Rod Pentode TRF

[Murphyv310]

Hi.
Well today I've done some more. The line up is now 1j42a, 1j24b. 1j29b which is the AF output. I'm reluctant to move off the 1j42a as the RF amp and regeneration.
Performance is indeed quite spectacular with strong audio driving a small speaker on the stronger stations. DX is easily possible using phones and its quite possible that you can damage your ears on the strong stations at maximum volume...... The disadvantage of no AGC.
I've done my usual YouTube video here, https://youtu.be/tszXBdxuEhA

HT is 54v and current drawn is just a tad over 5ma.
Im using both filaments on the 1j29b in parallel.

[BusyBee]

I had noticed that the 1j24 screen was very sensitive to hum test if not decoupled too. I wonder if they would work in space charge mode at low anode volts...? (With a positive bias on g1).

An excellent video as usual Trevor. Your success leads me to thinking about trying that front end myself, at my usual higher voltages. I may try a single loop on a rod aerial I have already and see how it goes.

It would be interesting to hear your results if you tried that Amie. These valves do apparently work more on beam principles so there may be issues with de-focussed beams but there is nothing better than trial and error

Tracy

[Murphyv310]

Hi.
Today I bit the bullet and tried a 1J24B in the RF stage. The result was mixed. Sensitivity was up but G2 regeneration didn't work. In fact the radio worked exceptionally well. Spirit Radio on 549 was very strong as was BBC R4 on 720. There was some bleeding though from 1035 my local and 810 BBC R Scotland. Winding more turns around the ferrite rod didn't help. Increasing the anode load resistor value brought back regeneration but output was then no better than the 1j42a and was less stable, the valve was fed from the 54v HT line, it also was badly microphonic. I then tried a 1j18b it did admittedly work more like the 1j42a but the reaction was far from smooth and was peaky, tuning was also displaced, perhaps caused by a different G1 to F capacitance. Again without reaction the output was excellent but did suffer with bleed from the strong stations.
I've now returned the RF stage to the 1j42a. It is lower output on the low level stations on speaker but plenty of volume on headphones.
So I'll be now leaving the radio as is.

Next projects in my mind is another pantry transmitter using a 1j37b. Also a superhet again with 1j37b, 1j24b x2 and perhaps a 1j24b, will see. Lastly a single valve FM radio, I've already made one with a 1j29b using super regen. Not sure as yet what way I'd go with it but may use a 1j37b. Don't hold your breath though since covid restrictions have been lifted we are back doing some charity work so time is more limited, this week though some external work is being done on the house so I've not been out and have had time to play.

[Amie]

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...

[Murphyv310]

One reason they should be in a mu metal shield.
A tape head demagnetiser works fine.

[Amie]

Thanks Trevor!

[BusyBee]

It's interesting Amie regarding your magnetisation. It did make me wonder about Trevor's variation in results with similar valves. As far as I remember when I experimented I didn't get anything like that. The field had minimal, but noticeable effects but was not predicatable. I think it was a 1j37b and would have been in a circuit of HT 60V. Thinking further on this, and the fact that they are beam devices, two things come to mind. The first is that the magnetic field orientation will likely have varying effect dependant on orientation (unlike a traditional valve in which the electron flow is radial). The second is the actual dimensions. The electron path is very short so a large magnet external to the valve will likely have little fine control, just having a swamping effect at high field, as you have found. To me it is really thinking like a CRT in miniature. On a normal CRT the beamlength is very long. In these it is of microscopic proportions so any field would have to be focused to an extent difficult except in a laboratory.

Another point I wonder about is, if any magnetic field is strong enough to affect the beam, if the parts are magnetic would a high enough field warp the elements? The things were (so it has been said) immune to an electromagnetic pulse. Maybe what we are coming across goes against that? I will finish now as I am perhaps going too far lol.

Tracy

[Murphyv310]

Hi Tracy.
Good point about an Electromagnetic Pulse. To be fair the effect is very marked on the 1j42a and less so on the other types.
I see another YouTube video coming on

[Amie]

Hi Tracy.

I think if you were you introduce enough magnetic force to distort the rods in one of these valves it would be a phenomenal EMP, and burst the envelope

I'm quite sure if there was enough sensitivity, as Trevor said with the 1j42a, then an alternating audio signal with a net magnetising effect of zero wouldn't cause a problem.

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

[BusyBee]

Hi Tracy.

I think if you were you introduce enough magnetic force to distort the rods in one of these valves it would be a phenomenal EMP, and burst the envelope

I'm quite sure if there was enough sensitivity, as Trevor said with the 1j42a, then an alternating audio signal with a net magnetising effect of zero wouldn't cause a problem.

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

Hi Amie,

I thought my mention of the EMP may have been a bit too far, but was really just a passing thought as I am not really conversant with the physics of them. I take your point about the AC field.

My thinking on the 'warping' of things was perhaps more with some of these really powerful mini magnets that can be obtained these days. I would think that any small magnetic structure would be affected and, with their small working dimensions, a very small flexion, permanent or otherwise, may cause what you have found. It does make me wonder whether the field is affecting the electron beam or the structure hence the beam indirectly? Perhaps an electrostatic field (compare oscilloscope CRT with scan plates and TV with scan coils)? Maybe I go too far again although this discussion is giving me one or two thoughts

Tracy