Unorthodox hybrid power amp from early 1990's - who made it?

[teemuk]

Out of plain interest, whose amplifier is described by this patent?
Enhanced vacuum tube power amplifier - Patent 5148116

The patent describes a hybrid power amp design from early 1990´s that uses a BJT + 6BQ5 pentode as a (common grid) voltage amplifier, MOSFET + BJT diamond buffer as a power follower, biased by a pseudo class-A integrated circuit licensed from JVC (“Super A”). Never seen anything like it before in the field of musical instrument amps.

I know that John M. Robinson (the inventor) was a few years earlier before this patent appeared on the payroll of Gibson Labs but this patent doesn’t seem to be for any of Gibson’s designs. Furthermore, it’s not assigned to Gibson Guitar Corp. (like other patents from the same era) and the schematic styling is different from the one Gibson used (during the same era).

Since it’s not a mere patent application it seems to imply that considerable investments were indeed made to make this thing true and to production but persistent Google searches with various terms found no references of such hybrid amp – or even about what happened to Robinson since he (assumably) left Gibson.

It’s little to work with but does anyone know anything about this thing?

[Steve Conner]

The details of the implementation seem a little odd, but I think the topology is basically the classic hybrid. A tube voltage amp driving a solid-state current booster with unity gain, and a DC servo to clean up the mess.

Warwick, Ampeg or whoever, use a similar circuit in their hybrid bass amps: a 12AU7 or 12BH7 in SRPP, driving a unity-gain MOSFET output stage.

IMO, these circuits are more suited for bass guitar or hi-fi, because the output impedance is low. They probably all sound the same, and moreover, sound the same as a solid-state power amp with an Alembic F2B in front of it.

The "common-grid" thing is interesting: he's taking feedback to the grid and driving the cathode. It's more usual to do the opposite, so that implies that his power stage must have had a gain of -1, and he had to reverse the input to compensate.

Possibly it was the guy's pet project, and he thought it was good enough to patent, but neither Gibson nor anyone else were interested in buying it.

[J M Fahey]

They probably all sound the same, and moreover, sound the same as a solid-state power amp with an Alembic F2B in front of it.

I think that's the key issue.
So many use tubes and then use every trick in the book to kill the "tubeness".
If you do not let tubes overdrive or "linearize" them with NFB or cathode drive them (this one or 99% of Music Mans) or just plain use them inside a conventional SS amp, with 40 dB NFB, as in ADA Microtubes, FORGET about tube sound, they are only gimmicks.

[teemuk]

Well, the main reason for my inquiry is only due to pure interest in historics of different amps and their makers. There's not many hybrid circuits that would use a power amp tube instead of plain 12AX7's, let alone a pseudo class-A biasing technology borrowed from HiFi amps. It makes this one quite special so I was hoping someone knew what the amp exactly was and who made it - assuming it ever was made, that is.

From technological standpoint, I've quickly tried this circuit in SPICE and it did seem to perform pretty well concerning "tubeness". At moderate levels of overdrive the EL84 soft clips (way more prominently than a plain 12AX7 that most similar hybrids use) and the amplifier's output impedance is high enough to make the gain somewhat dependent on load impedance.

[J M Fahey]

Hi Teemu.
As always, I fully agree with you, sure this one in particular gets closer.
Clearly, an overdriven power pentode will be closer to overdriven power pentodes, the typical Power Amp output device, than any overdriven preamp triode, so that's a big point in their favor.
What I complained about is the futility of spending money and effort to introduce a tube , with all the problems it carries, into an SS design, and then waste what they get.
You know I'm an old timer and can't handle simulation at all , so I must rely on old breadboards (the nail and wood kind, not Protoboards), hooking a signal generator there and scoping for results.
As an example, this is what I found saturating a cathode-driven-6L6 Music Man:
This is the actual power stage: a dual op amp driving two very high beta medium power transistors, which cathode drive 2 x 6L6

First: clean sinewave, don't remember if 440Hz or 1 KHz now, just before clipping.
Very clean, linear, "very SS". Most tube guitar amps do not provide such textbook sinewaves at all, unless biased way too hot and using new , very well matched tubes.

Now around 6 dB overdriving, still quite well behaved, most tube amps would start to show slope kinking near zero crossing, similar to crossover distortion (which they did not show at lower level), caused by grid rectification.
Here grids are confortably biased around +24V over ground, if I remember correctly, and quite linear current drive comes through the cathodes, so, no grid rectification possible.

Now around 10 dB overdrive at a higher frequency, probably 4 or 5 KHz. I'm usually interested in the audible range and guitar speakers don't go much higher than that.
The lower half slopes show a kink , as if part of the clipped sinewave became a proper squarewave, or went through a Schmitt trigger, a waveform I have often found on SS amps which are marginally stable.

Now back to a mid frequency and heavily overdriving (around 20dB), the waveform is very similar to an overdriven SS amp ; overdriven Tube amps look very different, can get quite flat topped but slopes are not that vertical, retain some of the original sinewave, and introduce heavy "crossover", you can still recognize the note you played. Well, tube lovers have always known that for sure.
The lower half shows some ringing, it's less stable than the upper half, we already saw some indication of that in the previous picture.
The leading edge peaks show that the load is inductive and current driven; it's similar to what a "Valvestate" type amp gives when overdriven and driving a real world speaker, not a resistor load.

This last image shows the amp mildly overdriven (around 6dB) at a lower frequency, probably around 80 Hz, the falling upper and lower tops show poor low frequency response, compatible with a real world output transformer.
Old capacitor coupled SS Acoustics showed about the same.

In a nutshell: MM made an enormous effort to use tubes, but got results closer to a very good, output transformer coupled SS amp.
Of course one trait of tubes, their becoming current sources (high internal impedance) when overdriven, when NFB ceases to act, was not lost.
Anyway I respect them very much, they sound very good for loud and clean sounds, and are amongst the best, most solidly amps ever built.
When I find the scope pictures, I'll show how ADA Microtubes kill whatever tubishness their internal 12AX7may provide.

[benndoma]

adjustable Hybrid Power Amplifier

Hi Folks,

digging the Internet for DIY Hybrid Tube Amplifiers, I was found this Tread.
Maybe my Hybrid Power Amp. Simulation can help you to build you own Hybrid Guitar Power Amp. as described above.

Output damping/Tube Load, Tube Operating Type (over-dazzle from Triodeconnection via UL to pure Pentode) and global Feedback are independent adjustable via 3 Potentiometers. If this sounds interesting to you, you invited to visit Möhrenbude.de, today I have translated the original german article into english for you:

Moehrenbude - Content

With best Regards,
Mario Benndorf

[J M Fahey]

Hi benndoma, thanks a lot for your posting, it looks very interesting and certainly rises a whole bunch of fresh questions.
Good luck.