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A place to discuss member's DIY audio projects & post pictures/schematics. NOTE: There is a limit of 2MB per attachment, and a maximum of 3 attachments per message. If you need to post more than 3 attachments, just add another message.
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Re: Bias Measurements 50 mA=500mV

January 28th, 2017, 8:09 pm

No. You set the bias without a signal -- but with the bias meter still plugged in you can hear the result immediately. Make the bias setting then put on a CD or LP and listen.

Don't like what you hear? Make a change -- listen again.

Re: Bias Measurements 50 mA=500mV

January 29th, 2017, 11:09 am

Walt real hit the nail by saying there is no hard and fast rules with any of this. You have to trust your ears and follow your own path.
I have a pp kt88 amp that one of the biasing suggestions was to set one tube of the pp pair at 15 to 20 ma and the other at 45 to 50 ma. I really liked what I heard but since this went counter to all I read I did not trut my ears and put them back to a matched state.
I have never heard or read anywhere about this method and would love to learn the reasoning behind this method

Re: Bias Measurements 50 mA=500mV

January 29th, 2017, 12:00 pm

That's a new one on me. I always went the full Monty with balance. Parts, tubes, left/right, etc. I've gone to the extreme of matching resistors and capacitors in a differential circuit to within 1/2% or better. Same with power tubes -- matched pairs and dialing in the bias to those matched pairs.

I no longer have push-pull anything to run trials with. Single-ended is my holy grail from the phono stage all the way through with the exception of the CD playback system that is filthy with op-amps except for the final drive that has transistor emitter followers with servo-loops to eliminate output coupling capacitors (Mark Levinson 36S DAC).

A word of advice. If you have a power amp. with bias pots buried inside the chassis it is a worth while project to replace those pots with top mounted pots that are easy to get to for dialing in the output tubes. The attached photos show what I did and it saves a lot of pain from upending an amp. -- removing the bottom cover -- and making changes on one's work bench. As a DIY nut job I was able to tube-roll to my heart's content -- no muss -- no fuss.
Music Angel with retrofitted bias pots.jpg
Music Angel with retrofitted bias pots B.jpg

Re: Bias Measurements 50 mA=500mV

January 30th, 2017, 11:30 am

For what it's worth, and that's not much, there is possibly another technique in bias/balancing that may be worthy of consideration.

Personally, I've always biased for as close to zero imbalance as possible. Mr. Berning has lectured on the need to keep that imbalance as close to zero as practical. He tells us that measured distortion increases rapidly with as little as 2 mA imbalance, as the core saturates. I take him at his word! He's spent an awful lot of time creating Class AB capable auto bias circuits just to remedy that ill.

But, Tubelab George had a very interesting post. I HAVE NO FIRSTHAND KNOWLEDGE, I just read it and am passing it along. Bias for as close to zero imbalance as possible. Then, using a dummy load and signal generator set to ~1kHz, listen to the transformer "sing". Adjust bias in one tube to minimize the singing. He claims that the result is improved sound. Kind of an AC balance trick I suppose.

Please don't flame me, if this is terrible, I'm just passing along something that sounded interesting and easy to try.


Re: Bias Measurements 50 mA=500mV

January 30th, 2017, 12:34 pm

I think Stuart nailed it as an unorthodox method of AC balancing. But changing the DC balance a tad is not the way to go.

If that manufacturer finds that AC balance is important (as many designers before him) then why not incorporate an AC balance pot in the design.

It certainly is not an expensive addition. The user manual can give instruction on how to do that with nothing more than a test CD (or LP).
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