[R-390] C227 Bypass Question

[Flowertime01 at wmconnect.com]

Dan,

The idea of the 47Pf cap in parallel with the C227, the .047 pf oil filled, 
chassis mounted bypass on the cathode of V201, rf amp tube has nothing to do 
with shoving the .047 up another 47Pf.

Think about today's three terminal voltage regulators. There you often see 
big filter caps and small caps beside them.

Its in the nature of caps. That big (; ? .047 just does not filter a lot of 
small pop corn noise. So the little 47Pf gets the little stuff while the big 
one provides filtering to stabilize the Cathode bias point on the RF amp. 

If you are doing a recap, leave that big oil job right on the deck. It looks 
good from the top side and fills the hole. Just detach the lead under the deck.

Place a new .047 under the deck. Place a good 47Pf under the deck also.
Neither of these two values are exact items. You need a big one and a small 
one because neither one will do every thing that needs to be done. They 
function as filters and act different. A .1 at 25 volts would work. If the tube 
shorts then a 300 volt cap would be in order. So a .1 at 300 volts to .033 would 
do. More is better. 

Some folks of certain mind set will argue for nothing lees than a NOS 
replacement.

Do go for a good 47Pf cap and not just a low cost ceramic disk cap. The 
function of that cap is to reduce some spurious noise in the amp. A better cap here 
is worth the trouble to hunt one down. Again the value is not an exact item. 
Some thing under a 100 RF and over 33Pf will work.


Would one of the modern, tubular, axial lead yellow poly caps be OK in this 
application? I seem to recall a general rule of thumb that suggests poly-type 
and 
Orange Drop-type caps as best for audio to IF and ceramic and mica for RF 
bypass 
applications?

Dan, again good caps are good caps. The problem is you do not want to be into 
the receiver every other month working on some problem. And you sure hate to 
be wondering if the problem is some part you just put in last year. It is 
worth the time to use better parts. This is the first RF gain stage and it sets 
the noise level for the entire receiver. Hear some of that hype form the gold 
plated audio world may be in order. Any thing that will help the noise in the RF 
amp would be in order.

So a 600 volt orange drop working at the cathode bias voltage of the RF tube 
would be in order. A couple good small quite PF caps that grab some of the 
hash noise would also be in order. Good installation with short leads would also 
help. Plenty of solder to provide a good conducting cross section would also 
be in order.

Roger AI4NI