Problem with the bias on a Bassman 100

[Mastervolume]

I have a bassman 100 on my bench right now and when I run a bias probe in one of the jacks I am reading 2ma.

I have 460ish on the plates. 320 on the out of the PI. -58 on the balance.

Screen resistors look ok on a meter.

I'm stumped.

Any ideas?

[John Phillips]

If the proper B+ is present at both the plate and screen-grid pins, it must be either a bad tube (although I assume you've checked that already) or a poor contact between one of the socket contacts and the tube - or a bad ground connection from the socket to the chassis. I would check that first, since it's made via the same screw that holds the socket in. If it's corroded or loose it might not pass much current.

I would also check again that the screen resistor is definitely OK - if they're the original carbon-comp ones these can crack and work intermittently... like when you're touching the lead wires with your meter probes . Best to replace them all with wirewounds for reliability anyway, if it hasn't already been done.

[Swarty]

The ground connection would be soldered to the chassis. Most likely a bum tube but could also be a bum solder joint on the socket or a crapped out socket. Per John's suggestion double check the screen resistor.

[John Phillips]

Originally Posted by Swarty

The ground connection would be soldered to the chassis.

Yes... you're right, they solder the tags down as well. I was thinking too much of Marshalls .

[Wakarusa]

Originally Posted by Mastervolume

-58 on the balance.

Where on the balance? (input? output?)
If you've got -58 on the actual output grids they'll be very close to cutoff.