[R-390] C227 Bypass Question

[Cecil Acuff]

That's good stuff Roger.....

I learned a bit on that one....it goes in the file here for future 
reference...

Cecil...
----- Original Message ----- 
From: <Flowertime01 at wmconnect.com>
To: <r-390 at mailman.qth.net>
Sent: Saturday, August 19, 2006 5:29 PM
Subject: Re: [R-390] C227 Bypass Question


> 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
>
>
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