5f4 blowing fuses

[scole]

the amp comes on ok. i turn off the amp, when i hook my meter up to check plate voltage it blows the fuse as soon as i turn the amp back on. i took the power tubes out and i can check voltages without blowing the fuse. are the tubes shot? when the amp came in it was working but the tubes were way out of bias.

[Enzo]

You sure the meter is not set up for CURRENT instead of voltage? The voltmeter set for volts should act like it was a 10Meg resistor, which is to say it pretty much won't affect things.

[scole]

hi enzo, with my meter you have to move the negative lead to another hole for testing amps.

[Enzo]

I try never to assume anything.

I can't imagine. The tubes in the sockets shouldn't matter, unless in inserting them the pins move enough for them to touch something.

[R.G.]

I don't really know, but it might act that way if the amp started oscillating when you probed the plates. Put a scope on it to look for oscillation. Or try shorting the grid leak resistors to kill any possibility of oscillation.

[Garydean]

When the amp is cold are you able to probe and take readings. Are you saying when you do a warm restart, the fuse blows? Can you see arching inside the tubes? Have you a few good tubes to swap? Fuse correct size, slow or fast blow?

Gary

[Enzo]

I would think that even power oscillating for all it was worth, it wouldn't pop the fuse instantly.