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Tube Bias (Read 910 times)
Archie
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Tube Bias
05/29/20 at 20:56:00
 
I have a bit of a mystery with my "Mystery Amp" that maybe another ZMA owner can shed light on.

I just put a quad of Gold Lion KT66s back in my Anniversary modded ZMA after maybe 5 years. These were lightly used. The last time I had them in was around the time I first got my ZMA. At that time I was biasing, as per Steve's recommendation, at 50 mA. Last night, these same tubes wouldn't go below 64 to 68 mA (left vs right). I ended up biasing at 70mA.

Can anyone account for this? Since they were last in, the amp has new meters and the full complement of anniversary mods.

My meters go to zero with the power off. I use a ZLC and even at the higher bias I don't think my transformers get too hot -- I can keep my hands on them for a long time.
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ZLC
Technics 1200G TT w/ Ortofon Jubilee MC cart
ZMC1
ZP3 (25th A Mods)
ZR2 (25th A Mods) -- currently not in use
CSP3 (25th A mods)
ZMA (25th A mods)
Homemade Big Betsy Speakers (F15s)
Silver Cabling
DIY Isolation platforms under amps & TT.
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GroovySauce
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Posts: 146
Re: Tube Bias
Reply #1 - 05/29/20 at 21:04:36
 
Im currently using my brothers ZMA 25th.

Gold lion kt66 wont bias below mid 60s.  Running them about 72-74 they sound great!

I recall reading that the bias range has been adjusted a bit with the ZMA running in the 70s is fine.

Sit back and enjoy the music.
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Rega P8 | SS Zephyr MIMC Star | Roon + HQplayer | MicroRendu | PSA DSD | ZP3 25th ZMC1 | ZRock2 25th | CSP3 25th | Torii MKiv 25th | Argon 7Ls | Iconoclast 1x4 TPC / UPOCC RCAs | KCC Hercules | Zenwave Speaker Cable | Curious Evolved USB .2m
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Archie
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Re: Tube Bias
Reply #2 - 05/29/20 at 21:09:13
 
It's possible my memory is faulty but back then we weren't biasing high.  Jeff sort of started that trend with his fore into KT88s and other tubes.

I am wondering if the Anniversary mods might have changed something.

But since your GL tubes bias high, I could be wrong.  Stone-of-Tone used the GLs as well.  Unfortunately, he hasn't posted here in a couple years.
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ZLC
Technics 1200G TT w/ Ortofon Jubilee MC cart
ZMC1
ZP3 (25th A Mods)
ZR2 (25th A Mods) -- currently not in use
CSP3 (25th A mods)
ZMA (25th A mods)
Homemade Big Betsy Speakers (F15s)
Silver Cabling
DIY Isolation platforms under amps & TT.
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GroovySauce
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Posts: 146
Re: Tube Bias
Reply #3 - 05/29/20 at 21:23:57
 
Very possible the 25th mods result in a higher bias window.

I just swapped back to Tung sol 7027a and they are biased right at 50mA and that is about as low as they will bias.
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Archie
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Posts: 2046
Re: Tube Bias
Reply #4 - 05/30/20 at 00:07:12
 
I did some searching of old posts and I found this in one of mine from December 2014:

"One of my stock Tung-Sol KT66 tubes was getting funky so I bought a matched quad of Gold Lion KT66s from Cryoset. I don't have the aural memory or the language to describe what I hear but I'd say they are an improvement over the Tung-Sols. One thing is that I can't bias them below 58+ ma."

So it seems my memory isn't what I thought it was and that the GLs always biased high. It is interesting though that the lower limit of bias is considerably higher post Anniversary mods.
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ZLC
Technics 1200G TT w/ Ortofon Jubilee MC cart
ZMC1
ZP3 (25th A Mods)
ZR2 (25th A Mods) -- currently not in use
CSP3 (25th A mods)
ZMA (25th A mods)
Homemade Big Betsy Speakers (F15s)
Silver Cabling
DIY Isolation platforms under amps & TT.
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GroovySauce
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Posts: 146
Re: Tube Bias
Reply #5 - 05/30/20 at 15:41:19
 
I put the GL kt66 in this morning. I prefer them over the 7027a.

I was wrong. They will bias around 54-56mA. I'm sure I was running them at 72-74mA for a few days though.

Right now I have the bias set at 60mA and they are sounding sweet.
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Archie
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Posts: 2046
Re: Tube Bias
Reply #6 - 05/30/20 at 17:28:12
 
And I pulled my GL KT66s out this morning! I prefer the Tung-Sol 7581As that I have in as well as the 7027As. I figured I'd give the KT66s another shot since it'd been so long since I'd heard them. The 7581As have a lower bias window than the GL KT66s by a lot. I tend to bias them on the high side at 64mA.
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ZLC
Technics 1200G TT w/ Ortofon Jubilee MC cart
ZMC1
ZP3 (25th A Mods)
ZR2 (25th A Mods) -- currently not in use
CSP3 (25th A mods)
ZMA (25th A mods)
Homemade Big Betsy Speakers (F15s)
Silver Cabling
DIY Isolation platforms under amps & TT.
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Steve Deckert
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If the 1st watt
sucks why continue?

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Re: Tube Bias
Reply #7 - 05/30/20 at 22:23:43
 

Couple notes on bias differences -  if the voltage at the AC outlet changes by more than a couple volts, it will change the bias of the amp slightly.  Also, if the amp got different meters when it was here, they may have been calibrated differently than the original meters.  My guess is if there were any differences between them the new ones are probably the more accurate.

Hope that helps.

Steve

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Archie
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Posts: 2046
Re: Tube Bias
Reply #8 - 05/30/20 at 23:07:57
 
Thanks Steve. Can you account for the high minimum bias of the Gold Lion KT66 as compared to, say, the Tung-Sol KT66? I'm sure the ones you originally sent with the amp biased below 50 mA. I was under the impression that the bias window of a tube had something to do with it's power output.

When setting bias, as far as amp health goes, is there any other consideration besides transformer heat?
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ZLC
Technics 1200G TT w/ Ortofon Jubilee MC cart
ZMC1
ZP3 (25th A Mods)
ZR2 (25th A Mods) -- currently not in use
CSP3 (25th A mods)
ZMA (25th A mods)
Homemade Big Betsy Speakers (F15s)
Silver Cabling
DIY Isolation platforms under amps & TT.
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NormD
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Posts: 206
Re: Tube Bias
Reply #9 - 06/14/20 at 22:42:24
 
My TAD KT-66's are biasing at 50ma.
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GroovySauce
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Posts: 146
Re: Tube Bias
Reply #10 - 10/08/20 at 16:06:23
 
Following up on the GL KT66's I haven't been able to bias them below 60ma for a long time now. I'm going to run them a little hotter now and see how they fair. They are starting to drift a little so they might be coming to the end of their lifespan.
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Rega P8 | SS Zephyr MIMC Star | Roon + HQplayer | MicroRendu | PSA DSD | ZP3 25th ZMC1 | ZRock2 25th | CSP3 25th | Torii MKiv 25th | Argon 7Ls | Iconoclast 1x4 TPC / UPOCC RCAs | KCC Hercules | Zenwave Speaker Cable | Curious Evolved USB .2m
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