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Re: svet EL34 vs JJ E34L in dynaco st70

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steven oda

In Reply to: Re: svet EL34 vs JJ E34L in dynaco st70 posted by navman on July 12, 2002 at 12:44:48:

NM,
If I remember correctly the standard EL34 bias for the Dynaco ST-70 is 1.54-1.56 volts.
You bias the JJ or Tesla E34L a bit hotter---around 1.65-1.70 volts or hotter. Depending on tone.

Follow Ups:

Re: One last question - navman 16:13:54 07/12/02 (3)
When I set my bias, it fluctuates a bit in the early stages but after playing the amp for an hour or two, if I let it rest, the bias is a lot lower (1.47 or there abouts). When is the best time to set it, within 15 min of starting the amp or after an hour left sitting with no music played through it.
- Re: One last question - Kico 17:03:08 07/12/02 (2)
  Let the amp warm up, 15 minutes should be fine so the tubes can reach operating temps and then set your bias with (no signal) or input. Then play your amp. Check again (no signal) after an hour or so and re-adjust if necessary, after that the bias should remain stable. If it changes as much as you say then there is something amiss in your bias supply circut, rectifier or power tubes.
  A stable bias only needs to be checked/adjusted every few months or so to compensate for tube usage.
  - Re: One last question - steven oda 18:50:38 07/12/02 (0)
    When was the bias power supply caps and diode last replaced? Be sure that this are up to par---as drifting bias can be a sign of old or weak caps.
    A failed bias power supply could be potentially harmful to the amp.
  - Re: Thanks - navman 18:43:43 07/12/02 (0)
    Since I seem to have an original Mullard, I am guessing it may be a little of spec and hence causing some of the fluctuation. I have a new one on order.

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