The Matamp NFB circuit

Oh good, my phone lost my blog post.

Well, there's an LC notch filter in the Matamp's NFB loop, which seems clever except for inductors' sensitivity to noise, adding phase shifts in a marginally stable NFB loop (that anode bypass cap isn't an accident, nor is the speed up cap parallel to the feedback resistor) - plus we have an extra stage in the loop thanks to the cathodyne.

Also I should point out that when you inject NFB into the driver before a cathodyne, any presence/resonance controls serve to partially bypass the driver's cathode for a double dose of whatever range you're trying to boost. The blackface Princeton gets around this by fully bypassing the cathode and then applying NFB between the regular Rk//Ck and ground.

So the Matamp idea is an interesting one, but it's clear why it didn't last. I prefer building stable amps to GHz oscillators, plus shielded inductors are expensive, so I'll probably not bother with implementing this. Maybe an overall NFB control. Maybe.

You know what's easier to keep stable, and lets you implement a notch filter with adjustable frequency and Q? A local feedback loop.

It's not a horribly unique idea - check out the Electro-Harmonix Tube Zipper sometime. There's also a German patent on a similar circuit. As far as active mid boosts go, though, it could be super interesting. Either you could add a gentle Matamp/Orange mid boost, or sharpen the Q and get a "cocked wah" sound, or even defeat the loop for a gain boost... If you wanted to get really clever, you could even sweep the frequency at a constant Q, but at that point just buy a Tube Zipper. Puretube (who also designed the Flanger Hoax) is one of my heroes. Give him monies.

I'll fiddle with this idea first using a Kustom Defender 5H, turning it into basically a massive pedal. It might be a huge waste of time, who knows. Plus the James stack is perfectly capable of creating a "mid boost" by itself...

But seriously, puretube is a brilliant fellow.

The Matange!

So, I've been planning my next project lately. This year, I'm going to build an Orange/Matamp-inspired head, starting with a cheap donor. Still not going to make the plunge into woodwork/metalwork as long as I can avoid it. Someday I might build a high-gain monstrosity, but I'm not a big fan of preamp distortion, so I don't have much motivation.
Fortunately, I have the relevant schematics on my phone!
Now, my thoughts:
Fixed bias quad of pentodes. Sure, though it pains me to turn away from my beloved beam tetrodes, but I'll give these screen-current-sucking babies a try.
Cathodyne phase inverter. Awesome; love it. It's going to need a big grid stopper, and I'll put big grid stoppers on the power tubes too. Definitely going with AC coupling; DC coupling is too big of a pain for little benefit with cathodynes. What can I say, I like em center-biased.
James tone stack. Absolutely. I'll probably add, or at least try, a mid shift control so I can move the notch around.
The "FAC" switchable coupling cap bass control. This is great; I've used it for a bass control after a pentode. The series-connected version does pop; Matchless uses a parallel "one at a time" version with large resistors in parallel to prevent this. Since I've tried the series version, I'll probably try Matchless's take on the Matamp original.
The Matamp drive control. This kind of seems silly. Switchable bright cap, eh, why not. Gotta keep some British in there! I don't see the point in padding down the gain when there are only three gain stages. Also that Orange inductor-based boost control... I'm skeptical. I'm also worried about noise. It's a tricky situation. I might try variable cathode bypassing instead, possibly tied to a treble cut.
As for the order of the tone controls, I think I prefer having the FAC after the first gain stage and the James after the second. Always good to cut bass early and treble late, though there are so few stages...
It should be an interesting project. If there's space for a third preamp tube, I may use a 6GH8 triode/pentode to add reverb. I'll use the pentode to drive the tank and half of a 12AX7 for the recovery stage, and then I can use that medium-mu triode in the 6GH8 for the phase inverter! Mmmmm, sweet, sweet low-ra cathodynes...
More when I actually write up a schematic.