[Amps] SB220 driving problem

[John Lyles]

Didn't see your name or call there so would be helpful to know that.  

The unmodified input circuit of SB220 has a rather poor Q which is great for bandwidth (untuned) but not very good for solid state exciter/rigs. Since the finals are driven into positive grid current, the wave shape of the grid current is non sinusoidal. You need to redesign that input, I believe Somis (Measures) has some info on this in one of his old QST articles. I did the change and it made a big difference in mine, using a SS rice box as an exciter. 

Secondly, yes, you don't need a full 100 watts of drive on all bands. You didn't say what band you were working on. Follow the Heathkit instructions about grid current and plate current ratio, and look at your output spectrum or a scope for distortion. You aren't helping matters (for the grid current) if it is overdriven. Fixing the input networks is probably your best bet. 

73
John 
K5PRO

> Date: Mon, 27 Oct 2008 18:12:58 -0400
> From: "pmihnovets" <pmihnovets at comporium.net>
> Subject: [Amps] SB-220 Exciter Foldback
> To: <amps at contesting.com>
> Message-ID: <200810272209.BES25715 at rg5.comporium.net>
> Content-Type: text/plain;	charset="us-ascii"
> 
>             I recently have been re-furbishing a SB-220.  It has the Harbach
> rectifier board mod and also the Somis parasitic mod. Grids are grounded
> directly to chasis with copper.  The problem I have been experiencing is
> that my Alinco DX 70 is folding back power after running full power output.
> ( approx.) 600 watts in the cw/tune position.  It also will fold back in SSB
> position.
> 
>  
> 
> I have tuned the input slug using a antenna analyzer.  I was coached in
> tuning the slug by grounding the cathode using two 100 ohm resistors and
> pressing the TR relay in manually.  I was able to get the slug tuned on 80
> at 1.5:1.  but still having the issue with the exciter fold-back. 
> 
>  
> 
> Is exciter driving the amp to hard with 100 watts??

> 
> Any help would be appreciated.