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previous: Craig Layout & Oscillation -- 9/24/1997 1:02 PM

...I almost forgot...

9/24/1997 2:40 PM
Craig	...I almost forgot... I almost forgot to mention the main trouble I'm having with this amp; low volume. I was having some pretty severe oscillation problems, but shielding seems to have taken care of most of that. But, I am still having trouble getting the amp to put out volume beyond a whisper. What happens is this: From 0 on both pre and master volumes... - bringing up the preamp all the way, then bringing up the master results in no sound at all - bringing up the master all the way, then bringing up the pre-gain results in low volume, but you hear the signal. As the pre-gain is brought up, the distortion increases, which seems natural enough, but at around 6-7, the volume gets squelched. At the moment, the plate voltages are as follows: 5U4G 675v AC 6L6GC 260v DC (seems pretty low, huh?) 12AX7 170/119v DC (inverter) 12AX7 130/116v DC (2nd preamp) 12AX7 113/70v DC (70v ?) (1st preamp) I verified that the neg. feedback is passed back through the 82K side of the inverter circuit (self-balancing paraphrase inverter). The other question I have is regarding the low DC voltages being produced by the rectifier. I rigged up a tube socket mounting bracket that I salvaged somewhere (original amp had diode rect). What may or may not be a problem is that due to the placement of the socket mounting screws on the bracket, the tube won't seat all the way into the socket - it sits up about 3/64 in. above the bracket surface. Could this be enough of a gap to affect the performance of the tube? The connections appear to be being made, because it's basically doing it's job; but I wonder if it's doing it WELL?

9/24/1997 2:40 PM

Craig

...I almost forgot...

I almost forgot to mention the main trouble I'm having with this amp; low volume. I was having some pretty severe oscillation problems, but shielding seems to have taken care of most of that. But, I am still having trouble getting the amp to put out volume beyond a whisper. What happens is this:

From 0 on both pre and master volumes...

- bringing up the preamp all the way, then bringing up the master results in no sound at all
- bringing up the master all the way, then bringing up the pre-gain results in low volume, but you hear the signal. As the pre-gain is brought up, the distortion increases, which seems natural enough, but at around 6-7, the volume gets squelched.

At the moment, the plate voltages are as follows:

5U4G 675v AC
6L6GC 260v DC (seems pretty low, huh?)
12AX7 170/119v DC (inverter)
12AX7 130/116v DC (2nd preamp)
12AX7 113/70v DC (70v ?) (1st preamp)

I verified that the neg. feedback is passed back through the 82K side of the inverter circuit (self-balancing paraphrase inverter).

The other question I have is regarding the low DC voltages being produced by the rectifier. I rigged up a tube socket mounting bracket that I salvaged somewhere (original amp had diode rect). What may or may not be a problem is that due to the placement of the socket mounting screws on the bracket, the tube won't seat all the way into the socket - it sits up about 3/64 in. above the bracket surface. Could this be enough of a gap to affect the performance of the tube? The connections appear to be being made, because it's basically doing it's job; but I wonder if it's doing it WELL?

Replies:

Gary You might want to put a scope on th... -- 9/24/1997 8:12 PM
gary Those voltages are low WRT the 675 ... -- 9/24/1997 8:43 PM
Bruce C The fact that you get no sound when... -- 9/25/1997 2:29 PM