Short circuit in Peavey Valve King VK212

Electrical Engineering Asked by Piotr Tyburski on November 12, 2020

This is a follow up question from:
Guitar amplifier repair: burnt fuse, capacitor, what else?

I continue to work on the same case. In another module I’ve found a short to ground where the red dot is:
enter image description here

What I did so far:

  1. Removed and checked C212, C210, C213, C211, C216 -> caps seem fine, short still in there with all of them gone.
  2. Checked D208 -> is fine.
  3. If any of the power tubes is removed -> no short.
  4. As soon as all are in -> short in the red dot point.

Can the tubes be causing this short? How to test it? Optically they look fine.

I have to add that before assembling I have switched the amp on and the tubes all seemed to glow normally. It was simply complete silence on the output.

One Answer

Remove power tubes one by one until the short goes away. Replace the one that causes the short to go away. Repeat after replacing the dead tube as there could be more than one bad tube.

Alternatively, pull all the tubes out and measure the filament resistance. Look for values that are out of the ordinary - for a 12V tube, the resistance should be around 40 to 50 ohms. Any value extremely higher (or open) is bad, any that looks like a short circuit is bad.

The point you are looking at connects to a flag marked "HTR" via R213. "HTR" is probably the heater supply for the tubes. If a tube goes bad, then the heater filament could short to ground and cause your problems.

Answered by JRE on November 12, 2020

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