I have 3 schematics, supro, Gibson n Rickenbacker, they each do it a little different, I worked on the Gibson n it did have some bleed-thru, the owner said "that was normal" n that was injected at the PI inter-stage tranny, no clue on the other two. Is bleed-thru "Normal" for cathode injected trem?
Fender had a number of tremolo circuits, mainly because there are problems with every method of creating the effect. The right choice is mostly about balancing the negatives against what you gain.
A lot of older amps (and the cheaper models of newer amps) applied the trem signal to a preamp tube cathode. 2 "problems" with this approach: in the best cases, there tends to be a pulsing hiss when you're not playing (often this is a minor complaint); additionally, the trem oscillator (and buffer if present) needs to output a
current which is comparable to idle current of the tube where trem is to be injected. That's because the trem oscillator or buffer will share a cathode resistor with the signal-carrying stage, and the trem circuit needs to pull a current through that resistor which can idle to signal tube closer to cutoff and back to ~double idle current (at maximum intensity). So you've got to worry about current to get a strong trem here, which makes relatively high-current 6SN7's seem like a poor choice; that's why I suggested copying a known-good plan (you probably won't realize all the subtle details).
Fender decided to improve this circuit and got what was probably his first patent (and maybe his only one on the tweed amps, I'm not sure). Leo made the
Tremolux, where the trem buffer pulled current through the cathode resistor shared by the two section of a paraphase inverter. This was pretty slick, as the speaker pumps an output signal based on the differential output of the push-pull output stage, but the trem is applied common-mode. This creates a volume variation, but cancels the trem signal from reaching the speaker (under ideal conditions). Net result: trem without pumping.
But not every amp can use a paraphase inverter, so what do you do when you switch inverters? You can't apply trem the same way to a split-load or long-tail inverter. Fender moved to applying trem to the output tubes to vary the bias, as in the
5F11 Vibrolux, then tinkered with
pitch-bending vibrato on its bigger amps.
The problems with these approaches are that the bias-vary scheme works best on amps that use 6V6's, with their smaller bias voltages (the trem doesn't have to be enormous to shake the output tubes' volume), and that not everyone plays with constant vibrato unless your name is Lonnie Mack. The vibarto circuit also takes a bunch of tubes and parts to make it work. So after a short run, Fender dropped that circuit, and the output tube bias vary reappeared on the small-ish
Princeton Reverb.
So what about trem on the big 6L6 amps? Aside from the brief flirtation with the
6G16 Vibroverb, Fender moved on to grounding the signal ahead of the phase inverter with an
optoisolator in its big 6L6 amps. The oldest method still lived on in the student-model
VibroChamp. I think Fender targeted a price-point with the VibroChamp, gave the Princeton the best trem (as long as you're playing a relatively low volumes; it can get swamped when you crank the amp wide open), and gave the big amps a trem that works at all volumes (though maybe doesn't give you the sweetest response).
Of course, now you can get a pedal with tap tempo trem, a selection of waveforms, multiple speeds at the stomp of a switch or sweep of a foot pedal, and numerous different rhythms to the trem. So it really boils down to which circuit works best with the rest of the amp you're designing given the shortcomings of each approach. Or which pedal sounds best in front of the amp...
Topic: injecting tremolo (Read 6469 times)