"The suggested mod uses high current
FETs (12 or 15 amp). Just doesn't seem correct.
Looking at the schematic it shows 165 ohms in series with each plate,
with a B+ of 53V, the current would be limited to 320ma. "

Correct. Actually there is nothing really "in series" with the plates. They operate electrostatically, the only load they impose is capacitive. the way they have it congigured is apparently they have electrodes at each end of the plates, for whatever reason. I do not see much advantage in it. the plates are not all that large, and 270 Mhz's wavelength is about 1 meter.

The schematic to which you linked is not what this thing uses. It already has pushpull drive coming in. The transistors that drive it have their bases directly connected and the emitters are fed. Maybe, MAYBE the chip only puts out a single ended signal, but I really doubt it. Even looking back to the delay line, which is literally a roll of coax, it isa actually to rolls of coax. That would seem to indicate the signal is already fully diffeential at that point.

However people there and at places like tektronix are anal like that. Of course they have their reasons, some of them have to do with rejecting interference, and some of them have to do with avoiding the generation of interference.

I just thought 2100 pF was a bit much, but then across a 1K resistor and feeding a 160 or so ohm resistor, it is not all that bad.

Incidentally, that schematic in your link makes a mistake that is one of my pet peeves. the fact is that it is incorrect to call a phase inersion 180 degrees out of phase. A phase shift is when you delay something. If you delay a 1 Khz wave by ½ a mS, that is 180 out of phase. If you send it through an inverting amplifier it is inverted, not SHIFTED in phase. Delayed by 180 degrees and inverted are only the same on waveforms that are bidirectionally symmetrical, such as a sinbe wave, or a 50/50 duty cycly square wave or ene a pefect triangle wave. Only things like that which only contain odd order harmonics, it does not apply to any waveforms that are non-symmetrical BOTH WAYS.

But I didn't come to rant about that. The point is that in most scopes, the phase splitter doe not work like that, which is like ones in old tube audio amps, but not even all of them. Some of them actually used a tube that was high gain and configured about like an inverting OPAMP. Two equal resistors, the junction of which is at the grid of the invertor stage resulting in unity gain. Most scope amps don't even do that, they just make either the emitters or bases of a pair of transitors common and feed one of the elemant while bypassing the other. this causes it to invert, and after a coule of stages of the the CMR is high enough. (common mode reduction, not rejection ratio)

If you have good enough transistors, one stage does it well enough, but in these instruments, after the first phase splitter, ALL stages are differential and they ALL have some CMRR. It all adds up, and you get a perfectly balanced signal.

Interestingly, they think this has been reinvented with the way the signal gets to the display on some TVs now, called LVDS. The same scheme is used in the USB, HDMI, firewire and other signal transmission methods.

It is about the same as the old twinlead anrenna wire back in the old days, just refined. It doesn't radiate alot and it rejects noise. by keeping the signal perfectly balanced and at a low lever, capacitive effects of the transmission line are also minimized allowing for greater bandwidth.

But it is still a rehash of the **** that came out befor eI was born, and that was 1960. Circuit boards and IC were still forthcoming back then.

So they haven't really invented much, they have just refined it some. boils down to, for one, you are right, the current needed is not that high, but then if it happens to have the good rating then it does. I guess it will not blow as easily. I found that in Digikey's selector guide. They are talking 12 and 16 amps, I don't need anything like that even if there is a friggin capacitor across those plates. thing is I was looking at TO220 type cases because I believe they will make it alot easier to build the thing. there is only so much room for mounting and I don't want to leave them hanging by their leads which might only be .040" in diameter, and then maybe use silicone to keep things from shorting out.

Anyway, back to the drawing board on finding a MOSFET. One thing I do have to do is ggo to Electronic ****house (ESI in Mentor). REALLY, I mean no disrespect calling them that, my Uncle came up with the name. He used to go there every Saturday for quite some time. That is a long story. His kids (my cousins) built their own stereos.

Anyway, I tried to go to their site and it was down at the time. I will try again and see. Since the scope is old enough for a military pension, maybe the parts shouls be older as well. However the MOSFETS will not be as opld beecause they did not exist then.

You know how I know that ? Because HP would have used them ! lol. Those engineers have no clue of costing, they are trained that way. If the thing costs ten grand, so what ?

OK, enough chit chat for now. I am offf to do something. No idear what but........