MikeS
09-01-2007, 01:15 PM
I picked up some 6L6's from a PS and a organ amp. Found some limited data for 6L6 for push-pull and single-ended.
I use a 12AX7 for driver for both sides. The plates and screens of 6L6 are at 250 vdc, with 200 ohm cathode R's bypassed with 220 uf 35 volt caps.
The data shows 2500 to 4500 load resistance for 6L6 in class A1. I use the 3500 ohm primary taps of a pair of James 6123 universal OPT's.
The 6123's have a screen tap of (43%?).
I've used the screen taps with 6V6, which in this circuit can be swapped out with just a change of primary load R and cathode R values.
I haven't used the screen taps with 6L6 yet.....
I come out at 140 milliamps for both 6L6 and 12AX7 as indicated on the supply at 250 volts. 12AX7 uses 220K dropping R's. Cathode bias is at around 1 volt for 12AX7, about 12.5-13 volts on the 6L6.
Roughly 5 to 6 watts per side of 6L6 can boogie. Without feedback is overkill. I'm using a loop from the 8 ohm secondary to the 12AX7 driver cathode. A 6800 ohm reisistor, with a 150 picofarad compensation cap.
This was kinda determined by ear, which is by no means optimum. The 6800 ohm R makes the amp less noisy, unglued sounding, and less sensitive. Which I suppose can be done by ear by beating down the gain, but then again....
The compensation capacitor requires a scope, the whole process of the feedback loop does.....
I need to cobble up some dummy load R's.
The circuit with 6L6 can boogie. Lower bass is rather strong, lower mid to midrange is good, but the upper mid and top end is kinda annoying.
It's probably some sort of ringing resonance or overshoot in the OPT's, which can be compensated for with a square wave and a scope.
So I guess I better do it.......
I use a 12AX7 for driver for both sides. The plates and screens of 6L6 are at 250 vdc, with 200 ohm cathode R's bypassed with 220 uf 35 volt caps.
The data shows 2500 to 4500 load resistance for 6L6 in class A1. I use the 3500 ohm primary taps of a pair of James 6123 universal OPT's.
The 6123's have a screen tap of (43%?).
I've used the screen taps with 6V6, which in this circuit can be swapped out with just a change of primary load R and cathode R values.
I haven't used the screen taps with 6L6 yet.....
I come out at 140 milliamps for both 6L6 and 12AX7 as indicated on the supply at 250 volts. 12AX7 uses 220K dropping R's. Cathode bias is at around 1 volt for 12AX7, about 12.5-13 volts on the 6L6.
Roughly 5 to 6 watts per side of 6L6 can boogie. Without feedback is overkill. I'm using a loop from the 8 ohm secondary to the 12AX7 driver cathode. A 6800 ohm reisistor, with a 150 picofarad compensation cap.
This was kinda determined by ear, which is by no means optimum. The 6800 ohm R makes the amp less noisy, unglued sounding, and less sensitive. Which I suppose can be done by ear by beating down the gain, but then again....
The compensation capacitor requires a scope, the whole process of the feedback loop does.....
I need to cobble up some dummy load R's.
The circuit with 6L6 can boogie. Lower bass is rather strong, lower mid to midrange is good, but the upper mid and top end is kinda annoying.
It's probably some sort of ringing resonance or overshoot in the OPT's, which can be compensated for with a square wave and a scope.
So I guess I better do it.......