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TUBE BIAS, socket to me!

BIAS: (I'm starting from zero understanding) 

I have never measured/adjusted bias in the 3 tube amps, 3 tube receivers, and 2 tube preamps I have acquired over 47 years. I just switched my current Cayin from 6550's to KT88's. Adjust bias? Adjusters inside, scary electrocution warnings. I could pay someone else to do it, i.e. Steve at VAS 1 hr away in NJ, soooo, 

What really counts? (personally I don't care about either heat or life, but would like to understand)

Heat?
Life?
Output stays Matched when adjusted?
Acoustic Performance?
_________?

Over the years, fronts off, bottoms off, I hose em down with contact cleaner/lubricant, compressed air, all controls and switchers, any adjusters, swish full spin back and forth. Kill any spiders, look for, replace the rare burnt resistor. 
Then leave any adjusters (whatever they are) in the middle position, button it back up.

Two tube testers, my big hickock always agrees with small portable one, test strength, shorts, matched strength old and newly purchased. Large collection of NOS, used. Often used test essentially same strength as new ones.

When they go, it's usually a short.
elliottbnewcombjr
oldhvymec
5,101 posts
 
https://www.pinoydvd.com/index.php?topic=139769.0

OP check this thread out pretty good, pics in all...

Regards
lewm
13,228 posts
 
As tubes age, they lose transconductance.  As they lose transconductance if the grid bias voltage is fixed (not varying with respect to cathode voltage), you gradually run out of oomph (for want of a better term).  The amplifier may begin to sound a bit limp.  But this happens so slowly that the user is often only dimly aware that the sonics are not quite as wonderful as they once were, with fresh tubes.  And eventually, one buys new power tubes.  In this way, I once got 10 years out of a set of power tubes in a Futterman OTL.

However, as the owner of an amplifier with adjustable bias and a built-in method for measuring and correcting bias current when it inevitably goes off, by adjusting grid voltage, I have to say it is a revelation to hear the amplifier restored to normal bias current after a periodic bias adjustment.  By "periodic" I mean once a year or so. If you can adjust bias safely and know what you are doing it for, I highly recommend monitoring it and keeping it up to snuff.  Also, owners of amps with some sort of autobias circuit need not be concerned.
I have a Hickok 533. I use it only to test for transconductance of small signal triodes.  I do not use it to evaluate power tubes because it does not have the capacity to subject such tubes to plate voltage and plate current parameters where such tubes need to operate.
tyray
674 posts
 
Holy Holidays!

McIntosh tube tuner/preamp MX110Z (1962-1969) / KT88’s

Cayin is A88TC version 1, which has 16 ohm taps for my 16 ohm speakers.

JSE Infinite Slope Model II speakers

@elliottbnewcombjr,

I had no idea...In my ignorance of this thing audio (hifi), I always wondered why some like everything tubes? Period. Now I understand somewhat why.

Could you explain the quality of reproduction of music you get when played on your turntable through the MX110Z preamp with the Cayin is A88TC version 1 and those JSE’s?

I should have bought that Fisher 800C when I had the chance years ago.

Is there any Fisher/Mc and Cayin you can recommend me? Thanks All, for the bias education too.



elliottbnewcombjr OP
5,562 posts
 
jjss49

Thanks for the info and link.

In my shop, I’ve got some nice chopsticks, one set hardwood, another set plastic, if I can find them.

And a grinder, and lots of other sharp weapons of destruction.

I know where to make the connections, evicently going for .4 or a bit less.

Multi-meter on what setting????
elliottbnewcombjr OP
5,562 posts
 
oldhvymec

I'm betting it changes nothing, but, I'm gonna do it, just to find out.

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