[R-390] Caps

[Charles P. Steinmetz]

Dave wrote:

>Yeah, I'd like to see what others have used if not all ODs, 
>especially with the ten IF .1uF as ceramic disk, nearly all of them 
>are bypass caps.

Ceramic is best.  You can be a bit flexible with the 
values.  Ceramics are enough better at bypassing than the original 
paper caps that you can probably even use 0.01 disk ceramics and get 
performance better than the radios had with the original 0.1 uF paper 
caps.  I bought a lifetime supply of epoxy-coated, 1 kV, 0.02 uF X7R 
disk ceramics long ago (when they were less than $0.02 each in 
quantity) and use them for all bypass applications.  If I were 
replacing a 0.1 uF bypass cap, I'd use from 3 to 5 of the 0.02 uF 
disk ceramics in parallel.

>Only C531, audio coupling and  C538 (filament bypass, which could 
>even be a 50V cap) are not bypass caps.  It's not easy to find a 
>.1uF high voltage in even a ceramic at a decent price.

I'd use 600v ODs for all audio coupling and a 0.05-0.22 uF ceramic 
for all heater bypass caps (though I'd use a 200v rated part, not 50).

>Does anyone use a larger value for C549, the audio coupling

I know some do, but the audio of a 390A is so lo-fi that I don't 
believe it helps.  It extends the LF response to the point that you 
are pushing the poor audio transformers further and further into 
their LF intermodulation region if the source has any LF content 
(luckily, most don't, but in that case you don't need better LF 
performance, do you?).  Overall, not a benefit, IMO.  If you want 
better audio, take a line-level feed from the diode load terminal and 
run it through the electronics of your choice.  THAT's an improvement.

>I'm also going to replace the 5000pf bypass caps in one of my IF's 
>because something else is wrong and I can't find it.  I'm pretty 
>sure I'm going to use X7R for those.

I'd use the same 0.01 uF (= 10,000pF) or 0.02 uF (= 20,000pF) 
ceramics you use for bypass elsewhere.  The SRF of current disk 
ceramics is much higher than the original paper caps, and worries 
about the SRF were the only reason the designers used smaller caps 
there, so there is no reason to go down in capacitance if you use 
disk ceramics.

Best regards,

Charles