Bias Wiggle Term on A Bassman 50?

[boyer]

Hi gang!

I've got a frankenbassman. It's power/PI/Bias sections are all AA864, and the preamps are a custom cooked solution. The former Bass channel is used as a distortion channel, using 3 gain stages, and the Normal channel is used as a clean, using 2 stages. This leaves me with one stage left.

What I would like to do, is incorporate the power tube bias wiggle trem scheme found in amps like the Traynor Guitarmate (YGM-3/YGM-4) series, or the Garnett Sessionman.

This is a trem circuit that uses one half of a 12AX7 as an oscillator to modulate the bias voltage being applied to the power tubes. I have successfully added this circuit to other amps in the past.

My problem here is that my Bassman has a bias balance pot, therefore, two separate bias feeds.

So I guess my question is, will this type of circuit even work, and if so, where is the best place to insert the oscillator? Between the bias level pot and the centre tap of the bias balance?

Another idea I had was to adapt the oscillator to make the cathode of the PI go up and down. No idea how well this would sound in practise, if it would even work at all.

Any thoughts would be appreciated!

Thanks,
Sean

[Chevy]

From what I can tell, the AA864 Bassman has a bias pot, but no balance pot. So you should theoretically be able to go ahead with a 5G9 (Tremolux) or other type of bias wiggler circuit.

[boyer]

Hi, thanks for the reply Chevy.

I have a converted Bassman 50, which does have a bias balance. I've also added a bias level in front of the balance, replacing the (IIRC) 15k resistor.

The more thought I've put into it, the more I haven't been able to see why I couldn't have this circuit wiggle the bias level AFTER the bias level pot, and BEFORE the balance.