Discussion in 'Amps and Cabs' started by Dr. Tweedbucket, Dec 17, 2009.
Is it just spin the wheel and hope you get the bias close to optimal?
AFAIK your good as long as your cathode resistor is within spec -- just plug and play
I had an experience with this in my Champ. The short answer is, it depends on the tube. Some amps will redplate with one tube and be fine with another.
What happened was, I got a 6v6GT from Tube Depot with a bias point that was really high. I had a JJ 6V6S with a low bias point and it worked fine. With the GT, I speculate that because I ordered one tube, they gave me a tube that had an unusually high current draw, that they couldn't match to a set, so they gave it to me. I returned the tube and requested one with a bias point under 30, and when I plugged it in, it worked fine.
Yeah, I know you can order different grades of tubes, but even so it sounds like guess work because every amp has it's own bias resistor tolerance (probably 10% on newer amps).
I ordered a set of JJ 6V6s from Eurotubes and he asked what they were going in. They went in an Orange RV50 and seemed to be on the money because the amp sounded great after ward, still, it seems to be an inaccurate science.
Tube amps are built so sloppy, tolerance-wise, then the tubes are all over the place, too. It's always a crap shoot. That's why the adjustment is there.
@KGWagner: well the only adjustment you can make in a Cathode biased amp is to swap out the cathode resistor, and add or change the screen resistor.
In theory, the limiting factor should be the tube in a cathode biased amp. I was working closely with an amp tech over at OSG when I was working on my Champ, and the way he explained it is that the current that flows through the plate and screen is related, and in a Cathode biased amp, one balances the other.
It was months ago and I only barely had my head wrapped around it then. But when he explained it, he made it seem like because of the way cathode biased amps are wired, the power from the PT is balanced between the different parts of the power tube, and that within one type of tube, any tube SHOULD work.
So why doesn't it? Well, they don't make tubes like they used to. Because production is now overseas, the amount of current a tube draws can vary greatly among a certain tube type.
JJ and Eurotubes to the rescue. Those guys are very knowledgeable and will hook you up with the right tubes for your amp. And JJ's are great tubes with tight tolerances. With the exception of a 6v6 (just don't like the 6V6S), I always buy JJ. NOS tubes made in the US typically are of higher quality and can be trusted, too.
My main amps are cathode biased. Really not a big deal. I usually order from Doug's tubes, and tell him the GT rating they came with. Not an exact science, but not a problem.
This all depends on the amp.
With my Guytron GT-100 if it didn't have a way to adjust the bias on the EL84s I'd either mod it myself so it could or I'd have sold it because it makes that much of a difference to my ears. Maybe on other amps its less noticible.
The Guytron has a Hybrid bias on the EL84s meaning that it has a cathode resistor to get that type of response but also employs a fixed bias as well so you can dial in the bias setpoint.
@GT100, how does that work? Is it like a pot in series with the Cathode Resistor? I thought about employing something similar in my Champ but found the sweet spot with just one resistor swap.
On my newly acquired Rocker 30, This is something I am contemplating. I haven't plugged the amp in yet, so this is somewhat theoretical.
On my last amp, an Electric mvu120, all 4 el34's had adjustable bias. I could change the nature of the distortion with my adjustments on the bias.
Even if the rocker sounds good to me, the tweeker in me will wonder if I could make it sound better with a hotter/cooler bias point.