Some kind of ultra-linear operation in the output stage of the ToneKing Gremlin?

[janou195]

Hi all,

I am working on the schematic of the ToneKing Gremlin. I reproduced it in the file attached. I wonder about the purpose and the functioning of the R32 and C18 thing. This has been discussed a bit in this topic https://el34world.com/Forum/index.php?topic=31890.0 dedicated to the PSU of the Gremlin: it is suggested here that this may be some kind of ultralinear operation.

Have you ever seen this kind of thing in another guitar tube amp? What is the functioning of this or do you know some references about that?

Many thanks,
Olivier

[shooter]

see OP

[Carlsoti]

There are SOME guitars amps that run UL outputs, but it's far from the norm. A '69 Sunn Sonaro I just parted with was very much a Dynaco Mark IV with an extra preamp tube and tone circuit. UL, loud, and clean, just as David Hafler intended. I've also got a pair of 60's Magnatone amps, both with UL output sections. Both of these are also pretty clean, even when cranked up. I'm sure there are others, but I've had my fill. My more recent buys/builds are leaning heavily the other way... If it came to me with NFB, it goes on a switch. None of my upcoming builds will have NFB.

[janou195]

Thanks for these inputs on the UL!

In the TK Gremlin, there is no use of an extra tap in the OT. So how does it work in this particular build?

[Merlin]

When you feed 100% of the anode signal back to the screen, the tube behaves as a triode.
With 0% feedback to the screen, you have normal pentode operation.
Intermediate amounts of feedback produce intermediate operation. The tube characteristics 'morph' as in the attached image.

The toneking is basically doing the same thing, but with a lower DC screen voltage than in a normal UL amp.

However, in your schematic, the anode is feeding 33k/(470k+33k) = 0.065, or 6.5% of the anode signal back to the screen. That is a very small amount of feedback! So the tube is operating almost in normal pentode mode.

Why have they done this? I have no idea. It is such a small amount of feedback that I doubt it has much effect on the tone. It almost looks like a designers' experiment

[Merlin]

The circuit is similar in principle to this one, but the toneking used a lower DC voltage at the screen grid.