Acceptable range for a matched quad of tubes Question?

sjk · Jun 7, 2012

Question for the amp techs, people in the know here. Scenario...I bought a quad of sed winged C El34's and a quad of Sovtek Kt88's from Apex surplus on ebay recently after someone here posted the link in the classifieds. It was one of those incredidible deals almost too good to be true type of deals, but the guy that posted the link had bought before from them so what the heck. $65 plus like $11 shipping for the Kt88's and $50 plus $11 shipping for the winged C's. Killer deal. So I biassed up the Kt88's with my weber biasright in a Vht Ultralead and all 4 tubes matched up within 1ma. Pretty good I'm thinking. Now I go to bias up the El34 C's in the xtc 101b and this quad did not match up as close, 3 of the tubes were within 1 to 2 ma but the fourth tube was reading at least 4 ma higher than any of the others. So I'm wondering what is the acceptable range for a tube or 2 to be off? Previously I've thought nothing of a new set being 1 or 2 ma off but this was the first time for me having a new quad off this much, of course I have only been biasing my amps myself for the last year so my experience is limited and maybe this is common. Don't know just wondering and looking for the answers.

JerryP · Jun 8, 2012

You're fine, don't sweat 4ma.
Jerry

Buckeyedog · Jun 8, 2012

ah man, you are lucky to only be off by 4ma!! Really, these guys are right. No sweat at all with that. Plug 'em in and rock out.

sjk · Jun 8, 2012

Good to know! Thanks guys.

rockstah · Jun 8, 2012

Within 10ma is even fine and having them not so close sounds better to my ears anyway.
Bias to the hottest tube and yer good to go.

Mark

Capulin Overdrive · Jun 8, 2012

baron55":2dzhngx0 said:
Yep Jerry is right don't sweat 4ma.

If you could go into a time machine back in the late 60's and 70's and 80's and put bias probes on amps, most would be shocked how far off matching was and nobody cared and some legendary tones were created.

Now power tubes have to be matched within some range of course so you don't have issues. The closer the power tubes are matched, the more even order harmonics are canceled out (those are the good ones BTW) and odd harmonics and the more linear the tone becomes.

Matching to the 1 ma or better is an audiophile thing. Because in a stereo, you want no colorization of the sound.

In many guitar amps that have long tail phase inverters. one or one pair of the power tubes is always worked harder and it wears faster than the other side, one reason you will see tubes matched perfectly one day, and then a month later they are far off.

In my own personal amps, I will let the power tubes be off as far as 10ma, biasing set by the hottest one. Most in the industry will allow for 5ma difference before worrying too much.

In amps I ship, I make sure the tubes are matched very tightly, not because it is necessary, but because the guitar public thinks it is.

Also one thing to consider, tube are not linear in current draw. You could have a pair within 1ma at 400VDC and then in an amp at 500 VDC they could be pretty far off.

So when tube vendors match tubes, they are doing it at a specific plate voltage and bias setting, so having a different plate voltage and bias setting will effect the current draw.