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In Reply to: RE: Cary SLI-80 tube bias posted by El Monte on March 02, 2016 at 21:23:00
It is VERY risky to set bias by ear! The bias current setting depends on the amp, not just the tube. For instance, 75 ma bias current and 400 plate volts gives you 30 watts of plate dissipation, that's well within spec. But 600 volts and 75 ma gives you over 45 watts which is quite close to spec and hard on the tubes and amp power supply.Tube bias settings are important to the tone, yes - but they are CRITICAL to the tube survival!!
Do check out the link below.
Edits: 03/06/16Follow Ups:
I found it very useful. By the way; do I remember you saying something like the Sli-80's design
wasn't particularly optimized for an el-34 power tube, or did I just imagine it?
You're close! What I said was I wasn't a big fan (or a fan at all) of the SLI-80 power supply. There is a lot that can be done to improve it and to make life much easier on the rectifier at the same time.
Can you put into layman's terms the deficiencies of the SLI-80 power supply?
Do the deficiencies manifest themselves as poor sound quality or is it wear and tear on the rectifiers?
I have had the same rectifiers in for probably 4000 hrs with no problems.
Do you service Cary amps?
Thanks
Marlin
I used to own a SLI80. When the Cary supplied current meter was connected it measured the bias current in both tubes of the measured channel. So, 80mA measured was the total for the output pair. No provision for balancing the two tubes. If I remember correctly, Cary recommended 70mA to 80mA total bias or 35mA - 40mA/tube. Fairly conservative for a class AB1 amp. Not crazy low like some of Bobby Carver's latest creations at 15mA/tube. :~D
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
I normally set the bias at 70mA, or 75mA with GL KT88 tubes. Someone told me to set the bias at 50 or 60 mA with vintage TungSol 6550 or GEC KT88 tubes, but TS 6550 proved better with bias set at 70mA/pair.
I remember the RFT EL-34s sounded good with bias set at 60mA/channel.
Not long ago I contacted Cary about bias for KT77 tubes and their recommendation was 50-60 ma. I believe my manual says bias for stock tubes, KT88 or 6550's, is 75-100 ma.
That is for more than one tube, yes?
"Little details" like that are critical. That's why people need to pay attention to the maker's bias recommendations. That's not to say you can't try different settings, but the "spec" is a very good place to start.
Yes Jim,
The Cary has a bias pot for each set of two tubes.
I try to keep the bias set at 75-85 ma on KT88, KT120, and 6550 tubes.
Haven't experimented with any higher or lower settings.
Edits: 03/04/16
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