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Bias meters mean...? (Read 476 times)
hifitubes
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Bias meters mean...?
07/06/13 at 02:33:45
 
I have a question about matched power tubes. I have some from Upsacle Audio, EL34 Mullard re-issue, that are perfectly matched on the bias meters at about 40mA.

My question is, my other pairs of Black Treasures and PSVANE Type II EL34 are about 4-5mA apart.

I have read this is not a big deal and we should try for within 10% plate....

So what do the bias meters actually reveal? If we had an amp that needed manual bias, wouldn't we try to match the mA setting? Are the meters showing what the tubes are actually biased at? Or what the amp's autio-bias "has to work with".

Or is it that the tubes are more mismatched than 4-5mA and amp when biasing lands on that particular value.

Hope I made my question clear. These other Chinese tubes aren't cheap and I know their performance can vary greatly from tube to tube.

thanks
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Lord Soth
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Re: Bias meters mean...?
Reply #1 - 07/06/13 at 13:20:57
 
I can't answer all your questions but I have some info which might be useful.

1. NEW (either NOS or new production) Tubes tend to drift. They need 24hrs of initial burn-in to have stable readings for the mA to settle and stabilize.
UPscale Audio does this necessary initial burn in before tube readings were made for tube matching, hence your tubes from them were perfectly matched.
Conversely, your other tube seller did not do this, hence the difference in mA reading.

2. Perfectly matched tubes are not really necessary.
A 20% mismatch was still within the acceptable range during the "Golden Era" of tube amps.
The other school of thought is that only a mA difference of at most 1 to 2 mA is the maximum allowable difference.
This is considered to be a modern concept.
In theory, only Push Pull amps require such closely matched tubes.
Single ended designs are more forgiving.

3. Yes, for Fixed Manual bias amps, mA readings are taken with a multimeter and adjustments are made.

Regards,
LS


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hifitubes
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Re: Bias meters mean...?
Reply #2 - 10/10/13 at 11:16:13
 
Sorry I missed your post.

So can I assume the meters show the readings of the tubes before the bias circuit?

Or the tubes are being biased, and the meters show the result, the amp doing it's best to get them both at the same operating point?
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marky
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Re: Bias meters mean...?
Reply #3 - 10/10/13 at 19:56:09
 
I think the meters show the actual tubes `as is/are`. My Rachaels had 4 identical tesla el34`s @ 38ma. One blew, and the 2 replacements I bought showed 37 and 40ma. And LS is right,  the replacements start off at 38ma and wander slightly, noticeable after the amps have been playing  for over an hour. Makes no difference when changing each pair over l/r amps....so it`s not the amps. So hopefully they`ll settle back to 38ma after more time. But it`s no prob, the auto bias sorts it all out and the sound is well balanced.
I like the meters.

ps. When the tube went wonky I witnessed it in real time by watching the meters. A bit hairy. The two meters would just take off slowly. One would go up to 60+ma and the other drop to 15ma. So it could be that I was seeing the bias trying to compensate....meaning the meters show the bias applied ... the current fed/drawn to them.....I don`t know.
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« Last Edit: 10/10/13 at 21:00:01 by marky »  
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hifitubes
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Re: Bias meters mean...?
Reply #4 - 10/11/13 at 06:44:59
 
I watched a bad tube do the same thing!

Runaway!

My Upscale tubes don't wander, probably due to their excessive warmup before testing. I know a friend with an Amplitrex who does the same thing.

cheers!
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