musicbox said:
Basically the deal is, he will set up the tubes so that the 6L6's are really hot, and the el34's are really cold, or perhaps the reverse. That way they draw the same voltage from the trany, and the amp runs ok.
It's the other way around. The 6L6GC's that he supplies in the integrated quads are very cool 6L6GC's that draw very small amounts of plate current. The E34L's that he supplies draw are hotter and draw more plate current.
The actual bias voltage (negative screen grid voltage) is going to be the same. It's just a matching issue and that's what he has done.
If I remember what was said correctly, Roger was griping about both sets of those tubes being really far out of spec. If you blended that vibe in a power section, you could argue with Roger's point of view by saying that yes, there is less headroom with the 6L6GC's, but there is more headroom with the E34L's, which in a way would equal things out to a certain degree and give the power section a unique tone.
Roger was mainly ticked off that Bob was recommending JJ E34L's for amplifiers like the Peavey XXX at the time. The screen grid resistors in the Peavey XXX could handle the JJ EL34's, but they couldn't handle the JJ E34L. Once a screen grid resistor goes, the power tube will not bias up due to there being no negative grid voltage applied to the screen grid of that particular power tube and this can happen in every power tube socket. In some instances, when power tubes cause components on the amp to fail, this can cause fuses to blow.
With all that said, it kind of makes you wonder about some Mesa amplifiers. If you buy used, always ask the person you are buying from if they have had any power tubes fail on them while the amplifier was in operation. It's always good to have some sort of bias probe on hand as well. You can detect this very easily with a probe of some sort because, like I said earlier, a power tube will not bias up if the screen grid resistor has blown. You will still be able to see the plate voltage, but there will be no cathode current reading (bias current - mA reading).
I mention all of this due to this happening to me once before with a Bogner XTC 101B, which I still own. The guy had a power tube go bad and Bogner sent him a new set out to him. He put the new set in there and played the amp for about two more hours and then never really played it a whole lot again. When I got the amp from him, it appeared to be fine when I first fired it up. Then I pulled the chassis on the amp to see where it was biased up to because the previous owner mentioned to me that he didn't have a bias probe and that Bogner had supplied him the tubes. So, Ine tube checked the bias. One pair of tubes were drawing 50+ mA. As for the other pair, one was drawing 25 to 30 mA and the other wasn't drawing anything.
I had to replace the screen grid resistor and buy new power tubes. It's not a hard fix if you can solder and have a steady hand, but it can be a pain it's still a pain the backside. After all of that, the amp biased up very smoothly and it's drawing 32 mA across the board with a healthy plate voltage between 525 and 535.
Dale