Standel 25L15 Schematic & suggestions

[blind Blake]

Hi everyone,

I'm looking to get a Standel 25L15 amplifier made - do you have any idea where I can get a legit schematic for that and also any tips on building it. I know it needs the correct JBL 15" Speaker for it to sound right. Also looking to run it on 240v UK voltage so I guess I need a custom made transformer? Then the cabinet, do you recommend someone who could build that for me?

thank you in advance

[dmeek]

Years ago someone asked me about this amp. Apparently it has the Magic Mojo. I went looking for the schematic. You would think it was one of the Secrets of the Masonic Lodge. No one wanted to share or even admit they had it.
I finally found an original of the preamp. Here it is.

[Helmholtz]

Wow, the circuit around the 12AT7 looks quite uncommon.
The right side triode is wired as a CF that is cathode coupled to the left side triode.
Also it seems to part of a feedback circuit including the tone controls.
So active EQ.

[blind Blake]

Years ago someone asked me about this amp. Apparently it has the Magic Mojo. I went looking for the schematic. You would think it was one of the Secrets of the Masonic Lodge. No one wanted to share or even admit they had it.
I finally found an original of the preamp. Here it is.

Thanks a million - this is gold!

[teemuk]

Wow, the circuit around the 12AT7 looks quite uncommon.

Uncommon to a guitar amp gain stage maybe but its a generic differential amp.

The right side triode is wired as a CF that is cathode coupled to the left side triode.

"Left side" is common cathode amp coupled to a common grid amp on the "right side" via shared cathode circuit that forces same current draw for both halves of the differential.

Since this one is not used for phase "splitting" like the bog standard LTP but for high (open loop) gain amplification we don't need two opposite-phase outputs; one side simply drives a common cathode amp that furthermore increases open loop gain. As it's output is unused the common grid amp side doesn't even need a plate load resistor.

Also it seems to part of a feedback circuit including the tone controls.
So active EQ.

Yes, frequency selective global negative feedback loop encloses the stages. The feedback is fed to the other input of the differential. The tone control setup is actually very much that familiar from power amp "Resonance" (bass) and "Presence" (treble) circuits that vary closed loop gain by controlling magnitude of feedback. So yes, they are "active" controls.

In solid-state power amps you often see this very same circuit architecture (sans tone controls) with the difference that the second gain stage is also used to drive a push-pull buffer. As a simple "discrete opamp" this circuit is also very common.

It's very low distortion and transparent (to the point where it clips abruptly) and the closed loop gain is extremely predictable and consistent, virtually independent on variation of device characteristics.

[Helmholtz]

Uncommon to a guitar amp gain stage maybe but its a generic differential amp.

That's the kind of feedback I was hoping for.
Thanks for your explanation, now I recognise the differential amp (and the basic operational tube amplfier).
Though a poor differential amp as the low value cathode resistor is far from being a good "current source".

With uncommon I meant that I don't expect to see such active tone controls in a guitar amp - other than wrapping around the power stage for presence or "resonance" as you mentioned.