can you explain why the entrance of your cf driver has a different design ? (compared to the published #002 cf driver circuit). In your schematic the grid leak resistors are 470k ( 1M in #002) and you have 220k grid stoppers (#002 has no grid stoppers there). I do understand the 100k for the Rk. This one can be calculated by the voltage and maximum plate dissipation of the triode. The voltage divider for the bias is also clear to me.
Can I use the two 1M and no grid stoppers as well ?
I can't recall where I got the 220k value for the grid stoppers, and looking back through the earlier thread it's not mentioned there either. The value is not critical, and neither is the 470k grid leak. I believe that came from another SSS schematic. A small grid stopper is a good idea to prevent oscillation, so I would use something like 1k at least. Much larger values are fine since there is no Miller capacitance. It might be worth some experimentation to see if there is any tonal effect.
I built an amp with the 160PS transformers. 700v on the plates and 350 on the screens with a pair of KT88's. It is LOUD and clean. I don't think I've ever had the pre and master past 2.
The new circuit works fine: B+ for the plates is +198 VDC, grids of the CF have -50.3 VDC and Cathodes -48.6. Less stress for the tube with ~250V plate to cathode.
PT somehow delivered higher voltages than I expected (+/- 206 VDC without load). I had to adjust the values of the resistors in the bias string. Range is -40 to -77 VDC.
The rest of the amp is again "under construction" - so I had no possibility to hear, if there is a dfference to the old circuit.
Lars
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Here's another tweak to the alternate negative power supply: Moves the driver stage bias trimmer to the chassis for easier access, and adds a balance trimmer.
Lars, glad it worked out for you! The regulation on small transformers is generally pretty poor. I got about the same output voltage from a similar unit when it was lightly loaded.
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Here is an eyelet board that combines the 60V tap-powered bipolar bias supply and HV rectifier on the same footprint as the CE chassis/J Boarders PCB. This is pretty tight, so I think I'd stuff and solder it from the middle out.
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Last edited by martin manning on Mon Nov 29, 2021 12:12 pm, edited 1 time in total.
There must be something intense buried in this doc... It locks up Corel when I try to import it. Any way it could be exported as a different file type? I hate to lose the vector though.
"Education is what you're left with after you have forgotten what you have learned" - Enzo