[R-390] A few SP-600 questions

[James A. (Andy) Moorer]

I generally agree, but there is one mod suggested by Kleronomos that should
be avoided. He suggests putting in a current regulator in the filament
supply for the RF desk. The idea is to stabalize the power to the VFO tube
filament to improve frequency stability. That is a laudable goal, but doing
it with a current regulator to a bunch of tubes that are wired in parallel
is asking for trouble. If one of the tubes filaments goes open, the current
regulator will pump 4 tubes worth of filament current into the remaining 3
tubes. The voltage across the filaments will go up accordingly. Bad idea.
Kiss off four tubes just because one died.

If you want to regulate the filament of the VFO, you should put in a voltage
regulator - not a current regulator. Unless, of course, you also plan to
rewire the filaments so they are no longer in parallel.

The reason the ballast tube in the R-390A works is because the tubes it
regulates are in series - not in parallel.

James A. (Andy) Moorer
www.jamminpower.com

----- Original Message ----- 


>
> Since you are playing with an SP-600, read issues 20 21 22 of "Electric
> Radio" magazine. There are a number of improvement mods, and the mc1496
> balanced mixer product detector is a winner.
> 73...Steve..KJ8L
>