Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: nateflanigan on October 21, 2013, 08:18:09 pm

Title: ?s about the output of AC568 bassman
Post by: nateflanigan on October 21, 2013, 08:18:09 pm

Hi, I'm doing a bit of work on a friends AC568 silverface bassman.  I converted the bias balance pot to the AB763 bias adjust pot.  But I was wondering about...

A) The two 2000pf caps going from pin 5 of each tube to ground
B) The two 150r resistors from the cathodes to ground

I figure I can ditch them since they're not in the BF schematics, however I'm curious to their intended purpose.

Thanks!

Title: Re: ?s about the output of AC568 bassman
Post by: HotBluePlates on October 21, 2013, 09:56:18 pm

Quote from: nateflanigan on October 21, 2013, 08:18:09 pm

A) The two 2000pf caps going from pin 5 of each tube to ground

It's not obvious unless you have a blackface and a silverface Bassman in front of you, but the layout of the silverface is different and has longer grid wires leading to the output tubes. The feedback circuit is also different, and I'm not sure but it could be delivering a greater amount of feedback. The caps to ground roll off highs where I presume Fender had at least occasional oscillation issues.

Additionally, it's a bass amp so great treble response was probably not seen as a need.

Quote from: nateflanigan on October 21, 2013, 08:18:09 pm

B) The two 150r resistors from the cathodes to ground

Notice the bias balance was added, but the bottom of one of the 100k bias feed resistors shows -45v where the AA864 showed -44v at a similar point in the circuit.

Adding the 150Ω resistors would increase the bias, and the AC568 would idle cooler as the new cathode bias adds to the essentially-same fixed bias. Idling deeper in AB means you have to apply a bigger input swing, but you also get a bigger plate output voltage/current swing.

I don't know if more power would be a result (I'd want to measure an example of each amp). If the AC568 feedback circuit applies more feedback, that might be a use for the bigger output voltage swing. The net result under that theoretical scenario is same-power, cleaner, with wider bandwidth (or tighter speaker damping for your bass amp).

Title: Re: ?s about the output of AC568 bassman
Post by: nateflanigan on October 22, 2013, 06:25:33 am

Great explanations, I figured the caps are for oscillation.  I'll try disconnecting those parts to see/hear if that changes anything.