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In Reply to: RE: 20B posted by banpuku on January 18, 2017 at 16:27:59
1. The only issue with using the 6C6 triode strapped is that to get near full output, the preamp volume knob needs to be turned all the way up. With the preamp at full throttle, the output is about 90% of max output (just a guess, but close enough for the sake of this conversation). So, is there a way to get the 6C6 triode configuration to get more output from the power amp? If so, how?
If your preamp puts out 17V like you mentioned and your 6C6 is giving a gain of 15 (fair guess for a resistive loaded tube with a mu of 20) then something is seriously amiss if you can get anywhere near full volume on the linestage since 3V into the amp will run the 6C6 into A2 current and clip the output tube.
Your removal of the cathode bypass cap on the 2A3 may be what is "saving you" since the clipping behaviour looks a lot more polite without it in place. (even though the distortion is still upwards of 10%)
In any case, going from tube A with a µ of 20 to tube b with a similar µ isn't going to change your lack of gain.
dave
Follow Ups:
I took some measurements using a 1k sine wave input at 0.683VAC. FYI: instead of 100k ohms, there is a 220k ohm resistor at the RCA input (I have been playing with different input resistors, FWIW).
Here are the results:
Input signal: 0.683V
6C6
grid 145Vdc
plate 145Vdc (grid and plate are tied together for triode)
filament 4Vdc
2A3
grid 8.77Vac (using signal tone input)
plate 286Vdc
filament 2.6VAC
VDC P-K 237Vdc (target should be 250-265)
The 2A3 plate voltage is low at 286Vdc, typically I see it at 300Vdc.
The 6C6 grid voltage is 145Vdc, I typically see 175-180Vdc when in pentode mode.
I am unsure why the 2a3 plate voltage is so low and is the 6C6 grid 145V voltage OK since this is triode strapped?
Pat
see you upped the bias to 4V and the driver is giving you a gain of 12.8. This puts you at just over 50Vrms (70V peak) into the grid of a tube biased at 50V so everything looks to be in line and I don't think you will get much more out of this situation. (unbypassed 2A3 inot quad esl63's)
This doesn't explain why a preamp that puts out an alleged 17V needs to be turned up all the way since that should cause some pretty severe clipping to an input stage biased at 4V with a gain of 13
dave
9
Pin 4 and Pin 2 (plate) are tied together.
where are you measuring 4volts on the 6c6?
The filament supply is 4V. I am using 2 LEDs in series to produce the 4V.
The heater of the 6c6 should have 6.3 volts across it.
I assume the two LEDs are between the cathode and ground.
The 4 volts is your bias voltage (cathode) not you heater (filament) voltage.
Tre'
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