Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: G._Hoffman on March 03, 2011, 03:56:43 pm

Title: AB763 Tremolo question
Post by: G._Hoffman on March 03, 2011, 03:56:43 pm

Could someone explain to me why the tremolo oscillator in the AB763 is hooked up to the negative bias power supply?  The tremolo is coupled through the opto, right?  So, we're not looking at a trem that plays with the bias.  And the trem oscillator tubes are both cathode biased.  So, what is it for?


Gabriel

Title: Re: AB763 Tremolo question
Post by: sluckey on March 03, 2011, 04:18:34 pm

Some LFO phase shift oscillators are slow to start up. Using the negative bias voltage like this will quick start the oscillator when the neg. bias is removed by the footswitch.
 
The negative bias insures the oscillator will be completely turned off when the footswitch is open (or not plugged in). Closing the footswitch kills the bias voltage to the oscillator tube, allowing it to oscillate. The abrupt change from hard cutoff snaps the oscillator into operation very quickly.

Title: Re: AB763 Tremolo question
Post by: G._Hoffman on March 03, 2011, 10:41:10 pm

Great, thanks.  That means I can ignore it for my new amp, which has a digital LFO.


Gabriel