Of Aerials, Aerial Amps, Russian Tubes and Video Amplifiers

[BusyBee]

I thought it was time for an update on some of my progress as I have been working on something (amongst many others) which may be of interest, or spur ideas. I know this is not specific to Vintage equipment but uses quite a bit of information from that era.

The project itself was for a new aerial system to replace my unwieldy large frame aerial, and uses a ferrite rod aerial plus pre-amplifier (loosly in conjunction with a smaller frame aerial).

As the writeup is large I have provided it as a pdf file, along with the spreadsheet described within.

I think it is fairly self explanatory but don't hesitate to ask / comment as you feel. I cannot rule out mistakes or false assumptions but the implemented system does work well as described.

Tracy

[ppppenguin]

Definitely a masochistic approach to design broadband amplifiers with valves when transistors make it so easy.

You've used Hutson's book as a reference. The other book that looks at valve video amplification is Television Engineering by Amos and Birkinshaw. It's volume 2 of the 4 volume set that you need.

Ultimately normal valve arrangements run out of steam for wideband amplification and then you need distributed amplifiers. Widely used in Tektronix 'scopes, with rows of ECC88 (6DJ8). I think they were first suggested by Percival at EMI before WW2. Wikipedia confirms my memory: https://en.wikipedia.org/wiki/Distributed_amplifier

[BusyBee]

Hi Jeffrey, Thank you for the references. I have one or two other books as well but I found the one I used easiest to follow.

Although I do acknowledge that it is probably a bit of an unusual path to follow these days I had no real problems following it, partly for the following reasons:

In the first, I seldom do anything in isolation and going back to my text, before building my test amplifier I did try one of my similar, although AF, modules which worked far better than I thought it would. I think that was down to the emitter follower on the output. I did notice in 'Hutson' that a cathode follower was well thought of so it seemed the way to go. One thing I have not mentioned was that I did some analysis of this module (more specifically the transistors) in LTSpice. I did not spend long on the valves using this software as it is not specifically designed for it and I was not sure of the accuracy etc of the models for valves I found online (transistor types being esily obtainable from the semiconductor companies). I did give some thought to RF operation of the emitter follower. For the valve part, there is really only the anode load,  anode choke and screen resistor which are significantly different than with the audio circuit.

Second, my circuit only really needed flat reponse from around 100kHz to 2MHz so is just taken from a video amplifier really than being one itself. In that way the capacitors I perhaps should have mentioned in the first part above are only relevant to this frequency range. The test circuit I built had a flat response over a much wider frequency range. Specifically my amp is fairly narrowband (perhaps bandpass?). If I were thinking about 20 -> 30MHz, for instance, I would not be looking just at video amplifiers.

That said, and thinking about transistors, I don't think it would have been so easy for me. I would be more likely to copy some indeterminate circuit online. I do find that much transistor circuitry can be much better, but in practice more complicated than initial views show.  I won't dwell here as I would need to look further but do agree with you in most aspects.

This afternoon I decided on a little test just to show (essentially) how right you are, which is an example for anyone contemplating similar.  Even before I started on this project I had bought, as I thought it would be handy at some point,  one of those cheap (£5) Chinese wideband amps (0.1 -> 2000 MHz) . This afternoon I wired it up for the first time in a slightly ridiculous simplified arrangement and got the displayed  results for radio Caroline with my comms receiver  (out of circuit showing direct aerial -> receiver and in circuit [amplifier working]). This test was valid but slightly ridiculous as the low input imedance of the amplifier does totally flatten tuning circuit reponse but as construction (solder power wires) and assembly took less than half an hour it shows for anyone that, for anyone just wanting a solution, valves are not the way to go. In practice the input would require matching to the aerial tuned circuit for optimum response.

No amplifier:
   

With amplifer:
   

Amplifier:
   

Tracy