Gibson GAV-1 Vibrato

[jon]

Can someone explain how this works I am having trouble following the circuit.

[martin manning]

It's along the lines of Fender's harmonic vibrato, I think: voltage amp -> phase splitter -> filter network for each phase (one hi-pass and the other low-pass, or maybe one is a band-pass... can't untangle that bit without more work) -> voltage amp for each phase (with an LFO alternately shifting the bias of these last two stages) -> half-shared split-load plate resistor to recombine the two signals -> three in series hi-pass filters to eliminate the LFO frequency content. Fender's version varies the bias of the phase splitter with the LFO, so that frequency content gets cancelled out in the output stages.

[jon]

If you were going to build this how would you lay it out? I am thinking about building it.

[martin manning]

Do you have a link to a schematic that is legible? I can't read many of the component values on the one I found. Any pictures of an original? Does this thing sound better than a Fender harmonic vibrato?

I'm curious about the filter networks- I see two shelf filters in there, and the signals are fed across the phases too.

[jon]

ill get a copy up.

As for sound....

not sure thought I would do a little experimenting.

[Jack Hester]

Do you have a link to a schematic that is legible? I can't read many of the component values on the one I found.

This one is clear enough to read component values:

http://www.el34world.com/charts/Schemat ... /GAV-1.pdf

Jack

[jon]

Schematic

[martin manning]

Ha! That schematic is from Pittman's book, which is right here next to me. Some other documentation for this thing is here: http://www.harpamps.com/gibson/ The enclosure is 6" x 6" x 3", with a small remote box that rides on the guitar's output jack. That gives a scale for the small drawing on the schematic.

[jon]

What would you expect the current draw to be?

[jon]

I did some calculations and using ohms law and the voltage drop across the plate resistors I came up with approx 11mA of current draw.

this seems awfully low to me. do I need to factor anything else in?

[martin manning]

I did some similar calcs and I think it's only about 5-6mA. It's that low because the plate voltages are so low. Note that there are some missing values and connections in the LFO section that you'll have to fill in.

[jon]

Going to buy a hammond 262D6 should be pleanty.

120V @ 53 mA
6.3V @ .9 mA

[martin manning]

Going to buy a hammond 262D6 should be pleanty. 120V @ 53 mA, 6.3V @ .9 mA

Like 5-10x plenty! If you could find something around 10mA that would do it.

Studying the filter network a bit more I believe this thing is more about the phase shift of the two signals after the splitter rather than the frequency content. I get only about 6dB difference in level at 10kHz tapering to nothing at 1kHz, but there is 120-ish degrees of phase shift difference throughout the middle audio range.

I noticed that a selenium rectifier is shown in the layout and a FWB in the schematic. Probably the original had a half-wave rectified power supply.

[kiwiandy]

This vibrato is almost identical to the one in a Vox AC30. The only major difference is that Vox shorted out one side of the signal chain to also get tremolo. It works very well, just listen to an AC30 to get the idea.

I don't know who first used the circuit commerically, but I think it may have been the Wurlitzer organ company. The attached article explains the theory in some detail. This version has an unusual oscillator, but don't bother with it- it puts out some very ugly distorted square waves!

You might also want to check out the EH Tube Wiggler schematic (easy to google). It uses a "very" similar circuit, but switches around some of the filter-shaping capacitors to get a bunch of interesting sounds.

[VacuumVoodoo]

I don't know who first used the circuit commerically, but I think it may have been the Wurlitzer organ company.

Correct. In an inspired moment Vox borrowed the circuit