[Amps] SB220 running plate current when no drive

[Donald Fox]

 Hi Steve:
I am pretty sure you are the proud owner of at least one tube with a grid to cathode short. Since the filaments are wired in parallel, you can remove (carefully) one tube at a time, and see which one is causing the issue. You don't want to operate it this way for long though, as besides the obvious plate current issue, the filament xformer will burn up in short order. 

Unplug the AC cord before you open it up, and remember to (safely) short one of the tube anode connectors to ground before touching anything inside the tube compartment.

You can make the job easier by placing a non conductive item between the HV shorting bar and ground, which will allow you to fire it up with the top open. Be VERY careful during this process though, and make darn sure the HV has bled away before touching anything inside again When you find the tube that has no plate current in stand by, that's the good one. 
If needed, you can run it on one tube, but of course the plate/load settings will be different, and output will be less. Also, be sure that the missing tube's plate connector won't short out to anything when you fire it back up on one bottle.

Before you button it back up, don't forget to remove the item you installed to disable the HV shorting bar.

Don N8ECH
     On Monday, October 5, 2020, 02:03:49 PM EDT, 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
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