06-14-2016, 08:20 PM
(This post was last modified: 06-14-2016, 08:43 PM by Roman Ships.)
(05-16-2016, 01:45 AM)Hangman Wrote: So the R35 and R36 cannot be used to adjust the bias. Remember that the DC current going through the grid is very small so there will be only a small voltage drop across those resistors, if you want to adjust the bias you need to put something similar in place for R49.
So, you've boosted the values of the reservoir cap up to an effective 75uf. using two 150uf capacitors in series. I can see that now. I didn't see all of those caps you have over on the side of the amp by the power transformer.
its hard to say why your fuse is blowing, are you certain that the grids are seeing the bias voltage? double check power tube socket wiring. does the fuse pop right away? or does the amp warm up a little and then the fuse pops?
-Edit-
I should change my wording from R35 and R36 cannot be use, to should not be used simply to adjust the bias. it would be more effective to use the pot+resistor combo in place of R49. This way gives you a better sweep, and the load impedance for the phase inverter doesn't change with the setting of the bias.
Finally getting back to working on it. The fuse blowing was just a dumb error, don't recall exactly, I had something grounded.
I fixed the bias circuits to put the pot + res combos in for R49.
All the voltages look good.
No sound. I was getting a faint fizzy sound for a bit... Messed with the eq and then nothing.
Anything obvious pop out to you?
I can post pics again, but it will be hard to follow probably.
Also, R49 is now 56k + 50k trim.
In both bias circuits, at full resistance I was only getting -52VDC approx...
That is, about 100k... With the trim down it would go down to about -42 VDC...
That seems wrong, I should be able to easily hit the -64V
Update, sound is fine. I had been staring at it too long and had something hooked up wrong.
Stil a little puzzled by the bias voltage.
However, at about -52 V, I am running about 23 and 26 watts at the plates respectively.
Biased in pairs now.
Maybe I should swap out my 56k for a 75 k, my combo exceeds the original 75k for R49, I have tried to measure the effective resistance across the resistor + pot combo but it doesn't seem to read well.