Fender Tremolo

[C Moore]

I was reading about how good the non-opto tremolo circuit sounds compared to the more common Fender Trem circuit. I am not sure I even understand how it works. The non-opto seems to be a cathode follower of sorts and I assume it modulates the power tube bias via those two caps? Is it the charging and discharging of those caps that accounts for the fluctuation of the power tube bias?
Does anybody want to take a stab at a basic explanation of how this Tremolo works, and why it might sound "better" than the other Fender Tremolo circuit does?
Thank You

http://www.ampwares.com/schematics/vibroverb_ab763.pdf

http://www.ampwares.com/schematics/vibroverb_6g16.pdf

[tubeswell]

Bias-vary "non-opto" trem with CF is quite astounding, esp when you have the tubes biased right.

The trem itself works by driving the grids alternately +ve and -ve interrupting the biasing of the tubes. The resulting intensity of the trem is quite hypnotic.

The LFO all happens in the first stage. The tube itself inverts the signal 180 degrees (inverting gain stage), and the 3 x RCs invert the signal a further 60 degrees each to give a full cycle.

The CF stage is there to give a really low impedance signal to drive the grid load resistors. It is absent on some Fender bias vary circuits and they aren't as smooth sounding (like the BFPR and the Brown Deluxe). Think late tweed tremolux (5G9) or the Brown (6G9B) Tremolux or the Brown Vibroverb (6G16). (The 5G9 fixed bias 6V6 PP with LTP and bias-vary trem really smokes BTW)

[Bear]

As I understand it, why it sounds different is because of the difference in how volume is varied. The opto types essentially act like a variable resistor, where the bias types vary the volume by changing the operating points in the power amp or in some amps the preamp. As the bias point changes, the tone changes a bit, making the effect sound a bit more animated.

[Andy Le Blanc]

Spend time at the schematic heaven site in the bargain bin.
There are several ways and variations to get tremolo and/or vibrato.
It comes down to just how and where go about coupling your low frequency oscillator
to the rest of amp. There's the bias hook up with a fixed bias, the grid return
with a cathode bias, a photo-cell in the signal path (either in series or shunt to ground),
and you can couple the thing through the cathode circuit of any pre amp stage.
I been liking yet another way, you can use say a 12au7 dressed as two diodes
to ground, you feed the oscillator signal to the grids and the other end is
hooked into the signal path with resistors. Ampeg and Univox used that
in a few production runs. Just another way achieve a tremolo.
True vibrato is much more complex, haven't found a simple circuit for that.
Yet.

[martin manning]

Does anybody want to take a stab at a basic explanation of how this Tremolo works, and why it might sound "better" than the other Fender Tremolo circuit does?

The opto isolator trem varies a resistance from the signal path to ground at the input to the phase inverter, effectively acting like a volume control that moves up and down at the LFO's frequency. The bias vary trem superimposes a sinusoidal variation on the bias voltage supplied to the output tubes.

I think the key difference is that shifting the bias point will cause clipping and distortion, which might add some pleasing harmonics as well as modulate the volume. Since it is applied to both sides of the push-pull output the clipping would theoretically be symmetrical, but it is not likely that both sides will saturate at exactly the same grid voltage, and some asymmetrical clipping will be included.

MPM

[tubeswell]

Since it is applied to both sides of the push-pull output the clipping would theoretically be symmetrical, but it is not likely that both sides will saturate at exactly the same grid voltage, and some asymmetrical clipping will be included.

This is indeed what happens. The range of effects you get as you increase the vol is quite amazing and intense. Add 'verb and it is a stunner.

[C Moore]

Thanks for all the info. I have a Ampeg B-25 that I am considering turning into a Vibroverb head. So that is one reason I raised this question here.
But doesn't the AB763 Trem also affect the grid bias of the power tubes as well as being tied into the phase inverter?
Thank You

[C Moore]

Thanks for all the info. I have a Ampeg B-25 that I am considering turning into a Vibroverb head. So that is one reason I raised this question here.
But doesn't the AB763 Trem also affect the grid bias of the power tubes as well as being tied into the phase inverter?
Thank You

[martin manning]

...But doesn't the AB763 Trem also affect the grid bias of the power tubes as well as being tied into the phase inverter?

No, the connection to the bias supply is only used to stop the oscillator by referencing it's grid to a below-ground voltage, which puts the triode well into cut-off.

MPM

[C Moore]

...But doesn't the AB763 Trem also affect the grid bias of the power tubes as well as being tied into the phase inverter?

No, the connection to the bias supply is only used to stop the oscillator by referencing it's grid to a below-ground voltage, which puts the triode well into cut-off.

MPM

Thank you so much, the mystery is now unveiled.
Now I need to figure out why/how I keep double posting...???

[M Fowler]

MPM[/quote]
Thank you so much, the mystery is now unveiled.
Now I need to figure out why/how I keep double posting...???[/quote]

Slow down the mouse speed on the double click option.

[M Fowler]

Delete I have problem with quotes

[Cornelius]

Delete I have problem with quotes

You have a slash in your first 'quote' statement; it shouldn't be there, only in the second 'quote' statement...

[angelodp]

What components in the circuit of a Brown G64 will affect the nature of the Vibrato in these amps. Is it possible to pull more strength in the effect. I notice that with full intensity and speed at about 6 or 7 the the Vibarato is at peek effect. As you dial back either intensity or speed the effect really drops off, very unlike the tremolo in a Blackface that has a range in speed and intensity. The Brown Vibrato is more like getting to that sweet spot.

[martin manning]

What components in the circuit of a Brown G64 will affect the nature of the Vibrato in these amps. Is it possible to pull more strength in the effect. I notice that with full intensity and speed at about 6 or 7 the the Vibarato is at peek effect. As you dial back either intensity or speed the effect really drops off, very unlike the tremolo in a Blackface that has a range in speed and intensity. The Brown Vibrato is more like getting to that sweet spot.

There are two versions of the Fender "harmonic vibrato." If your amp is a 6G4, as opposed to 6G4-A, it is the earlier one. The phase-shift LFO itself is basically the same in both; other differences are as follows:

The earlier one uses the output straight from the LFO to modulate one side of the filter section and uses a gain stage to create an inversion of the LFO's signal to modulate the other side.

The later circuit uses the LFO signal to drive a cathode follower, and then the output of that drives a cathodyne phase inverter to produce the two signals used to drive the filter section. It has an extra triode stage, five in all, and that means a total of six small tubes in that amp. If yours has five, then it is the earlier circuit. All of this may have been done to improve on the range and intensity problems you note, and this LFO arrangement with the CF is the one Fender used from that point on. The high/low filter network is a bit different in the -A, too.

Major surgery would be required to upgrade to the -A, but if the oscillator in yours is simply dropping out (stops oscillating), you may be able to improve things by using a fresh tube and replacing the cathode bypass cap on the oscillator. The circuit relies on high gain to operate, so if your components have degraded this might help. I have read that the usable oscillator frequency can be lowered by increasing the value of one or both of the 0.01u ceramic caps in the feedback loop (stock values 0.02u, 0.01u. 0.01u; increase 0.01's to 0.02). Maybe someone else can comment on this.

Incidentally, the current version of the tube LFO (as used in reissue amps) has the cathode follower, and in addition the CF is included in the feedback loop for the oscillator. This is said to produce more stable operation.

MPM

Edit: Looking over the phase shift oscillator design notes on R. Aiken's site (thanks!), the cathode bypass cap on the oscillator might better be increased to 150-220uF to hold the gain up all the way down to the lowest required oscillation frequencies (~2 Hz).