Hi! What is the highest acceptable value of 47pf capacitors in the base-collector connection? In my recycled stock I only found 120pf. Can I put 2 in series...60pf? It's difficult to buy this online because they only sell packages with 50...100 pieces, and these days I can't go to physical stores... 
I would try the series 60 pf caps. My similar ST120 PC14 board I used radio shack grab bag inmates. A tube of cardboard with a coil of wire on each end, red and brown dot. Might be 51 pf ??? Probably 65 year old parts. Never oscillated.
If you tried one 120 pf cap, it would probably just slow down slew rate a little.
If you tried one 120 pf cap, it would probably just slow down slew rate a little.
You may be able to find a suitable one from previous threads here -
https://www.diyaudio.com/community/threads/transistor-tone-control-schematic.177879/page-2
https://www.diyaudio.com/community/threads/active-discrete-baxandall-tone-control.356569/page-2
https://www.diyaudio.com/community/...ames-baxandalls-topology-wanted.195993/page-2
https://www.diyaudio.com/community/threads/transistor-tone-control-schematic.177879/page-2
https://www.diyaudio.com/community/threads/active-discrete-baxandall-tone-control.356569/page-2
https://www.diyaudio.com/community/...ames-baxandalls-topology-wanted.195993/page-2
Hi! In the previously mentioned CCE SA-130 amplifier, the power supply is 55 VDC. The circuit has a capacitance multiplier to power the preamplifier stage, which reduces the 55 VDC to 40 VDC. The AX6 that will replace the original amplifier will be powered with 67 VDC. Below is the original capacitance multiplier circuit. My question is what resistor values should I change so that with Vin of 67 VDC I have a Vout of 40 VDC?
Keep the current constant in R296. You see 14 volts of drop in 5600 ohms, giving 2.5 mA. You want to drop 26 volts with the same 2.5 mA - that’s 10.4k. Use a 10k and put a 390 in series. That way if you want to adjust it up or down you just change the smaller resistor.
Keep in mind that you’ll be raising the voltage on the BC337. It has to tolerate START UP conditions, where the caps are uncharged, putting the full power supply voltage on it. I’d put in something with a bit more voltage rating than that BC337. It’s the 16 rank, so it doesn’t need stupid high gain (up to 800 for the -40) so any of the usual VAS transistors in a TO-126 would be my choice. Also keep in mid pin out if you do swap them out.
Keep in mind that you’ll be raising the voltage on the BC337. It has to tolerate START UP conditions, where the caps are uncharged, putting the full power supply voltage on it. I’d put in something with a bit more voltage rating than that BC337. It’s the 16 rank, so it doesn’t need stupid high gain (up to 800 for the -40) so any of the usual VAS transistors in a TO-126 would be my choice. Also keep in mid pin out if you do swap them out.