[Amps] SB200 problem

[Tom W8JI]

> Wonder if anyone has run into this before:
> I've recently had trouble loading the FT1000D into the 
> SB200 on some bands.
> Even with the tuner in the 1000 on, there are times when 
> it doesn't load
> properly.  The tuner cycles and cycles and cycles some 
> more, but no luck.
> Without the tuner, there's very little output from the 
> 1000.

You should never use an auto-tuner in front of a grounded 
grid linear.

The input impedance of a GG amp changes with drive level, 
especially on higher bands where the amplifier is less 
isolated from input to output.

> To make things really interesting, when the 1000 is run 
> barefoot, but with
> connections through the amp, tuner on the 1000 off, and 
> into a dummy load,
> output of the 1000 is fine on 80-20.  On higher bands 
> there is a great drop
> in output.  On 10, output is about 100 watts (normal is 
> 200).

That's because the relay inthe SB200 is not compensated for 
SWR on the higher bands. Normally connecting a 27pF 
500-1000V capacitor from the shorting jumper on the NC 
contacts to ground will fix that.

> So I figured the problem is with the relay in the SB200 
> presenting a high
> SWR on the higher bands.  I changed the relay, but the 
> problem is still
> there.  WHEN THE RELAY IS BYPASSED RIGHT AT ITS TERMINALS, 
> OUTPUT GOES TO
> NORMAL, BUT WITH THE RELAY IS IN THE CIRCUIT, EVEN 
> UNENERGIZED, OUTPUT
> FALLS A LOT.

That's normal. It is because of the lead length in the path 
through relay. The inudtance is too high. You can change the 
relay 100 times with the same relay type and you simply 
reinstall the same problem.

> Have you ever seen this?  If so, what can be done?

All the time. You fix it by compensating the relay with a 
small mica cap.

73 Tom