[AMRadio] Caps

Jim Candela jcandela at prodigy.net
Sat Dec 30 11:47:47 EST 2006

    I am guessing that you will be fine going ahead with ceramics since most of the SP-600 circuitry won't care. That said, there are differences between capacitor types, and within the ceramic family you have Z5U, Z5V, X7R, COG (NPO), and maybe some others. I listed those from worst to best. It is hard to find the COG (NPO) types in higher values above about .001 uf, and that is where X7R is the next best choice. 
   So what are the differences? In short it is temperature stability, and voltage stability along with the ESR. With high impedance circuits where the bypass capacitor is .01 uf and greater, it might not make a whole lot of difference which type you choose. Then again we see a big difference if the impedance is lowered where we then need to bypass some RF current. Here is a case in point:
On the Central Electronics 20A the mixer L-C tank is high Q, and is bypassed with a .005 silver mica on the B+ end. To lower the Q, a 10K resistor is across the coil (L). On later 20A's, the bypass is .005 ceramic Z5V, and for the same L-C tank Q, the parallel tank resistor had to be increased to 22K. If you take the newer version with the 22K, and replace the ceramic .005 uf with a silver mica, the Q increases to the point that instability occurs.
I played a lot with that circuit on one of my QRO 20A projects, and tried every type of capacitor I had to see how they behaved where ESR was important due to the high Q of the tank circuit. Interestingly polypropylene had the lowest ESR, and therefore the stage had the highest gain. The difference was easily 6 db between the ceramic and the poly with the mica in-between. Mouser and others carry polypropylene and they are quite inexpensive. I also had RF floating on the unshielded B+ wire to the VFO, and a polypropylene capacitor knocked that down better than any other type I had. It is interesting to bridge existing bypass capacitors to see the effect on stage gain, or circuit Q. Sometimes there is gain stage regeneration from another stage where the B+ is not well filtered. This might be normal and then somebody arbitrarily starts changing the bypass capacitors. The circuit behavior might change, and maybe for the worse. Be careful!
Back to the SP-600, you should expect to need a re-alignment after re-capping, and you might re-cap one stage at a time to see if anything bad happened. I got my SP-600 from Ozona Bob, W5PYT. He re-capped this one with Sprague Orange-drops all except two above the RF Turret. Those failed later, and I realized why he skipped them over. Quite a job to get at those dudes. There are several types of Orange Drops, and I believe that some are polypropylene.

----- Original Message ----
From: Rick Brashear <rickbras at airmail.net>
To: AMRadio at mailman.qth.net
Sent: Saturday, December 30, 2006 9:57:24 AM
Subject: [AMRadio] Caps

Thanks to everyone for your suggestions and advice on the paper versus 
ceramic capacitors in the SP-600 rf strip.  It seems the majority advise 
the use of ceramic and I'll most likely go that route.  I also agree 
with the thought that anything I put in there will be better than what I 
have.  :-)

Thanks guys! 

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