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Re: EL84 driver

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In Reply to: Re: EL84 driver posted by john2 on May 06, 2004 at 12:42:15:

The bias voltage necessary is a whole lot lower than that needed for KT88s. You could use a 24vAC transformer to make the raw supply and then use the same configuration as in the KT88 schematic. To make the choke loading work, all of the pot and resistor values will probably have to be reduced by a factor of 4, too.
C2 = 100uF 450v Panasonic TSHB
C3 = 1uF 400v
Input impedance is 100K
It will work well, but won't sound as good as it does with the interstage. It will sound really good, though, in any case.
The schematic I sent you shows triode connected EL84s. Pentode is OK too, but not as much to my taste as triode. I don't like the sound of UL.
There is a bit of it on some older forum messages. Essentially it amounts to using the lowest drive impedance practical to drive the output tubes, preferably transformer coupled. Also I think that doing the phase splitting in a transformer, rather than actively, results in a cleaner sound. Call me if you would like to hear more. I'm a better talker than a typist!

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Re: EL84 driver - john f 12:05:25 05/07/04 (3)
Kevin, thanks again.
Could the 6DJ8 be changed for ECC99 or 6SN7 in your scheme to run parallel triode EL84's assuming input and interstage trannies. I would like to know more re neg bias setting for EL84 output stage. I notice also that your driver stage schematic shows coupling for tube grid without neg. bias? (sorry I can't call easily as I am in England)
- Re: EL84 driver - KevinC 12:25:49 05/07/04 (2)
  You could use the ECC99, but the 6SN7GT is, alas, too wimpy to do the job properly, in my opinion. Great tube, but it has limitations.
  What more do you want to know about biasing EL84s? Do you mean you want a schematic to look at?
  The driver stage schematic doesn't assume a bias configuration, but you could choose cathode biasing by grounding one end of the secondaries and standing the cathodes of the EL84s on individual or common cathode resistors or "fixed" biasing, by connecting that end of the secondaries to negative voltage sources and standing the cathodes on individual 10 ohm resistors to facilitate bias measurement and adjustment.
  With a $0.08 per minute plan like I have it's easy to call Europe and not break the bank account. Try it.
  - Re: EL84 driver - john2 02:48:42 05/08/04 (1)
    Thanks. Sure, a pointer to shematic showing neg. bias figures for this tube would be great
    - Re: EL84 driver and bias supply - KevinC 09:52:45 05/10/04 (0)
      Check out the same EL84 amp schematic link again. It has been updated to add suggested bias circuitry for four output tubes.

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