AB763 Bias Trem Trick

[zodiac_analog]

Hi y'all just wanted to share this. I recently rebuilt my 1980 Deluxe Reverb and wanted to add that sweet 6G16 bias vary trem. I've done this quite a few times in the past on various amps with mixed results....most who've done this know that the main issue you run into is that you get one of two things: A. nice warm bias or B. nice sounding tremolo. So this time I came up with a small simple solution of adding a switch to the back of the 250k intensity pot which parallels a resistor from input to output of the bias pot. Through experimentation I've found a 15k ground resistor and 12k parallel resistor to work best but I'm sure those numbers will need to be fudged with for each amp. On this amp at ZERO intensity negative bias is set at -35, when switched on (1) it's bumped to approx -43. Anyway it sounds glorious and just thought I'd share. Apologies if this has already been discussed to death...I'm not online much!

[Lectroid]

I’ve only been doing this a few year, but this looks like a great technique. Very cool. Thanks for sharing this.

[stratomaster]

I've seen stuff like this to compensate for switching between solid state and tube rectification.  I haven't seen it used on a bias trem before.

My usual recipe for deep bias trem on a PR circuit regardless of idle bias setting is to LED bias the oscillator and add a diode on the Intensity pot.  I've had to drop the the 1M series resistor feeding the Intensity pot in one case, but not by much, to 750k. This makes the tremolo plenty deep at any reasonable idle bias.

It also has the benefit of drastically reducing bias oscillation at minimum intensity, so just rolling the trem down sounds identical to switching it off.  You can also measure bias without having to switch off the trem.

[zodiac_analog]

My usual recipe for deep bias trem on a PR circuit regardless of idle bias setting is to LED bias the oscillator and add a diode on the Intensity pot.  I've had to drop the the 1M series resistor feeding the Intensity pot in one case, but not by much, to 750k. This makes the tremolo plenty deep at any reasonable idle bias.

Very cool! The diode on the intensity pot is new to me... I'd love to see a photo or schematic to see how it's implemented. Luckily (unlike the PR) I have a full tube to play with, so the issue seemed less about the range and more about the offset...I could absolutely be wrong but that's what stuck in my head from past experiments...I'd really like to try this switch on a bigger 6l6 amp next, as that's where most of my past failures were...

[jbrrrrr]

I'm on the "bias trem research" team myself at the moment, tinkering with this Princeton/Deluxe Reverb hybrid that I had built for me a while ago.

Very cool! The diode on the intensity pot is new to me... I'd love to see a photo or schematic to see how it's implemented.

https://www.tdpri.com/threads/6g2-trem-thump-diode-fix.1154195/ - here's a thread on TDPRI about the Intensity pot diode with a Falstad simulator circuit to play with - the tldr is that when active, it seems to lop off the positive half of the waveform acting on the bias, which doesn't seem to have any negative impact on the amplitude modulation lope, but *does* get rid of any thumping issues which would otherwise be present at a hotter bias point.

[jbrrrrr]

the tldr is that when active...

...and as is usually the case when someone skims quickly, it seems that there's a lot more nuance to how that diode affects the waveform and what that sounds like in the tremolo effect.  Worth reading through the 3 pages if it's of interest to you.  Ignore my laughably reductive smartypants synopsis.