Bias modulating tremolo - ineffective at max volume?

[waspclothes]

Hey guys,
I recently tried out an amp with bias modulating tremolo (a Traynor YGM-3), and the tremolo was VERY pronounced at low volumes. But when I dimed the amp, the tremolo wasn't even noticeable unless playing softly.

I'm under the impression that this is totally normal, and here's why and please correct me if I'm wrong -

The bias modulating tremolo shifts the bias point by x volts, regardless of the input magnitude. This x volts might be enough to shift the output tubes into cutoff (or saturation?) during the dips of the tremolo for smaller signals coming out of the PI. But the same x volts might only have a very small effect on a large signal when the amp is cranked.

The Fender LDR style is configured as a voltage divider, so maintains a ratio regardless of amplitude at that stage of the circuit. So output compression and stuff aside, the tremolo amount should feel constant regardless of how cranked the amp is.

Yay, nay?

[Chuck H]

Yay. A bias trem that can actually cut off the tubes during heavy clipping sounds overly exaggerated at lower volumes. But that's what the intensity knob is for The last bias trem I built can only just imply trem when clipping hard.

But I'm vaguely remembering a thread about some Traynor trem circuits that default the trem some at higher outputs to protect the bias condition. But fact check that (ie:I'm vaguely remembering).

[waspclothes]

Yay. A bias trem that can actually cut off the tubes during heavy clipping sounds overly exaggerated at lower volumes. But that's what the intensity knob is for The last bias trem I built can only just imply trem when clipping hard.

But I'm vaguely remembering a thread about some Traynor trem circuits that default the trem some at higher outputs to protect the bias condition. But fact check that (ie:I'm vaguely remembering).

Hi Chuck, yeah this Traynor's tremolo was non-existant when playing hard at 10.

[pdf64]

The Fender LDR style is configured as a voltage divider, so maintains a ratio regardless of amplitude at that stage of the circuit. So output compression and stuff aside, the tremolo amount should feel constant regardless of how cranked the amp is...Yay, nay?

Trem modulates the gain.
If there's a heavy clipping stage after the trem (eg power amp overdrive) any trem fx will kinda seem a lot less apparent, to the point of disappearing during the initial section of the note envelope; then the trem intensity seems to 'fade in' as the note decays.
It's quite a cool thing to work with, as you can play softly / with guitar vol low, and get a lot of trem, then as things build up, the trem can be 'ducked out' by picking action / instrument volume being turned up.

A weird thing is that the same effect seems to happen (to my ears at least) with vibrato fx and an overdriven amp, ie heavy power amp overdrive seems to make the frequency modulation fade out, and then become more apparent as the note fades out.

[Chuck H]

It's quite a cool thing to work with, as you can play softly / with guitar vol low, and get a lot of trem, then as things build up, the trem can be 'ducked out' by picking action / instrument volume being turned up.

+1
And I think this is one reason many players prefer bias trem to signal chain trem. Otherwise, bias trem has many disadvantages over signal trem. Including intensity inconsistencies, as noted, crossover distortion, that garbled sound that gets worse as the tubes are biased colder and especially bad on amps biased to less current to accommodate high plate volts and the requirement of careful circuit adjustment to avoid over dissipation in amps that will be clipping the power tubes.

Manufacturers have typically reserved bias trem to lesser models. Using it where it can add "a feature" to small, hot biased AB1, class A and single ended amps. With most higher end, higher power vintage designs employing signal trem. The players preference for bias trem has inspired some experimentation by custom builders and Fenders custom shop using the circuits is higher end, higher power models. I consider my own experience designing it into a 2x el34 amp with 470Vp a limited success and probably wouldn't do it again with such an amp unless it was specifically requested by a customer.