Paramount 300B cathode bypass cap rating

[triode]

Hi,

I want to upgrade cathode bypass capacitor from electrolytic to film cap (maybe Solen)
What is the minimum voltage rating for cathode bypass capacitor for Paramount 300B (old version without soft start, 12AT7 driver)? Would 250VDC rating safe?
Also what is the minimum capacitance needed for "reasonable" bass performance? should not go lower than 47uF?
I'm asking the minimum value since there is not much room under the hood (the big Mundorf parafeed cap already taken a lot of space)

Thanks in advance

 

[Paul Birkeland]

If you stick to the 300B version, the bias voltage isn't going to go much higher than 75V.  You may wonder then why there's a 450V cap there.  Partially I believe that it is because we stocked that particular 47uF/450V cap, and also because we usually stocked the Panasonic ED electrolytic, which is a fantastic capacitor with good temperature tolerance and long life.  If your output tube had a catastrophic failure and shorted out, that particular cathode bypass cap wouldn't mind so much.

If you put a 160V rated cap in that position, you'd probably be safe, but you'd be less protected from catastrophic failure. 

It's also worth mentioning that the parafeed output transformer returns to the filament, so the sonic impact of that cap isn't a high as it would be in a regular old series feed amp.

I did manage to fit these in my Paramounts, but I didn't have other giant caps in there competing for space:
https://www.parts-express.com/jantzen-audio-47uf-400v-crosscap-capacitor--027-948
You should also be aware that the hot end of where that cap attaches is also available as a terminal on the output transformer (5 or 10, can't remember which), and you may find it easier to anchor the cap on top of the OPT and just solder some wires onto the ends of the cap and connect them where they need to go.