Only one bias adjustment screw on RM50 head question.

[Balou]

So there's only one adjustment screw for the two tubes even tho there's two test points. How am I supposed to bias them individually? I have a RM100KH and there's four so Im confused. Ive got one at 37 and one at 33 Milivolts, is this going to be OK? (They are 3.8 matched)

 

[jmgman70]

You'll be fine.As long as the tubes are closely matched.

 

[kc2eeb]

3.5 is starting to be in the "mis-matched" zone. But consider this...
Up until recently, like this century, no amp company bothered to match
output tubes at all. I love to go back in time and measure all the Fender amps made in the '60's. The closest they matched was probably being made by the same company,

 

[Jaded Faith]

3.5 is starting to be in the "mis-matched" zone. But consider this...
Up until recently, like this century, no amp company bothered to match
output tubes at all. I love to go back in time and measure all the Fender amps made in the '60's. The closest they matched was probably being made by the same company,

Excellent point. The advice given is correct. The RM50 and RT2/50 use one pot to bias the pair, making closely matched tubes more critical for these amps. But again, see kc2eeb's post. I wouldn't sweat it.

 

[Balou]

K, thanks. Its funny cause I didnt even think about it when I got it and just been used to the KH. If theres anything else someone has to add please do.

 

[Julia]

A lot of stuff like "must have matched quad/duet" and "balanced" phase inverter became "a necessity" about the time Al Gore invented the Internet (Al Gore being ridiculed for saying that is the result of an Internet legend -- he really never did say that he invented it).

Prior to that time no one really paid much attention. From what my amp tech tells me the tubes will even out on their own in a push/pull circuit. He uses matched and balanced tubes because they are tested.

 

[electricevil]

I have a pair of Svetlana 6550 tubes in my rm50. they are 8mv out from each other. .035 and .043

I would guess to think this is pretty far out. am i risking anything by using tubes this far out?

blako

 

[JKD]

I have a pair of Svet 6550s that are the same...around 7mV out...as they heat up they drift a couple closer...I just live with it since it sounds OK.

 

[kc2eeb]

What you may want to try is, after you set the bias, leave the multimeter
connected and play the amp. You will see the current rise in proportion to the volume, typically a pair of 6L6's can draw over 100 ma (you read mv)
and so a 3-4ma difference at idle won't amount to much as you play.
These amps operate in class AB when properly biased. What that means is that one tube amplifies the positive going portion of the wave (conducts
on positive) and the other tube amplifies the negative going portion of the
wave (conducts on negative). i.e. Push-Pull. So when the tubes are mis-matched one will amplify (draw current) slightly more or less than the other. Can you actually hear it? Hear what?
Yeah, and if you under bias the tubes they will conduct past 180 degrees
and start acting like Class A.
Ever wonder why AC30s run so hot? Class A is conducting ALL the time!
Instead of fixed bias (one you adjust with a bias pot to a fixed idle current), the cathodes go through a cathode resistor bypassed by a cap to ground.
There is no negative bias applied to the control grid.