[Amps] SB-220 rebuild advice

[R L Measures]

On Jul 21, 2006, at 11:30 AM, Christopher J Galbraith wrote:

> Hi gang,
>
> I am getting ready to rebuild a school club's SB-220 amplifier and  
> could use
> some advice.  So far we plan to replace the power supply  
> capacitors, diode
> stack and metering circuit, add a soft-start and keying interface  
> (using
> essentially all of the Harbach SB-220-related products).  The  
> amplifier
> itself was a gift from an alumni who built it in the 70s, used it  
> for a few
> years, and placed it in a closet for the next roughly 30 years.  It  
> looks
> like it is brand new and has the original Eimac tubes.
>
> Questions:
>
> 1. Is there a procedure to out-gas, or otherwise prep these 'dormant'
> 3-500Zs for operation?  Is it reasonable to hope that they will  
> work after
> all of these years (they were good when put in storage).

There are several, expert-recommended de-gassing procedures, but my  
advice is to high-pot the tubes and measure the leakage @ 8kV before  
and after proceeding with one to see if there was any improvement.
>
> 2. Is replacement of other stock parts recommended for preventative
> maintenance?  In particular, I am wondering if the 5.1V/10W zener, the
> carbon comp. resistor in the parasitic suppressors, and some of the HV
> ceramic bypass caps are good candidates.

The Q of the stock suppressors is c. 4.5 @ 100MHz. It is possible to  
build suppressors with a Q of 1.5.  If you want the tubes to have  
more VHF amplification, go for the higher Q.
>
> 3. Is it advisable to directly ground the 3-500Z grids, instead of  
> the stock
> arrangement of grounding through a parallel RFC and capacitors?

Direct grounding moves the grid resonance down about a MHz from stock.
>
> end

cheers, Chris

R L MEASURES, AG6K. 805-386-3734
r at somis.org