[R-390] Bypass capacitors, R390 and SP-600 [WAS: saga continues...]

[2002tii]

Roger wrote:

>Some 300 Volt items will work well. Move up one decimal point .01 to .1 and
>such. You just get some more by pass. Cost is not the issue any more. Size
>is not the issue any more. Exact part number is not an issue any more. You
>need a good cap that filter out more than it lets in or does not filter out.

Going up in value with film bypass caps is probably not a good idea, 
because it moves the self-resonant frequency of the caps down (and 
unfortunately, modern wound film caps do not have appreciably lower 
inductance than their forbears).  As we have discussed here before, 
however, you can get MUCH BETTER bypassing performance by switching 
from film caps to disk ceramics, owing to their much higher 
self-resonant frequency.  In this case, going up in value is OK (but 
definitely not necessary).  As I have said before, buy yourself a 
large bag of NEW .02 or .05 uF/1000 V disk ceramics and use them for 
all tube radio bypassing applications -- you will never be 
sorry.  Keep all capacitor leads as short as possible -- as Roger 
noted, feel free to use a different ground point to achieve 
this.  And use spaghetti tubing as necessary to insulate the leads.

Do NOT use ceramics for audio coupling applications -- for that, use 
film-and-foil types.  And I do mean film-and-foil.  Stay well away 
from all metallized film capacitors in high-voltage applications, 
regardless of their voltage rating.  Quoting one of my previous messages:

>Besides the performance differences (metallized film caps generally 
>have significantly greater ESR, as well as exhibiting greater 
>nonlinearity due to the higher field gradient across the thinner 
>dielectric), film-and-foil types are many, many times more reliable 
>than metallized film types.  Further, metallized film caps are 
>designed to be "self-healing" in the event of a dielectric 
>breakdown.  Sounds good, right?  Unfortunately, it's only good 
>marketing.  What this means is that if [rather, when] the dielectric 
>breaks down, the capacitor will arc until it vaporizes enough of the 
>metallization to clear the short.  That means whatever the cap was 
>intended to keep DC off of -- this could be your mechanical filter, 
>or the grid of the next stage -- has to to tolerate the "healing" 
>current while this vaporization happens.

Finally, I thought I saw a message recently about re-capping a 
Hammarlund SP-600, and the poster quoted a number of replaced caps 
that sounded very low to me.  My recollection was that there are 
nearly 60 paper bypass caps in an SP-600, and every single one MUST 
be replaced due to their 99-point-something percent failure 
rate.  Chuck Rippel's web site puts the number at 57.  So when you 
are replacing paper bypass caps in an SP-600, make sure you get them 
ALL.  Don't forget the ones in the IF cans, the RF module, or the 
turret, and look for the ones hiding under the wiring harness.  While 
you are inside the turret, clean and lube the contact pins with DeOxit.

Note that Hammarlund early on in SP-600 production adopted an 
engineering change that substituted disc ceramics for the tubular 
paper caps for the very reasons discussed above.  However, for some 
reason, they apparently went right on making the radios with the 
paper caps.  Trust me -- they work much better with disk ceramics.

One other modification you will want to make (if your SP-600 isn't 
new enough to have been built this way) is to move the feed for the 
IF output cathode follower buffer.  Look at the IF output jack on a 
scope with a strong signal tuned in.  If it is about one volt and 
nicely sinusoidal, you don't need to do anything.  If it is five 
volts or more and/or severely distorted (clipped negative) you will 
want to change it to reduce IF harmonic leakage into the RF.  The 
newer schematics show what to do (there were three versions of the IF 
cathode follower buffer circuit, to my knowledge -- older radios will 
not have the tapped inductor necessary to implement the newest 
version, so copy the middle version).

Best regards,

Don