Grounded grid Barn Burner

[KA3EKH]

I do a lot of weird stuff, not all of it Ham related and one of the recent projects was repairing an Amplifier Research 100-watt broadband amplifier. The spec sheet says its broadband and flat from 100 kHz to 100 MHz and tolerant of gross output matches. This amplifier is about thirty years old and no longer supported so that?s why I am working on it.
No documentation and had to draw up a basic schematic of the amplifier and this is the output stage in a ?simplified? version, the real thing has additional decoupling and unlike the drawing you are seeing it uses twelve 8122 tubes all in parallel.
The one Ohm resistor shunting to ground is what throws me, figure this amplifier is dissipating huge amounts of power to get the 100 watts of output and has no tuned circuits so its efficiency is marginal at best. Maybe five or ten percent?
Just thought I would pop it up here for others to see how to build maybe the most inefficient amplifier possible but it is broadband.
 

[KD1SH]

   I'm wondering what that amplifier was originally used for. Probably some sort of industrial use, like drying wood, maybe. I believe there was at least one industrial wood drying machine that used 810 triodes.
   That actual amount of energy dissipated in that 1 ohm resistor would depend, of course, on the reactance of both the capacitor and the inductor in series with the resistor, with the reactance of the inductor decreasing as frequency decreases and the reactance of the capacitor increasing as the frequency decreases.
   The schematic shows no circuitry for applying a regulated DC bias to the cathodes. The inductor from the cathodes to ground would need to have a very high reactance throughout the operating frequency range and relatively high resistance at DC. Since the bias will be whatever is dropped across the DC resistance of the inductor, the bias and thus the operating point would, I'd think, be all over the place.
   That 8122 tube would seem to be a good tube for amateur amps, at 400 watts of plate dissipation, but they don't seem to be very common these days.

[K9MB]

I do a lot of weird stuff, not all of it Ham related and one of the recent projects was repairing an Amplifier Research 100-watt broadband amplifier. The spec sheet says its broadband and flat from 100 kHz to 100 MHz and tolerant of gross output matches. This amplifier is about thirty years old and no longer supported so that?s why I am working on it.
No documentation and had to draw up a basic schematic of the amplifier and this is the output stage in a ?simplified? version, the real thing has additional decoupling and unlike the drawing you are seeing it uses twelve 8122 tubes all in parallel.
The one Ohm resistor shunting to ground is what throws me, figure this amplifier is dissipating huge amounts of power to get the 100 watts of output and has no tuned circuits so its efficiency is marginal at best. Maybe five or ten percent?
Just thought I would pop it up here for others to see how to build maybe the most inefficient amplifier possible but it is broadband.
 

I don?t know what this is, but one look at the curves for an 8122 shows that a grounded grid and screen voltages of several hundred volts will render put this in a saturated state pulling over one ampere per tube even at 600volts on the plates.
See curve for 400volts on the screen at zero grid voltage in image.
Am I missing something?

[MikeKE0ZUinkcmo]

Make sure you are nice to the screen grids, because they quickly go PFFFFFTTT at around 8mA.

They worked well in AB1 with around -30 or so on the control grid.

[KD6VXI]

Sounds similar to a cable TV head end amp.

Super wide band with a near flat frequency response.

A Connor those where floating around about 25 years ago.  CB amp builders snapped them up for the parts.  Most of those where 250B based IIRC...  or maybe the coaxial F series.

--Shane
WP2ASS / ex KD6VXI

[WA1QHQ]

Those types of amplifiers are what I would characterize as laboratory equipment. I have personally used them for electromagnetic compatibility testing, where you would excite various parts of a peice of equipment to test it's suceptibility to electromagnetic radiation to see if it met the various mil standards for suceptibility. I am sure they have other applications but drying wood is probably not one  of them.

[W1ITT]

Amplifier Research equipment was lab and industrial stuff.  I used to use a 150 watt solid state AR amp in a high power network analyzer setup so I could tune HF curtain arrays withing a few hundred feet of other such arrays running 300 kw or more.  I also had a 25 watt solid state AR amp and neither one ever failed me.
The 8122 got its few minutes of amateur radio fame in the National NCL2000 amplifier and the NCX1000 high power transceiver.  They are hard to find lately so should be treated delicately.
73 de Norm  W1ITT

[Opcom]

The amp is called a distributed amplifier. Artificial delay lines are used to isolate the stages from each other while connecting them to the load. usually operated class A because there's no flywheel. Can be pulsed for the usuall reasons.

I have about 6GB of technical information on those, decided to study them for a few months. Also, own an IFI 400, which is a 50KHz-220MHz amp with six 4X250Bs. It's half of the schematic shown, which has 12 tubes.

Also a 100KW unit attached.

Tektronix made much use of the scheme for vertical deflection amps in its better old tube scopes.