Kay 704 Schematics? I have four of them, all different. You have another one???

[R.G.]

Hiya. I'm a techie, born and bred. I take short breaks from my day job by reading techie stuff.
 
I was intrigued by a post in a different forum from a guy who wanted to build a new clone of the Kay 704, which was one of Kay's Vanguard 700 series. These were the first all solid state amps in the US.

I found a Version 1 (V1) and a Version 2 (V2) on the net and mind-wandered through them. Then a tech friend traced out one that was in the shop, and found that not only did the traced circuit in the amp not match the pasted-on schematic inside the amp, it did not match either the V1 or V2 from the net. What the heck??

I think Kay was trying hard to get them to work reliably coming off the line in the face of field reports of issues, and tinkered them as they went. That makes me think there are possibly more of them, all hiding under the "Kay Vanguard 704" label.

Do you happen to have one? Got the time to open the back and get the best-practical shot of the pasted-in schematic? It's got my curiousity up.

[J M Fahey]

I bet changes are small, more tweaking than "new design" and all must or should be functionally equivalent.
If you wish, post them here and let us see the actual differences.

[R.G.]

I'm not confused by them, just curious. There is a strong family resemblance, but the changes are not all trivial.

To be fair, I'll probably end up writing up a guide to the whole family. I'm just trying to figure out if they did even more bizarre things along the way.

As of now, the precursor 700 was a two-tranformer class AB, single supply. The next earliest was a stacked AB output stage, and they ran it from a bipolar DC supply. Next was a more complex driver circuit, and single DC supply, with the speaker strung across the power supply on two capacitors, apparently on the hope that this would let the speaker float to the voltage at the junction of the two output devices.

The next iteration, and probably all later ones, put one cap in series with the speaker.

The tremo/vib LFO was a relaxation oscillator in all of them, but not the same in any of the variants I've seen. The LFO made a rectangular (not square, except by accident) output that was heavily filtered to get rid of the edges and make it more ramp up and down. The ramps were referenced to different DC voltages and injected to different places in the circuit. This makes me think they were looking for a reliable trem/vib LFO.

The earlier ones used only two transistors from input jack to driver transformer. The later ones added a high(-er; these were germanium) gain transistor to the input jacks. More gain? Maybe.

This all kicked off my curiousity; I wonder how many devia... er, variants    there are.