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High-Voltage Entertainment in the Home To Do Pre-amp power supply The pre-amp power supply is just too complicated: dual-mono after the 6CJ3, two capacitance multipliers, four regulated high-voltage power supplies and three regulated heater supplies. But, there it is. The caps on the filter supplies ring. Periodically, the voltage seems to sag on the B+. I need to find the appropriate caps for the filament supplies; I think there is too much ripple current for the filter caps, which is why they ring. The main PS transformer may actually be too small, and I need to systematically go through all seven voltage regulators with the volt meter and the O-scope and make sure they're doing what they need to do, to make sure there is no power-supply induced noise in the pre-amp. Headphone amp The gain needs to be increased; the 12B4As should be replaced with 6EM7s. Phono Stage The
front end of the phono stage, currently parallelled 12AY7s,
is too noisy. I have tried different tridoes (e.g., 6C45pi), and
I believe that options like triode-wired EF86 or 7788 won't result in
further improvements. Possible solutions include adding a FET
cascode (i.e., under the cathode of the 12AY7s) or replacing the VA
stage with a pentode with feedback, or an OP-275. After so many
modifications, the grounding is kind of ugly; I have tried
every systematic grounding scheme I have read about, but I have yet to
try the trial-and-error de-bugging stage all such schemes always seem
to end up with. Maybe the whole thing should be rebuilt using
terminal strips on one of the 6"x12"x1/8" aluminum subchassis plates,
like everything else, which might make rationalizing the grounding
easier. Plus, if I could squeeze the headphone amp onto the line
amp chassis, the phono stage might fit in the pre-amp box.
Amplifier Right now, the power amp is sufficient for loud jazz, but wimps out at appropriate rock & roll volumes (i.e., loud enough to make the neighbors either complain or appear at the front door with beer). I need to go through the stages and find out which stage is not making the cut. The 6SN7 should be producing 240V P-P, but, since the power stage is biased at 100V, I have probably reached the limit of what can be done with 320VDC B+, but I need to check if this in fact the case. Amplifier Power Supply Needs to be optimizied using PSUD2, and maybe a nice pair of new power transformers, since the current power transformers are just a little too small, causing the B+ to sag. SV 572 Push-Pull What I really need to do is finish the push-pull amp built with SV572-10s. Horns And then, we need some nice horns... |