testing zener diode question

[tonyp]

Hello, Can a zener diode be tested while in the circuit? It's a Sunn Model A amp and the zener is supposed to have -22V . I measure -22V as soon as I put the meter probe to it but it drops down to-15V pretty fast. It's across a 50uf cap.  Trying to figure out why one 6550 tube is red plating after about 5 minutes. Thanks

[shooter]

short answer No, lift one leg of the cap and re-check the -22

[tonyp]

Thank you for the info!

[SEL49]

If that circuit was behaving properly, there would be 22V across that zener all day, every day. There are three things that could cause the voltage across the zener to drop to 15v...

1. Bad zener.
2. Something on that -22v buss is drawing too much current causing the voltage to drop below the zener threshold.
3. The voltage coming into the zener is dropping below the zener threshold.

It should be pretty easy to determine what is causing the voltage to drop.

[Latole]

It help if we see schematic.  Here :  https://el34world.com/charts/Schematics/files/Sunn/Sunn_model_a.pdf

One tube is red plating ? It may be worn out . Bias circuit / Zener feed both tubes. If zener or bias circuit is defective chance are both tubes redplating.

Reverse both tubes.
If the same is red plating , it is the tube the issue

Or

1-Remove all power tubes
2-Did you read - 55 volts at 6550 tubes grid pins ?

A defective tube (s) may drop voltage.

[tonyp]

Thank you all for the help. More than I expected!! I did measure -55 at the grid pins.  I'll have to reverse the tubes and check that out.

[Latole]

Thank you all for the help. More than I expected!! I did measure -55 at the grid pins.  I'll have to reverse the tubes and check that out.

IMO one tube is short.
Replace both by matched pair