Ah, got it! Missed the bleeding obvious: I saw what I expected to see (a dual ganged volume control), not what was actually drawn (a variable R)
mea culpa
Here’s the drummer’s opinion:
The Roland JC-120, as previously mentioned, is a great fit for what you want. Loud and clean for practice. Due to age, some of them no longer have working “chorus” effect inside. (I never cared for it anyway)
When the boys show up with beer, delicately place the 120 atop a 4X12 cabinet. This gives you a “6X12” and moves enough air to cause permanent hearing loss. Loud.
If you must have tube distortion, although that is opposed to “clean headroom”, I suggest a pair of Super Reverb amps. That’ll be eight tens pushing air. Feed them with an analogue delay to give you as much rich power anyone could need outside an arena.
The Roland JC-120, as previously mentioned, is a great fit for what you want. Loud and clean for practice. Due to age, some of them no longer have working “chorus” effect inside. (I never cared for it anyway)
When the boys show up with beer, delicately place the 120 atop a 4X12 cabinet. This gives you a “6X12” and moves enough air to cause permanent hearing loss. Loud.
If you must have tube distortion, although that is opposed to “clean headroom”, I suggest a pair of Super Reverb amps. That’ll be eight tens pushing air. Feed them with an analogue delay to give you as much rich power anyone could need outside an arena.
A dual ganged volume control is the obvious way to go. It is simple, straightforward, behaves properly, makes complete engineering sense, and actually works as it's supposed to. Therefore it is not sufficiently exotic or "cool" to be worth building. 😀
On the other hand, when a man who claimed to be able to hear the difference between Teflon and vinyl wire insulation creates an awful master volume circuit that is actually a variable bass cut, and then tragically dies at age 51, sending prices of his amplifiers into the stratosphere, his circuit becomes popular on guitar amp forums all over the Internet. 😉
Printer2 made a very good point about global negative feedback fighting all attempts to insert any kind of post-phase-splitter master volume into a guitar amp. With only a few dB of NFB in use, I'm not sure how much this will actually change the behaviour of the amplifier, but it might be interesting to try to find out.
-Gnobuddy
I wonder if a single triode valve (tube) stage placed in front of a powerful class-D solid-state amplifier module might not be a simple and effective way to get what the OP wants. "Clean" in a guitar amp is usually not the same as Hi-Fi clean. The latter is what you get from DIY solid-state guitar amplifiers, and they usually tend to sound too clean for most types of guitar music.
I will add the caveat that I don't play jazz guitar, and perhaps the jazz cats really do want sterile-clean guitar amps.
It is also possible to make a single JFET behave rather like a triode, aka the "Fetzer Valve". Using a Fetzer Valve JFET stage at the input, followed by a class-D module, would be even simpler than the hybrid valve/SS idea.
All that's needed to make a JFET behave like a triode (for small signals, i.e. well away from clipping or overdriving) is to add a small unbypassed source resistor, between ground and JFET source.
Having the correct value of this resistor is the whole secret; first you measure the Vp of your JFET, then its Idss, and then you calculate the magic value of source resistor very simply, using the equation:
Rs=0.83*Vp/Idss.
The attached image shows a very simple test fixture that lets you measure both Vp and Idss for any N-channel MOSFET. Simply switch the DMM to its volts range to measure Vp, then to its milliamp range to measure Idss.
(In the latter position, the DMM itself has a very low resistance, shorting out the 1M source resistor, and allowing the JFET current to increase until stopped by its own internal resistance - in other words, allowing the current to rise to the value Idss.)
-Gnobuddy
I will add the caveat that I don't play jazz guitar, and perhaps the jazz cats really do want sterile-clean guitar amps.
It is also possible to make a single JFET behave rather like a triode, aka the "Fetzer Valve". Using a Fetzer Valve JFET stage at the input, followed by a class-D module, would be even simpler than the hybrid valve/SS idea.
All that's needed to make a JFET behave like a triode (for small signals, i.e. well away from clipping or overdriving) is to add a small unbypassed source resistor, between ground and JFET source.
Having the correct value of this resistor is the whole secret; first you measure the Vp of your JFET, then its Idss, and then you calculate the magic value of source resistor very simply, using the equation:
Rs=0.83*Vp/Idss.
The attached image shows a very simple test fixture that lets you measure both Vp and Idss for any N-channel MOSFET. Simply switch the DMM to its volts range to measure Vp, then to its milliamp range to measure Idss.
(In the latter position, the DMM itself has a very low resistance, shorting out the 1M source resistor, and allowing the JFET current to increase until stopped by its own internal resistance - in other words, allowing the current to rise to the value Idss.)
-Gnobuddy
