Third set of Q4-7 is installed. I'm changing some other resistors right now. Current stays similar across R29 but the voltage seems to always be different by around 1V to ground.
I'll mark up a schematic with what I've been seeing. There seems to be a pattern here but I can't figure out the cause.
I don't remember how long I left the VAS test running the other night. I may try that one again and let it run for a bit. This difference in VAS current seems to get worse as things warm up. On initial start up they are very close.
My BipSumo boards have now become TubSumo boards and I think the old TubSumo boards are going to get driven over. On the new boards Vas current is slightly lower and DC offset still isn't great. My heater voltage is close to 7.5VAC. Is that too high? I'm using a 6.3VCT 4 Amp transformer. Self regulation is pretty poor obviously.
What is the voltage at the servo output - right at pin 6 of LF411?
When it can't balance out the offset, it normally goes to the limit (some 14V or so). Is this the case?
If this is the case, try to make R13 =1K, R19=22K
It will make servo loop more "influencial". Let's see if it improves the offset.
It will make servo loop more "influencial". Let's see if it improves the offset.
One board is pretty good. Offset is only -3mV. Pin 6 is +2.7V on it. The other board is hovering around -40 mV. Pin 6 is around +2.5 on it. It seems more like the servo is holding it at that offset. Both boards are fairly steady, only drifting a couple mV. Would this be a mismatch of R18 and R24?
I matched up a pair of resistors for R18 and R24. I guess you still need to be picky with 1% metal films sometimes. Offset is gone.
Replacement LatFets are in. Now I just need a chance to fix up some output boards and try it out.
Replacement LatFets are in. Now I just need a chance to fix up some output boards and try it out.
7.5VAC for heater is too high ... aim for 6.3VAC, as recommended by datasheet. Add series resistor to lower the voltage to around 6.3VAC.
Yes, over-heated cathode - not good for tube's life...
P.S. Under-heated - not good for performance 🙂
P.S. Under-heated - not good for performance 🙂
Are simple resistors good enough for lowering heater voltage or would it be better to rectify and regulate them?
Last edited: