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[Amps] SB220 "A" mods

Subject: [Amps] SB220 "A" mods
From: "John Lyles" <>
Date: Fri, 28 Mar 2008 04:19:44 -0700
List-post: <">>
You won't get any concensus here on a single path to upgrade this venerable and 
easy-to-mod power amplifier. However, IMHO:
Highly recommended:

1) drill the small holes to be able to lube the fan every couple of years with 
light oil.

2) check your HV filter caps, and possibly replace if they have high life or 
fail this test. Unfortunately they can dry out even on the shelf, so when you 
fire up first time, at least you should re-form them individually with a 450 
VDC power supply and check the leakage. Check their capacitance too. Any 
leakage and they should all be replaced with a new set. Replace the carbon 
equalizing/bleeder resistances to a lower value, higher precision metal oxide 
power resistors. I believe this is covered in the QST/AG6K articles and 

3) Check the HV diodes for any overheating or cracking. Possibly replace with 
1N5408s from a new batch. If using RC equalizers, check the R's 

4) Change input matching to get better match across each band, for SS rigs to 
drive it.

Fun to do, but not mandatory for reliability:

1) Add a standby switch that cutsoff tubes with HV still on

2) Fast QSK, replace the open frame RF relay from the RF path with vacuum relay.

There are more mods such as replacing the phono plug on back, better line cord, 
MOV on AC mains, parasitic suppressors, but they are just if you have time to 
hack your amp more. 

I can send you schematic and performance tests for mine that I updated in 1993 
with many of these ideas. Certainly this list isn't exhaustive for all 

There seems to be some disagreement on different sites about the best
> mods to incorporate into this amplifier, specifically regarding
> self-oscillation of the 3-500z tubes.  As a HF amp n00b, I thought it
> best to ask some questions before I dove in.
> When you've all done rebuilds on this amp, what did you do and what
> could you personally recommend for it?  I will likely want to operate
> these amps when I am finished, so it's very important to me that this
> is done right.  Anything you could tell me would be appreciated.

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