Hoffman Ab763 Bandmaster build problem

[theholyweed]

Hey folks, Wonder if someone could give me any advice? Im working on a Fender Bandmaster build based on the Hoffman Ab763 Layout...Im having an issue with the bias, When the amp is taken off standby the bias is super low then as soon as a signal is fed in (or the bias pot is adjusted) it shoots up and stays pegged at 45ma with the bias pot not making any difference, however when the PI valve is removed the bias works properly....Im getting 280v on the plates of the PI (seems too high) and 78v on the cathodes...any help would be mucho appreciated!

[sluckey]

Disconnect the NFB wire from the speaker jack. Any better? If so, swap the OT plate leads.

[theholyweed]

No difference, I seem to be getting some kind of oscillation happening now? bizarrely the bias issue seems to have sorted itself out after changing valves (although they all checked fine on the tester) with the amp connected to 8ohm dummy load the bias is adjustable from 2ma upwards but when amp is plugged into 4ohm speaker it bottoms out at around 20ma... the amp does work although when turning volume up loud-ish and playing with hard dynamics the notes initially ring out fine but their volume is abruptly cut...the tremolo also appears to be working but is adding a "thumping" noise on each oscillation....Starting to pull my hair out over this build!

[sluckey]

8Ω or 4Ω load should make no difference in setting the bias. Bias should be set with no signal applied and tremolo switched off. What procedure are you using to set the bias?

Did you follow the Hoffman schematic and layout with no deviations? If not please post your schematic. And post some hi-rez gut shot pics. Maybe we can spot something.

[theholyweed]

I usually set the bias to 70% plate dissipation using my bias probe...The bias issue doesnt seem to be changing now regardless of load after switching on an off a few times...Used the hoffman ab763 2 valve build with no deviations apart from the component changes as listed in the BOM for the bandmaster circuit.....

[shooter]

using my bias probe

did you install 1ohm'rs on the power tube cathodes?  That's the easy bias test/set point.  IF NOT, I would recommend it highly! 

[theholyweed]

Aye there's 1 ohms on the cathodes to ground per the layout, I just use my bias probes as its easier...Iv'e worked out the oscillation is around 83Khz on the output...any ideas?

[shooter]

my bias probes as its easier

unless it's bad 

I wouldn't be concerned about oscillations til without pa tubes in you can get a good range on the grid pin of each tube, then for fun set it cold, put in tubes, use a good meter and measure VDC (=current in mA per Mr. ohm) across the 1 ohm and just watch awhile.  NO input