I went back to my notes and checked the cathode voltages on my 299B pre-amp tubes (V1 and V6), and all the tube voltages were well within tolerances. My cathode voltages were right around 0.6 VDC, compared to 0.5 VDC on the schematic. A note on the schematic: the 299B schematic in the AK database and generally available is an H H Sams reproduction. I have not been able to find an original Scott 299B schematic, but it's worthwhile noting that the Scott schematics for the 299A (with virtually the same pre-amp circuit and voltages) shows 0.8 V for the V1 and V6 cathodes. The Sams schematics are known for having errors present and this might be one. If that's the case, your cathode voltages may be spot on. In fact, the Scott schematic for the 222B with the same pre-amp circuit and voltages shows 1.0V on the cathodes.
So, I'm thinking you don't need to sweat the cathode voltages at about 0.8 VDC. I'm thinking this is not the cause of the noise you're hearing. Another thought: I always expect some tube rush in the phono stages of vintage integrated amps, particularly at 3/4 to full volume. It's not really meaningfully present at normal listening volumes in the amps I've worked on and listened to. Both of the 299B's I've done had Telefunken front end tubes--known for low noise.
From your posts, the noise appears to be worse than this. If that's the case, this is one of the best threads I found discussing tube noise when I was researching it a while back:
http://www.diyaudio.com/forums/tubes-valves/174246-reduced-heater-voltage-reduce-noise.html
It suggests that the main cause of hiss/rush comes from the 'shot effect' that can be magnified by excessive cathode heating or tube design. It seems to me that this boils down to a couple of things for you, since you've already done things suggested in the thread like replacing plate and grid resistors. First, try more tubes. The RCA's and Sovtek 12AX7 LPS tubes should be low noise, but you never know. Have you tried swapping your line amp 12AX7's with the pre-amp tubes? Maybe give that a try.
Another thing to try (as BinaryMike suggested) imight be to further lower the filament voltages for the 12AX7's served by your bias/filament supply by fiddling with the RC chain resistor just ahead of the filament take off. I think your last reading said you were about -48VDC here, so you might try taking it down to -46 or -45 VDC.
Brent, that's about all I've got on the noise front, but would love to hear other thoughts as well if I'm off base or am missing something.
Good luck with your efforts! I admire your persistence in trying to run this down.
Dave
So, I'm thinking you don't need to sweat the cathode voltages at about 0.8 VDC. I'm thinking this is not the cause of the noise you're hearing. Another thought: I always expect some tube rush in the phono stages of vintage integrated amps, particularly at 3/4 to full volume. It's not really meaningfully present at normal listening volumes in the amps I've worked on and listened to. Both of the 299B's I've done had Telefunken front end tubes--known for low noise.
From your posts, the noise appears to be worse than this. If that's the case, this is one of the best threads I found discussing tube noise when I was researching it a while back:
http://www.diyaudio.com/forums/tubes-valves/174246-reduced-heater-voltage-reduce-noise.html
It suggests that the main cause of hiss/rush comes from the 'shot effect' that can be magnified by excessive cathode heating or tube design. It seems to me that this boils down to a couple of things for you, since you've already done things suggested in the thread like replacing plate and grid resistors. First, try more tubes. The RCA's and Sovtek 12AX7 LPS tubes should be low noise, but you never know. Have you tried swapping your line amp 12AX7's with the pre-amp tubes? Maybe give that a try.
Another thing to try (as BinaryMike suggested) imight be to further lower the filament voltages for the 12AX7's served by your bias/filament supply by fiddling with the RC chain resistor just ahead of the filament take off. I think your last reading said you were about -48VDC here, so you might try taking it down to -46 or -45 VDC.
Brent, that's about all I've got on the noise front, but would love to hear other thoughts as well if I'm off base or am missing something.
Good luck with your efforts! I admire your persistence in trying to run this down.
Dave
