10-09-2021, 05:48 PM
(09-09-2021, 12:22 AM)Amie Wrote: 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
