[Amps] SB220 running plate current when no drive; Problem found

[Steve AA7V]

Hello to the group;
First, thanks to the many folks that took the time to answer my inquiry.
Your responses are greatly appreciated.
Second, I removed one of the tubes, powered on the amp and the behavior
seen before was gone. I found and installed a spare and now the amp is back
to normal ops.

The faulty tube is 6 years old. I dont know what the normal life-span is
for a 3-500z but I had hoped for a few more years.

73 to all
Steve AA7V



On Mon, Oct 5, 2020 at 11:03 AM Steve AA7V <aa7victor at gmail.com> wrote:

> Hello to the group;
>
> My SB220 has started to act up.
>
> Immediately upon power on, the 3-500s both start drawing plate current.
> The meter shows approximately 250 ma of current. It is apparent some
> component has failed.
>
> Also, it appears the grid current meter is showing a negative value (if
> this is possible) as the needle now rests on the left peg.
>
> I am not adverse to troubleshooting to the component level but would
> appreciate any advice as to how to narrow my search to a specific set of
> probable causes.
>
> If I am unable to fix this amp, does anyone have any recommendations
> towards a person that works on SB220s?
>
> Regards
>
> Steve AA7V
>