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Old 13th May 2020, 9:47 pm   #1
sorell13
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Default Two vacuum tube radio kit FM radio from Aliexpress

Hi,

I bought this kit:-

https://www.aliexpress.com/item/32850244023.html

and I am trying to make it work, however without any success. My bad luck is that I did not learn anything in the Uni regarding vacuum tubes. Can anybody help me to make it work somehow?

Just to give some details:
- I have a background noise in the headphones but nothing happens when I modify the value of the variable capacitor;
- my antenna is around 2m long;
- it drags 90mA from the 1.5 V DC PSU (made my me using an LM713) and 12mA from the 67V DC PSU (also made by me using a handmade transformer, bridge and caps);
- lamp 1A2 does not get warm but it has light while lamp 2Ps is warmer but does not have too much light (I think it is something written in Chinese about this but I do not really understand).

Any advice is much appreciated!
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Old 14th May 2020, 12:00 am   #2
Radio Wrangler
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Default Re: Two vacuum tube radio kit FM radio from Aliexpress

Super-regen doing slope demodulation. If you've checked everything and had a play with that 20k pot (acts as the gain control for an unstable input amplifier) then there's not a lot left to do. It's very marginal at best. I would rate it unsuitable for learning about valves with. It'll need a strong signal from a relatively nearby transmitter and a reasonable aerial. It is also likely to radiate interference as you play with the controls...

Now there's an idea. If you have a normal, known good FM radio nearby, can you get it to produce interference on that receiver (tuned to a weak-ish station) it would at least prove you had enough gain and could cross the band with the 2-valver.

Those valves are very low power intended to make dry 1.5 and 90v batteries last as long as possible in the generation of portable radios before transistors displaced valves. The do not get hot that you'd notice. There is barely any glow.

Have you done n ohmmeter check to see that the filaments are intact.

Look up data on those valves and is the sum of their heater current ratings equal to 90mA? as an alternative check.

Look out for loose pin contacts in cheapo valve sockets.

David
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Old 14th May 2020, 9:34 am   #3
llama
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Default Re: Two vacuum tube radio kit FM radio from Aliexpress

There are cheap FM *transmitters* on ebay - intended for getting an audio input into an FM car radio. If you bought one of those as part of your test setup you'd know that you had a nice strong FM signal available. Then back it off to see how well it works with weaker signals.
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Old 14th May 2020, 11:41 am   #4
kalee20
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Default Re: Two vacuum tube radio kit FM radio from Aliexpress

Probably not the best recommended introduction to valves is a flaky kit running a complex circuit!

You say you learned nothing about valves at Uni - are they off-syllabus?

I'll bet super-regen is off-syllabus too. Of Armstrong's amazing inventions (FM, regeneration, superheterodyne, maybe others), the super-regen is the least-applied, and to me, the most astounding. Deceptively simple, and absolutely appalling mathematically (especially for slope-detecting FM!)

The two valves should draw about 150mA from 1.4V supply (if you have the option, 1.4V is better than 1.5V). The 2P2 (=DL92) operating without any negative bias is a bit naughty, excess HT current and distortion could arise. Personally, I'd disconnect the earthy end of the 1MΩ resistor and connect a 3V lithium cell, +ve to the ground line, -ve to the resistor, to bias the grid at -3V (no current is drawn).
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Old 14th May 2020, 6:26 pm   #5
sorell13
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Default Re: Two vacuum tube radio kit FM radio from Aliexpress

Thank you very much to all of you for your kind replies.
I will try with a small FM emitter; it seems that everything is all right, filaments are fine, no loose pins; there is no FM radio nearby, I think this is the issue.

Can anyone recommend me a simple radio maybe with 2,3,4 valves that I can try to make and to learn the principles ? Something with an already designed PCB in order not to loose time to draw it), I would like just to make it fast with a press and peel foil. The lower voltages, the better.

Also, yes, it is sad, these days they are not teaching anything related to those in the Uni. Everything is off-syllabus - regeneration, superheterodyne, even the basic valves theory. Of course, in my father's old courses I can find all these.. It is sad, tubes are used these days in professional electronics - in lasers thyratrons and for switching high currents and voltages usually..
Many thanks for all your help !
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