Brain Teaser Circuit - What are doing 6SJ7 + 6SQ7 tubes on Gibson GA-Custom ???

[kagliostro]

I was watching some old Gibson schematics (they are so interesting and there is a lot to borrow there)

when I've seen the GA-Custom schematic ................

there is an entire part of the circuit on which I'm not able to follow the signal path nor to understand how that part of the circuit can function 

the schematic is this

http://el34world.com/charts/Schematics/files/gibson/Gibson_GA-CUSTOM.pdf



the odd (to me) part of the circuit is formed by the 6SJ7 tube + 2 x 6SQ7 tubes that are on the left bottom of the schematic

I'm not able at all to follow the signal pat or to understand the function of that part of the circuit

Franco

[sluckey]

2 x 6SQ7 tubes that are on the left bottom of the schematic

That is a tremolo circuit. The dual pot on the left is the speed pot. The dual pot on the right is the intensity pot. The trem signal is applied to the grids of two 6SJ7 preamp tubes. The top 6SJ7 preamp does not get any tremolo.

[kagliostro]

Thanks Steve

I had a small idea about it was a tremolo, but the fact that the normal channel has only one jack and that channel had 3 jack descouraged me to think in that way

---

Have you an idea about which can be the purpose of the 1k pot connected via the 9.1k resistor to the cathode of the left 6SQ7

and which the purpose of the switch in parallel with the 15K resistor trat connects to ground and to the 51k cathode resistor on the right 6SQ7 ?

Franco

[sluckey]

The pot sets the gain of the left tube. The switch controls the gain of the right tube. I suspect (just a guess) the pot and switch are internal adjustments. A tremolo oscillator can be tricky to get oscillating reliably. The gain of the circuit has to be good enough to sustain oscillations. This early design circuit 'may' have not been ready for prime time. That circuit certainly didn't become the standard tremolo oscillator.

[kagliostro]

Thanks for the explanation

Franco

[shooter]

there is a lot to borrow there

Franco, I have borrowed from Gibson twice, one trem and, my favorite, using a transformer as PI.  I suspect Gibson did the same borrowing 

And thanks Steve, I got the trem, but,

the pot and switch are internal adjustments

was hangin me up.

[PRR]

FWIW, the oscillator is a Wien Bridge, notorious from Terman and two guys H and P in a garage in Palo Alto.

Terman's idea, implemented by his students, was to stabilize the oscillator with an incandescent lamp NFB gain network. They got a patent, started a company, sold a few, designed a very cool case, got lucky, the company took off.

If you can tolerate some waveform distortion, you can hand-trim a Wien just past the start of oscillation. Amplitude rises until the wave flattens top or bottom enough to "lose gain". When I have hand-trimmed prototype Wiens, this is VERY fussy. The lamp-NFB makes it a little less fussy and more likely to work for years.

The other interesting bit: V2 of the Wien's amplifier is worked as a Cathodyne, two outputs. I have never seen that. If V1 has enough gain, no reason it can't work. In fact there is a near-Wien (a patent dodger) which uses a cathode follower (or WCF) at V2.

The two outputs do something to a pentode, which does not seem to be "in" the audio path, but closely coupled. My eyes are crossing trying to follow the lines. There is a "clever" 2-pentode cross-couple which varies audio gain without large variation of total DC current (thump). {EDIT} No, this is just a differential amp, signal in one side, silence to the other side, and out-of-phase LFO to the two grids. The signal side goes in and out of conduction, but the sum current stays near-constant.

The three inputs have nothing to do with it. Maybe they thought the whole band would plug into the one tremolo.

[kagliostro]

Thanks, fantastic explanation PRR


 


Franco

[HotBluePlates]

... the sum current stays near-constant. ...

So translation is "elaborate scheme to reduce pulsating noise from the trem"?

That's what I thought I saw when looking at this circuit. And it's then a variant of Fender's 5E9 Tremolux circuit. The phase inverted trem output also reminded me of Fender's Harmonic Vibrato (though it's being used differently there).