New EL34's won't bias

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ClassicRock88
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New EL34's won't bias

Postby ClassicRock88 » 04 Jul 2017, 13:15

I got a new to me Germino Headroom! (See the Valve Amp section.). So far it's really cool but I ordered a new quad of TAD EL34B's to run in it. It runs 450V and I can't get the idle current down below 40 ma per tube. This works out to about 75% MPD. Suggestions?

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Re: New EL34's won't bias

Postby Anitoli » 04 Jul 2017, 17:35

Depending on how the bias circuit is laid out it might need to have an increase in one or the resistors to bring the bias voltage up.

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Re: New EL34's won't bias

Postby surfnorthwest » 04 Jul 2017, 19:29

Being a brand new amp I would give Greg a call and get his opinion as it may be a simple tweak, as expensive as that amp is the last thing I would won't to do is start modifying it. I suspect the issue is with the "B" as in El34B opposed to straight EL34s. I know the EL34-B is a higher impedance and has a max plate wattage of 25 watts.
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Re: New EL34's won't bias

Postby Dave W » 05 Jul 2017, 00:25

I try to run all my amps at about 40ma per tube even if I do not measure plate voltage when biasing. Most amps run 400-450v and 40 is nice number. Not running cold for sure but not hot, just warm. I would think its the grade-rating of the tubes and the bias pot can only get them down to 40. I could be wrong but that's what I would guess. Your new tubes are probably on the far end of the spectrum as far as their rating. Just my guess. You could contact the tube seller and explain and ask for a replacement set. See what the amp designer says to as mentioned in previous post and maybe that info will help if need to deal with a replacement set from seller. You could also just run them at 40 and leave it.

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Re: New EL34's won't bias

Postby ClassicRock88 » 05 Jul 2017, 06:13

Thanks guys. I got it figured out! The guy who installed them clearly had no idea what he was doing and didn't check the range of the bias pot when he first installed the tubes. If he did he would have realized that the trim pot on the new amp was backwards from what he was used to and he was actually trying to cook the tubes. When he decided to say "oh well I'm gonna see how hot these will go" he realized that the idle current decreased as he turned the trim pot toward what he thought would increase the current. Oops.

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Re: New EL34's won't bias

Postby Dave W » 05 Jul 2017, 22:17

:headbanger :yea

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