Squeal when I check plate voltage.

[pinkphiloyd]

This started off as a 5f4 I built awhile ago. I've been reading on design and so I came up with my own preamp section and built it. It sounds great and everything seems to work fine. But for kicks I thought I'd check my voltages. Everything on 6l6 #1 is spot on. But when I check number two, I get a loud, horrible piercing squeal when I check the plate. The other pin readings are dead on with the other power tube. What's up with that?

[Cliff Schecht]

Your DMM and leads have a resistance, inductance and capacitance associated with them. All that is happening is you are introducing an unwanted parasitic into the amp that causes that section to oscillate when probed. No big deal unless the amp itself is unstable and oscillating (a scope will quickly reveal this).

[pinkphiloyd]

Well, sweet, I just ordered a scope last week! Why is it occurring on only one tube? Thanks, btw.

[Cliff Schecht]

Can you repeat the results with anything other than a DMM lead (i.e. a chopstick or something non-conductive)? Does the amp have negative feedback? It could be any number of factors really but nearly all of my amps do something similar when I touch the different high voltage nodes (especially around the plate resistors). It's not something worth worrying about unless the amp itself is unstable..

[pinkphiloyd]

Okay, cool. I'll poke around with my scope when it arrives. Which I'm hoping is today. They shipped it USPS.