express external bias-pot option question?

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Gibsonman63
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Re: express external bias-pot option question?

Post by Gibsonman63 »

M Fowler wrote:Separate bias for each tube since the matched tubes we get today don't stay matched for long.
Thanks. That's a new one on me. Maybe I need to be checking bias more often.
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M Fowler
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Re: express external bias-pot option question?

Post by M Fowler »

Well I imagine the amps circuit isn't exactly balanced either so that would be a reason the tubes are not the same. Even so matched tubes is no guarantee of a perfect match when the amp is operating.

Some enjoy a little mismatch for tone purposes I have read. :) But what the heck do I know, I'm no expert in tube matching or tube amplication engineering. :)

Mark
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NickC
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Re: express external bias-pot option question?

Post by NickC »

M Fowler wrote:Well I imagine the amps circuit isn't exactly balanced either so that would be a reason the tubes are not the same. Even so matched tubes is no guarantee of a perfect match when the amp is operating.

Some enjoy a little mismatch for tone purposes I have read. :) But what the heck do I know, I'm no expert in tube matching or tube amplication engineering. :)

Mark

Having a separate bias for each tube sounds like a great idea. If a mismatch is desired, dial it in. If not, get each output tube at the exact desired bias point regardless of other circuit/tube vagaries.

Is there any downside to it?
Gibsonman63
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Re: express external bias-pot option question?

Post by Gibsonman63 »

Plus you can run unmatched output tubes.
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M Fowler
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Re: express external bias-pot option question?

Post by M Fowler »

Yes the whole point for me is I want to run individual bias pots to set each tube and not to soley rely on match tubes sets. I would still get a matched set but then I could keep them idling correctly not just each pair.

Mark
analog_stud
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Re: express external bias-pot option question?

Post by analog_stud »

That would be the way to go. In the existing "express" BOM files found here it, indeed, calls for two trimmers in the external biasing options' list of parts. I'm going to assume that's for each tube? I've still yet to see exactly how this is implemented in the circuit. Has anyone done this (trimmer for each tube)?
yalesmith
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Re: express external bias-pot option question?

Post by yalesmith »

Has instructions been posted anywhere? i cant seem to find any.

Thanks




That would be the way to go. In the existing "express" BOM files found here it, indeed, calls for two trimmers in the external biasing options' list of parts. I'm going to assume that's for each tube? I've still yet to see exactly how this is implemented in the circuit. Has anyone done this (trimmer for each tube)?
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M Fowler
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Re: express external bias-pot option question?

Post by M Fowler »

Notice on the attached the 220K/220K resistors are separated going to their own bias circuit and pot.
dual bias.gif
Yeti-50-30-June-2015.jpg
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