Sola Sound SS100 schematics here

[Alex R]

Herewith schematics I drew for the Sola Sound SS100. One came in, burned boards everywhere and a general basket case, and as there are no schems out there I drew some. Hope they help someone.

These amps were made in the late 60s by Dave Reeves' firm Hylight Electronics, before Hiwatt production got going, when he was building the early Sound City amps. Sola Sound amps were made for Macari's in London.

Interesting that we have a master volume channel with an extra gain stage to drive it, and the other with a cathode bypass cap boost switch. Bias individually adjustable on each of the four EL34s.

Cap-driven bias supply, but it needed stacked caps to handle the voltage.

The amps use PCBs. Interesting to see so many components of the same value - economies of scale in buying I guess.

[Alex R]

...one of the bias trimmers is shown connected to a screen instead of a grid. That's not gonna be the last mistake in theae drawings, I'm no draughtsman as is fairly obvious.

[Helmholtz]

The PI wiring looks wrong. As shown V2b would be inactive, not receiving a drive signal.
Also plate resistors are missing.

[Alex R]

Grrr. Thanks, I'm really bad at this! I should have another go, there being no other schematics about.

[Alex R]

Ok I had another go. But wouldn't those two 1M resistors on the PI plates just cancel the signal?

[Helmholtz]

Ok I had another go. But wouldn't those two 1M resistors on the PI plates just cancel the signal?

That wiring makes sense. It is a clever feedback arrangement that makes sure that the two out-of-phase plate signals have equal amplitudes within a few %.
The input for V2a is the small difference (2% to 4%) between the plate amplitudes, so the error signal.
Because of the NFB around V2a its closed loop gain is unity and its plate signal is an inverted copy of the V2b plate signal.
A similar PI was used in the VOx AC100: https://www.voxac100.org.uk/schematics.htm

Look up "Self-balancing Paraphase PI" aka "Floating Paraphase".
Here's an older thread: https://music-electronics-forum.com/...se-stage/page2




V2b is missing a grid leak to ground.

[Alex R]

Ah yes thanks Helmholz a little learning lies ahead, I didn't recognise the circuit, except it seemed to resemble the old tweed octal Pro. Will look your suggestions up. Oh the PI input grounds via the MV pot, it's on another drawing.

It does look to me as if someone clever (with the initials DR) has designed an amp to a tight budget, with minimal component count and as many as possible of the same value.

No NFB I notice. Might be quite a beast. Be a while now till I get back to this. Don't think I've seen an amp with so many faults and failures before... which may be a tribute for the design's ability to endure a number of faults.