Rod Pentode TRF

[Murphyv310]

It's interesting, but my curiosity doesn't extend to operating them while detonating a small yield nuclear weapon in the potting shed

Why ever not Amie?
Youll need to give it a go, mind and put it on YouTube though

[Amie]

OK then Trevor!

I'm  thinking the guys who have  the rod pentodes  on sale may be able to put me onto a  supplier of enriched uranium: I don't think that yellowcake UF6 is a natural mineral resource around here! 

[BusyBee]

Thanks for the information Trevor, the comparison between the types is interesting.

I'm actually very surprised at your comments about the rod pentode being microphonic I have found them quite good in that respect, though at massive gain even the interconnections in a circuit can become microphonic.

Tracy... Yeah, I see your point about the valves working on beam principles, I have planned a comparison, I have one or two subminiature RF pentodes (straight) that I want to try as comparison with the Ruskies...

I did try their, (1j24b), sensitivity to an external magnetic field: it did not seem to work as an introduction of  signal, also bringing a tiny bar magnet near it stopped it working completely, I'm assuming that the rods are nickel, and became magnetised, as I had to put a coil of wire onto my old solder gun and degauss the valve to get it working again... I could see how it could work if I'd found a way to sensibly introduce the field but it could only be AC and low frequencies, as they seem to retain magnetism...

Hi Amie,

I was reading through various articles on rod pentodes today which stated one of their advantages is their being less microphonic than conventional types (and far less noisy too) so maybe it was a rogue valve that Trevor had?

This afternoon I wanted to try an oscillator circuit I have been thinking about and decided that the unscreened 1j37b I have already setup on a board would be suitable as it's early days and I just wanted to test principles. I set the circuit up with both g1,s together. All worked as required.

While I had everything setup, and with the oscillator running at around 3.5MHz I got a fairly powerful magnet I have (around 2" x 2" x three quarters of an inch) and tried moving it in for effect. Although testing was rather crude I did determine:

The results I got were similar to yours in that the oscillatory output from the anode stopped when the magnet was within an inch or so of the valve.

The output level did vary downward as the magnet was moved in.

The results I was getting appeared to agree with the normal laws of fields, ie as the magnet was moved in the signal reduction rate increased ever more rapidly.

The effect did not appear to be affected by magnet positioning perpendicular to the valve from any direction but from either end there was some minor effect but very much reduced (no cutoff).

Measured oscillator output appeared to be fully restored (even from total cutoff) when the magnet was taken away (on test the magnet was never moved to actually touch the glass of the valve).

From my feelings about the results I think that it would be possible to modulate the output to some extent as there does appear to be variation with field strength. Obviously more testing is needed. I also did not appear to have any permanent (even fleetingly) magnetisation but I presume frequency checks (hysteresis etc?) would show this.

Tracy

[Amie]

I'm beginning to believe that not all Rusky rods behave the same, I must admit I did shove the magnet pole up close to the side of the valve, and it is a small but powerful one... .... Actually just went and tested it... Its a neodymium magnet about 6mm diameter 200gram pull away...

I don't have them here, but I have some large reed actuator coils that may serve as a modulator, I will think about the way it may affect the electron beam in the valve. I have the impression it is like a flat "ribbon" across the section between the rods to the anodes.

I do know what you mean about the very small clearances I did read some of Michael Watterson and Joe de Suzas work, I would think that it would be easier to warp the structure if overheated rather than mechanically bent.

I don't like to destroy these things but I must admit being impressed with Trevor's lopt stage using the 1p24s with the rods glowing!

If I get hold of any of the magnetically susceptible, I may try a minimalist pantry transmitter...

[Murphyv310]

Hi Tracy.
Only on one occasion did I notice a permanent drop in output. I was doing the same and the phone rang, I left the magnet a couple of millimetres away from the valve for at least half an hour. On removing the magnet output was down, not massively by any means but yes a drop..
The use of a tape head demagnetiser passed over the valve a few times and a gradual increase of distance brought it back up. Again it was the 1j42a in my pantry TX. from memory it was like a drop from 5v p-p to 4.8.

[Kalee20]

I haven't had any problems with microphony, or with screening, of 1j24b's.

I did use a turn of 0.002" copper foil with a grounding lead soldered to it, around me 1j24b's in a 2-stage 470kHz IF amplifier (the valves being 2" apart - I just measured it!), but on looking at the sweep response and taking off the foil, there was almost no difference - and the very slight difference there was, could be nullified by a slight tweak to the anode tuned circuit IFT, which I'm therefore putting down to slightly increased capacitance direct from anode to foil.

This is obviously electrostatic screening, rather different to the steady magnetic screening which a mumetal shield would provide, and Amie's findings are not to be ignored - but it does seem unnecessary in my circuit.

[Murphyv310]

I'm beginning to believe that not all Rusky rods behave the same, I must admit I did shove the magnet pole up close to the side of the valve, and it is a small but powerful one... .... Actually just went and tested it... Its a neodymium magnet about 6mm diameter 200gram pull away...

I don't have them here, but I have some large reed actuator coils that may serve as a modulator, I will think about the way it may affect the electron beam in the valve. I have the impression it is like a flat "ribbon" across the section between the rods to the anodes.

I do know what you mean about the very small clearances I did read some of Michael Watterson and Joe de Suzas work, I would think that it would be easier to warp the structure if overheated rather than mechanically bent.

I don't like to destroy these things but I must admit being impressed with Trevor's lopt stage using the 1p24s with the rods glowing!

If I get hold of any of the magnetically susceptible, I may try a minimalist pantry transmitter...

Youll not get any more minimalistic than using either a 1j37b or 1j42a. Although Michael said that I was using the wrong way to do it I got good success with the 1j42a on Medium wave.
Have a, look here......... https://youtu.be/v1D0D8qTHjs

[Amie]

Hi all .

What I did was pretty extreme, I don't think that unless you are going to stuff these bottles next to a mains transformer or similar its not going to matter, a low level phono amplifier stage then may need screening possibly?

[Amie]

Youll not get any more minimalistic than using either a 1j37b or 1j42a. Although Michael said that I was using the wrong way to do it I got good success with the 1j42a on Medium wave.

How is working wrong?

Do or do not do, there is no try

[Murphyv310]

The issue was it was against convention.
Theory says its inefficient and could be better.
Being fair in comparison to a minimod it's far better in the respect of audio quality, less hissy and a tiny component count.
I'm going to try the 1j37b in a very similar circuit.
The original was running for over a year virtually continuously with it going off when the Ni-Mh AA went flat. The PP3 only dropped 0.6v in that time.