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Joel 27th June 2003 05:40 PM

Class B designs with high fidelity?
Anyone out there have a class B amp design, preferrably using a 6N7 output stage? I searched and found a guitar amp by Tim, but that was it in the forum.
I'm really looking for something that would stack up in the listenability department.

Sch3mat1c 27th June 2003 06:36 PM

Well... wrap some mean NFB around it, that'll make it work... ;)


SY 27th June 2003 06:46 PM

With a 6N7, no, but there are some other fine class B designs around. I use a variant of Dave Berning's topology, first seen in the Audionics BA150, then later the EA230. The latter is a pretty easy amp to duplicate and it runs cool as can be (6JN6 outputs, if memory serves). The screen drive Class B method has penalties in requiring rather high drive voltages and currents, but the linearity is superb.

Joel 27th June 2003 06:54 PM

Thanks for the replies guys. Yes, Sy, I was thinking about the drive requirements for your average class AB2/B stage, and I was wondering: why couldn't you take two 6V6's (for example) and bias them down to say, -30V or so on the grid - until they're only drawing about 1mA each - and then feed them from a standard phase inverter? Your quiescent current would be only 2mA, and because the grids are always negative to a degree you would not need any grid "drive".

Is this a possibility, or a truly bad idea? :)


ps. Did you notice that the RCA tube manual calls the typical characteristics of the 6N7 "Class B Power amplifier", yet the tubes are never biased into cut-off in the example they give. They are shown drawing 17mA at zero volts Vg, and no-signal... what the? :confused: This is class AB2.

SY 27th June 2003 07:17 PM

I haven't done it with a 6V6 yet, but with EL34s, there was a terrible linearity problem. And I couldn't get them to be stable (in a DC sense) with a 3 ma idle current.

Joel 27th June 2003 07:38 PM

Interesting. You mean that the DC plate voltage and bias voltage was fluctuating at idle?

I guess if this worked great we'd see TONS of amps out there running this way. But maybe not... because you'd still need a heck of a well regulated PSU in order to go from zero to 70 or 80mA of current demand in a millisecond, and not have the voltage crash hard.

SY 27th June 2003 07:40 PM


Interesting. You mean that the DC plate voltage and bias voltage was fluctuating at idle?
No, the DC plate voltage in my EL34 amp is regulated. So is the bias voltage. I was talking about idle current, i.e., the no-signal cathode current.

Sch3mat1c 27th June 2003 07:40 PM

The thing with tubes is they never completely cut off. At -1000Vg (as if) there'll still be a few nanoamperes. IIRC class B is classified as idle dissipation = 1/10th full-signal.
By -30Vg on a 6V6 you'd be in light class C, unless screen voltage is like 4 or 500V. :eek:
Correction, more like >300V.


Joel 27th June 2003 08:07 PM

Ok. Well, what if they were biased slightly higher, and we used an ultrafast diode to clip the bottom of the waveform off so that the tube doesn't ever go down into that horribly non-linear region Sy is running into?

What is the fastest, distortion free solid state diode that conducts at millivolts?

SY 27th June 2003 08:35 PM

Well, dumb question- if you want to go the Class B route, why not use a more suitable tube, rather than trying to shave a square peg down until it fits the round hole?

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