Drive in G-G Amps.

[k4kyv]

The 30L-1 I have was bought by me many, many years ago from a good, reputable amateur radio store. (Amateur Radio Center here in Baltimore) One of the RCA 811A's had a hole burned in the plate, a couple of the others had noticable "hot spots" in their plates. It came from a local ham that was a "Collins Conniseur". It still has the same tubes in it to this day, and still makes full outpoot!

Which goes to prove that they are not hard to burn holes in the plates if one is not careful when tuning up.

That old 833-A that had been kicked round the shack for years, with the huge shiny spots on both sides of the plate structure and the sunk in spot on the glass where a hole was almost melted in the envelope, is one of the best modulator tubes I have ever used. Before I installed it in the Gates, I could just barely make 100% positive modulation.  With that tube in place, I can now modulate up to about 135% positive.  

Normally a BC1-T wouldn't be capable of modulating up that high under the best of circumstances, so in case anyone might be wondering, when I converted the transmitter I replaced the big plate voltage rheostat with some fixed wirewound resistors to drop the PA plate voltage a few hundred volts below that of the modulator plates.

Regarding the grounded grid amplifier, some of the driving power passes through the tube and appears at the output at all times, because the input and output circuits share a common load impedance from cathode to ground.  But I have never heard anything about it not being possible to overdrive a G-G linear.  On the  contrary, just listen across any band to the slopbuckets who are running amplifiers.  I'd say you can overdrive a G-G amp and cause splatter, even damage the tube if you get too carried away.  Even though most of the driving power might bypass the tube, if you have enough total drive to begin with, I'd say you could still have enough left to trash the band and possibly the tube.

Most likely what saves tubes in many such cases is that the ricebox itself is capable of only so much output and flat-tops beyond a certain point.  This trashes up the band but spares the grid of the tube.  Splatter on the bands probably results as much from flat-topping in overdriven exciters as from overdriven leen-yars.

[WD8BIL]

On the  contrary, just listen across any band to the slopbuckets who are running amplifiers.  I'd say you can overdrive a G-G amp and cause splatter, even damage the tube if you get too carried away.

And I'd say most of that overdrive is asking too much of the plate supply. They end up transmitting squarewaves when they run out of headroom then "snap" off the signal as the amp gets driven negative.

[WA1GFZ]

I think it will square wave on the positive peak

[WD8BIL]

Yes Frank, it will. And the slope at that point is, for all practical reasoning, straight up and then down. It's the downward slope that snaps things off.

[KM1H]

The GG amplifier needs lots more drive than grid driven due to the inverse feedback built into the design. The payoff is much lower distortion and stability with simpler circuitry.

Regarding the grounded grid amplifier, some of the driving power passes through the tube and appears at the output at all times, because the input and output circuits share a common load impedance from cathode to ground.  But I have never heard anything about it not being possible to overdrive a G-G linear.

Both above are correct and since the input and output of a GG stage is in phase the input power passes thru minus what is needed.

And I'd say most of that overdrive is asking too much of the plate supply. They end up transmitting squarewaves when they run out of headroom then "snap" off the signal as the amp gets driven negative.

That is true with the wimpy PS amps such as the SB-220 which will simply limit max power no matter what the drive and the grid becomes a dummy load and others on the band suffer. Others such as the 65# Dahl in the LK-800A will let you keep raising output power with more drive as long as you keep the load control tweaked to limit grid current.  At some point the 3X 3CX800A7's scream enough.
Ive had a few of the dual voltage specials with the 3CPX tubes come in here after a contest where they were run at 5kw out (thats dead key power from that PS) for 48 hours from a 150-200W xcvr and the blower run in the low speed position. Tubes are melted to the chimneys and solder has flowed out of the tank circuit connections. Often the tubes are still fine after a bead blasting to get the crud off and a change to ceramic or Teflon chimneys from whatever is in the Eimacs.

Carl
KM1H

[Ed/KB1HYS]

I thought that in and out were out of phase by 180 and that was why a GG amp did not require neutralization?

It would seem that there is a lot more to this relatively simple amp than is published.  I did some research before this came up and everything I read just said that GG amps pass unused power through to the output.  There wasn't much talk of overdriving them.  This could be why so many folks do and hose up the bands.   

Ah well, I am learning more every day, and that's the thing.

[WA1GFZ]

Pulling the cathode negative in a GG amp does the same thing as driving the grid positive in a grounded cathode amp. Too much grid current has the same effect and you saturate the tube in either configuration.
A GG is more stable because the grid is at ground providing isolation between the plate and cathode.

[k4kyv]

Plus, the feed-through effect acts as a swamping resistance on the driver stage.  It takes more power to drive the amplifier, but most of that power is not wasted, but passed through the final to the antenna.  This maintains a more constant load on the driver, to improve regulation of the driver voltage.  A slopbucket leen-yar is exactly like a class-B modulator for AM; the source of driving voltage needs to be extremely well regulated to avoid distortion due to the varying load impedance over the a.c. cycle of the driving signal, whether audio or rf.  The constant load placed on the driver by the feed-through reduces the percentage of variation of the total load impedance seen by the driver.

[KM1H]

A GG amp without an input network to provide a flywheel effect prevents a continuously varying load to the driver. This may be tolerable to a tube driver but doesnt work well with SS.

In addition it has been proven by Orr and others that an input network with a Q of 2-3 or a bit more improves the final IMD by 5-10dB over a simple .01 coupling cap.

Carl
KM1H