Bassman Plate Voltages

[Pryde]

Hi folks. Thought I would start a new thread as I am working through all the issues with this AA864 Bassman

Schematic:
http://vintagefenderamprepair.com/wp...-schematic.pdf

Layout:
http://vintagefenderamprepair.com/wp...864-layout.pdf


Got the green/black OT wiring completed to the speaker jacks and powered up to test bias and vP. On the first power tube (far left looking at back) I am getting plate voltage of 345 and 0 volts bias current. The other power tube I am getting 440 vP and -12 m/a bias current using a bias-rite probe (tested and it works fine with other amp).

I tried another set of good matched tubes with the same result so the problem lies in the first tube circuitry.
Inside the chassis I am getting 450vDC to ground on both pin 3 sockets.
-50vDC of the middle leg of bias pot
-36v on both pin 5 sockets

Not sure why vP and bias current readings are all out of whack with the bias probe between the 2 tubes? Help appreciated.

[g1]

Where exactly are you measuring the 345Vp ? In some part of the bias probe? If so swap them and see if the problem stays with that probe.

edit: I see you said the probe worked fine in another amp. Then it sounds like you may have a bad socket or loose socket contact.

[Pryde]

OK not sure why but there are a couple resistors to ground from pin 8 on each power tube. The lower vP pin reads 30v and the other one reads 0v (which it should be)

Should I just remove these resistors and tie pin 8 direct to ground per schematic or should I change the bad one that is allowing 30v to pin 8?

Jeez this amp

[g1]

What value are the resistors? Some of the SF amps had them, or someone may have installed them for checking bias.
If they are 1 ohm, I would suggest you replace the bad one. If they are something else, what are they?
For time being yes you can just tie the cathodes to ground.

[Pryde]

What value are the resistors? Some of the SF amps had them, or someone may have installed them for checking bias.
If they are 1 ohm, I would suggest you replace the bad one. If they are something else, what are they?
For time being yes you can just tie the cathodes to ground.

Thanks. They are 1 ohm 2w resistors. They appear to be added on. I went ahead and tied the cathodes to ground per the schematic and all is well with the voltages. Thanks for the help

[g1]

Then they are an add-on for measuring bias. With them in place, you can measure DC mV across them and it will tell you what the current is in mA.
A good question would be why the one burnt. Possibly a bad (or intermittently shorting) power tube.

[Pryde]

That is exactly what happened. a power tube shorted to ground via some wild solder job someone did at some point. all is well now. thanks again everyone for the assistance.