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Mixing tubes in an auto-bias tube amp

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Posted on July 9, 2020 at 09:42:35
Mick Wolfe
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Posts: 3365
Location: AZ
Joined: October 10, 1999
Contributor
  Since: September 4, 2000
 For those familiar with auto-bias tube amps/circuits. Have any of you inmates had any success mixing a couple of different tube types or just same tube type/different manufacturer in your amp? In other words, come up with your own mix/match output tube recipe. I'm thinking it's just best to stick with a quad of your favorite type and brand and call it a day, but I had to ask.

 

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RE: Mixing tubes in an auto-bias tube amp, posted on July 9, 2020 at 15:12:35
6bq5
Audiophile

Posts: 4393
Location: SF Bay
Joined: August 16, 2001
 Auto Bias circuits are not lab-bench circuits capable of accommodating any tube, in any combination-
If the amp was set up for 6550s - and 4 of each per side-
You will get the best results with 4 from the same Manufacturer that are reasonable grouped as a 'matched' quad-
substituting a KT-90 or 6CA7 will not work out well-

The amp MAY be able to accommodate a quad of either tube- but not knowing the amp, I would hesitate to encourage-
Happy Listening

 

RE: Mixing tubes in an auto-bias tube amp, posted on July 9, 2020 at 18:22:53
Mick Wolfe
Audiophile

Posts: 3365
Location: AZ
Joined: October 10, 1999
Contributor
  Since: September 4, 2000
 Thx. That's exactly as I see it, but had to confirm. Pick a quad (or whatever) of a respected output tube that's optimized for your amp and enjoy the self bias feature. I'm sure the tube cocktail I've seen in ads for self biasing amps would be a sonic train wreck.

 

RE: Mixing tubes in an auto-bias tube amp, posted on July 10, 2020 at 06:55:14
Ric
Audiophile

Posts: 364
Joined: October 14, 2001
 When I had a tube problem in the past, I called my manufacturer at VAC and talked to the tech, asking him if it was ok to mix tubes. He was fine with that, as long as they're on the same side (two of the same on each side). My Avatar has 4 bias adjusters, so no problem, and yes, you can mix and match. I've been having poor luck with Sophia Electric EL34's, so now I have them on one side and some Valve Arts on the other.
Give it a try, you might like what you hear. I don't know anything about autobias, but seems like it should work regardless.

 

RE: Mixing tubes in an auto-bias tube amp, posted on July 10, 2020 at 08:42:55
6bq5
Audiophile

Posts: 4393
Location: SF Bay
Joined: August 16, 2001
 Ric,
You amp has individual pots for YOU to adjust the bias of each tube-
Very different from the OP question-

You also talked with the Manufacturer - great decision, and they are aware of the limits of their product-
Happy Listening

 

RE: Mixing tubes in an auto-bias tube amp, posted on July 10, 2020 at 09:43:42
Mick Wolfe
Audiophile

Posts: 3365
Location: AZ
Joined: October 10, 1999
Contributor
  Since: September 4, 2000
 Thx Ric. But as 6bq5 mentions, your biasing set-up is a bit more advanced in design. That said, it appears you are using only one tube type. EL34's I take it?

 

RE: Mixing tubes in an auto-bias tube amp, posted on July 11, 2020 at 06:52:03
Ric
Audiophile

Posts: 364
Joined: October 14, 2001
 Yes, just the EL34's. I did not talk to the tech about cross pollinating.

 

RE: Mixing tubes in an auto-bias tube amp, posted on July 17, 2020 at 07:34:17
SETdude
Audiophile

Posts: 3944
Joined: January 20, 2000
 Mick, I tried it in a Dennis Had Inspire amp. The amp can handle it but I preferred using a pair or quad of the same tube.

 

RE: Mixing tubes in an auto-bias tube amp, posted on July 17, 2020 at 08:20:39
Mick Wolfe
Audiophile

Posts: 3365
Location: AZ
Joined: October 10, 1999
Contributor
  Since: September 4, 2000
 Thx. Yep, that seems to be the consensus.

 

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