catstrat 6/13/2018 4:45 PM
D.C. Tube heater circuit.
This is a general question. I have an amp where V 1 and V2 are heated up by D.C. Voltage. It has 4 rectifier diodes and 3 filter caps that are 6000 micro fared At 10 volts. How critical is it for the replacement filter caps to be exactly 6000 micro fareds. Or can I vary the value? I know it can be a higher voltage but not sure of the value.Well, I tried to retire but too many people need amp repair. Thanks.
Enzo 6/13/2018 5:18 PM
It absolutely does not matter the value. First off it is just a guitar amp, so not much of anything has to be exactly something. But aside from that, it is just DC for heaters, nothing in teh signal path. 6000 seems an odd value to me. 6200 or 6800 or 4700 would be standard values. Surely 4700 would be fine and I'd have reached for 2200 myself if I had no starting idea.

If V1,2 are 12AX7s wired for 6v, then we have 600ma current draw. Not only that, it won't have a varying current draw. SO all we need is a relatively smooth DC good for an amp or so.
catstrat 6/13/2018 5:51 PM
Thanks Enzo. Being an odd value Enzo, well,it is a Mesa Boogie.
catstrat 6/13/2018 8:40 PM
I was just thinking about the purpose of filter caps in a D.C. Circuit to be used to heat a tube. Most tube amps use a.c. Heated tubes anyway. Even if there is a.c. Ripple in the d.c. Heater circuit would it really matter? Even some Mesa amps use d.c. Heaters and those circuits don't have filter caps. I just don't see the wisdom. Maybe I mis understand the reason.
Enzo 6/13/2018 9:19 PM
Well, don't assume everything has a real reason. A lot of things get done for the reason no one ever questioned it.

As I see it here, if you rectify the AC but don't filter it, instead of pure AC on the heater, you now have a similar sized pulsing DC signal. SO while we want DC ther, it would still be alternating off and on at 120Hz. I suppose ther is still potential to couple to the cathode.

What's a cap cost, a buck?

Frankly I'd be more inclined to leave them all AC, and elevate the string to some DC voltage.