Hi Ken,
> I've got a modified SB-220 that is unable to operate on 15 or 10m
> without popping the (added) 10 ohm plate HV resistor. It puts out
> 600-700 W on 80 - 20 without a problem.
Where is the resistor located exactly? It should be between the
supply line and the plate choke's cold end.
>I've replaced the grid bypass
> cap and RF chokes with very short 1/2" wide brass straps (3 per tube).
Did the resistor pop before you did that?
> Whenever I tune it up on 15 or 10m, the grid current is very touchy and
> will soar if I don't adjust the amp loading and xcvr drive. I can load
> up on 15m to about 225W @ 225 mA grid current at plate resonance.
Maybe you aren't loading the amp heavy enough. That will cause all
sorts of problems. The loading cap should be pretty far open on 15
and ten meters.
>The
> anode parasitic suppressors are stock Heath. The grid circuit has been
> modified, as mentioned, and also uses a pair of capacitors in the input
> circuit (one for each tube).
I don't understand the capacitors?
> Any ideas? Old tubes? Parasitics? Help?
VHF parasitics will show up on any band, if the plate capacitor is
rotated through its range. HF or near HF oscillations are dependent
on the bandswitch setting. If the problem is only on ten and 15, I'd
look towards the plate choke or one of another many causes.
Anyway, it would help to know where the resistor is at and what type
it is. Is the bypass cap at the bottom of the choke good? Is the
loading correct? Is the plate choke in good shape and original?
73, Tom W8JI
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|