KW distributed amp 2-28 MHz

[Opcom]

This has always been very interesting to me. I have an old "Instruments for Industry" model 400A / M-400A Broadband RF Amplifier, which is a distributed amplifier using six 4CX250B tubes. IFI, or there succesor, has no data because there was a fire years ago. I am not sure how much power it makes, but it is likely very wideband. I have some pics, sorry none of the plate circuit right now, but basically there's a termination at one end and a coil between each anode, till it comes out the N connector. The power transformer was rated for about 1.5 amps/2500VCT, but I do not have it any more. There's also a distributed amp thread on the GB list right now.

pics of my little amp here:
http://www.bunkerofdoom.com/lit/ifi400/ifi400.html
I guess I should do something with it.

Attached is some material I found, it explains pretty well how these things work.

[ka3zlr]

This has always been very interesting to me. I have an old "Instruments for Industry" model 400A / M-400A Broadband RF Amplifier, which is a distributed amplifier using six 4CX250B tubes. IFI, or there succesor, has no data because there was a fire years ago. I am not sure how much power it makes, but it is likely very wideband. I have some pics, sorry none of the plate circuit right now, but basically there's a termination at one end and a coil between each anode, till it comes out the N connector. The power transformer was rated for about 1.5 amps/2500VCT, but I do not have it any more. There's also a distributed amp thread on the GB list right now.

pics of my little amp here:
http://www.bunkerofdoom.com/lit/ifi400/ifi400.html
I guess I should do something with it.

Attached is some material I found, it explains pretty well how these things work.

Dude, you got a Dell, (figure of speech)

 Hey man those puppies are gud for 165 Plate Agitated Watts..

 Yessum,  I really think you shud do something with that..Yes Sir...I really do...

[Opcom]

wideband ECM amp for those with annoyingly loud stereos comes to mind.. Ha. But the power requirements are a little extreme?

165 watts. from what, 2 to 400MHz? You are familiar with this beasite? What more can you tell me about it? I'm williing to listen..

[WA1GFZ]

I know that amplifier inside and out we have 2 of the "406" 1 KW versions at work and I rebuilt the finals of both of them. They are traveling wave amplifiers.
Your amp is the driver for the 1 KW final. Our amp does 1 KW 10 KHz to 220 MHz. It is very clean and runs class A if you want to or lower bias which is easier on the tubes. The 1 KW final runs about 780 volts at just under 7 amps
with 24 4CX250s. Six of them should do 250.
Check your screen resistors. One of our amps fried most of them. Also the bias bypass cap on the bottom of the input load shorted in both amps.
BTW that output coupling cap is all you need, no tuned circuits. The load resistors on the input and output must be functional. Great driver and room heater.
fc

[WBear2GCR]

Really?

6 x 4cx250s do only 250 watts?

Maybe in class A?

How about in class C? Just thinking out loud.
1.5 amps is strapping B+ @ 2500vdc...

                         _-_-bear

[WA1GFZ]

We usually run ours in class AB around 2 amps resting current.. 24 4CX250s in parallel is a low output Z. Also there is a load resistor across the output at the first tube while the last tube is just a coupling cap and RF choke to feed dc in. So you lose some power in the resistor. This helps with making it BB and handle weird loads. I'm thinking 6 in parallel is a higher output Z but in out amp at work the input to the final is 50 ohms so maybe the power required is lower. I know this configuration does not have high gain.  The 6 tube stage might want a bb step down transformer to get a better match. We have never had any problems with our driver so not sure and suspect we only need about 100 watts of drive.
We dump our amp into some weird loads the worst a biconical antenna 30 to 200 mHz. RG214 gets pretty warm from high VSWR. It is a BB dipole about 2 meters long with a balun on the input and no matching.

[Opcom]

Seeing as six could do a good job at 780V/1.75A, (1365W input), yow! that is a heater. 220MHz limit? Dang.. would otherwise be a great ATV amp for 440. There is much video equipment here.

The AB1 "RF Deck" idea of 2000V/1.5A has crossed my mind many times and the appropriate page of the RCA TT-4 manual is well worn. By the book that's 1770 watts output.

I have never run one at 2500V, I suppose it would take it on SSB, AM might be awful hard on them. -but by the looks of that "Class A" blower, the tubes could be kept fairly cool. None of the parts in that box seem to be rated for those kind of voltages. Plus, it might be a chore to tame six of these 400MHz-capable tubes in parallel.

The broadband 'as-designed' use seems a better use. Not sure if I have the iron just lying around. I checked and mine has 50 ohms as the line termination for both the output and the input.

What screen voltage are we needing here? It's not marked anywhere.
Can it really do only 220MHz? Any tech manuals for my little amp around? (hint hint..)

[w1vtp]

.....
What screen voltage are we needing here? It's not marked anywhere.
Can it really do only 220MHz? Any tech manuals for my little amp around? (hint hint..)

I think I used to calibrate one of those beauties in my former life as cal tech.  I'll ask the guy who is in charge of the lab if he still has the manual and yes, I still have the 2 KW 30 dB attenuator that I used for the calibration

Al VTP

[WA1GFZ]

Jacking the plate voltage will raise the plate Z. also the coupling cap may not handle the higher voltage. I would have to look but I seem to remember the screen voltage is normal for AB. I'll check our amp if I get a chance today.
This amp is intended for lab use so modifing it may mess it up. All the tubes are not in parallel but traveling wave amp. There is an inductor between each grid and plate as a big delay line. I think you want to run the DC input so the tubes are at around 100 ohms DC input.

[Opcom]

Guys I really appreciate your help on this. I have plenty of tubes, will keep collecting them up..