Current draw, matching tubes and bias probe.

[tony777]

I bought a bias probe for the purpose of matching my tube collection, and today I had some time do go through it.
Though I am not a technician for amplifiers I work with electrics, but in the distribution, engine controlling area, so I am a bit familiar with power so to say.

Anyway.
I used my MarkIIC+ simul class which has plate voltage at 423V, and used a dual bias probe, so I can switch between the two measured tubes and it makes reading a bit easier. What surprised me was most was the difference between the current draw between Mesa labeled tubes. I have a collection of STR415's which differed from 23ma to 42ma, but most of them were around 35ish. A str420 was down to 25ma.
I have some TAD 6L6 and all of them were below 30ma. And so on. My Svetlana 6L6's were from 37 to 42.

But I found out that my Mark IV did not have matching tubes, they had a difference of 12ma, at the 6L6, and if that wasn't enough I have a pair of TAD EL34 which had a difference of 8ma. So I checked the TAD in my MarkIII, which had almost the same difference. Luckily I coud swap them so I got matching EL34's in both amps. The EL34 had a current draw from 37 to 44 in the class A socket.

So I continued with my EL34s
My STR416s had drew 56ma.
Winged =C= one had 20 and the other had 34....pretty far off huh?
I had 4 Sylvania 6ca7's which were faulty, one drew 13, and the other three continued rising until they were above 80, and then I chickened out and switched them off.
Even my barely used STR447 were faulty. One of them was shorting when I was turning the amp on from stanby.
So I don't have enough EL34's anymore.

So on to my question.
I put 6L6 in th class a socket. A svetlana pair of tubes draw 60ma. Is this value correct when a 6L6 is in the class A socket?

Well it has been a fun experiment, and after my MarkIV got matching pairs of STR415 and a matching set of EL34 TAD's sockets it really blew me away about how good it sounds.

Any comments or information is very welcome.

 

[stokes]

60ma's will have you in "class A" territory at about 450 plate volts,so,yes you are close.Close is as good as it gets with these amps in "class A",without opening a can of worms here,there is more to "true" class A than just a certain bias range.As for the Mesa tubes being so far off in tubes of the same grading,that is par for the course with these tubes.They are tested and graded to be "safe" in a particular amp,so you dont have to re-set the bias,but "safe" can be from 45ma's down to 15ma's.Its a useful system for somebody that doesnt want to bias their amp and just plug and play,but it doesnt give you ideal performance,just "safe",and usually quite cold,in Mesa amps.If you have tubes that draw 35ma's and replace them with what you think are the same "grade" tubes but the new ones only draw 22ma's your tone is going to change,not necessarilly for the better.It is for this reason that pretty much every Mesa amp I've had on my bench I've done an adjustable bias mod on.In every MKIV simul amp I've seen,the current draw on the EL34's was very low,like 8 to 12ma's.FYI,the adjustable bias mod is not as effective in simul class amps,there is still a lot of compromise you have to do between the two sets of tubes,so you cant really get them both to ideal settings.When matching tubes,I look for no closer than 5 to 7ma's,I think the slight mis-match sounds better in guitar amps.In hi-fi amps you are looking for a "pure sound reproduction",so perfectly matched and tighter tolerance in component values is preferred.http://www.ionpool.net/arcade/tech/Weber_Bias_Calculator.html this is a link to a bias calculator,very useful.

 

[tony777]

Thanks Stokes for your information, much appreciated.

 

[kippiejr]

Don't forget the tubes push pull off each other. So changing an outer will effect the inner. It takes several tubes to get a equal mA rating across. Not always good, not always bad. Use your ears then read the mA's. i like 39mA across in 60/100 and in simulclass models..