybe worthwhile test for those who will make this pre non the less. Its a system synergy and preference area the tone thing should we not forget. Same goes for the tubes brand choice, output cap choice, etc.

P.S. Don't know about white LEDS, but red, green, yellow, IR, are surely the low noise ones.
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