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boiss · (This post was last modified: 13-09-2013, 03:10 PM by boiss.)

This was the first one which was based on a second hand PL36 from an old TV chassis that I found by the side of the road. The power transformer is a 2 x 15V toroid, one winding is used for the Filament and the other is full wave rectified and presents around 24V HT. The PL36 is operated as a pierce Xtal oscillator and works with crystals from 0.7MHz to 2MHz. Audio from an FM radio or other source is applied to the screen grid and the volume setting used to control the mod depth. Using the Earphone socket modulation levels were around 60-70% without noticeable distortion. I used this for testing a lot of old pre-war battery sets and just to be able to hear the music I wanted over my old radios.
Power output varied with frequency from 1mW to just under 5mw. Because there is no filtering, harmonics are also present, A 6 inch bit of wire for an Aerial (So much better than Antenna) and the job's done, range a few meters, with a longer one then the next room is also possible.
Just dug it out, It is a bit the worse for wear (after 7 yrs of being forgotten) however I plugged in the valve and 1MHz crystal, put some mains on it and off it went, see the Sony photo with the led on !
I'll see what I've got on the other one as I've borrowed the B9a valve socket. It was built in an Inverted Ally baking tray. Has variable mod control, triode strapped EF80 as modulator and the Oscillator (Fit for purpose according to RCA) is an 813 with DC Filament and 28V HT. Power output <1mW into 50 Ohms

Refugee · 13-09-2013, 03:44 PM

It looks like it would work well with a bit of coax with a couple of turns of inner around the anode lead and the outer connected to the earth on the radio and the inner as normal.

**tiv48** · 13-09-2013, 04:32 PM

Do you still have the circuit ?
Rob t

boiss · 13-09-2013, 04:52 PM

I'll trace it out tomorrow pm but it's really simple and nothing is critical, It worked straight off with components I had lying around. the box was a tourist souvenir thingy with nautical knots on top,and was the most expensive bit. I have found that most O/P valves work really well on low volts ie 6V6s, 6L6s, El34s, DL91 etc.
The PL,UL etc ranges are quite good if you have mains transformers with odd volts so you can run the heaters and rectify for HT off the same winding.
12V upwards, although I once made a superegen with a 6V6 on 9V. On 25V they're really lively. With pentodes you can swap the grids around like g1 with +V and signal on G2 or G3 etc.
Staying under 30V also means that providing you stay clear of the mains terminals you are not going to get a lethal belt and the risks are basically the same as with transistor equipment.
Have fun
Mike

boiss · 13-09-2013, 04:59 PM

(13-09-2013, 03:44 PM)Refugee Wrote: It looks like it would work well with a bit of coax with a couple of turns of inner around the anode lead and the outer connected to the earth on the radio and the inner as normal.

I'm sure it would work quite well like that, provided the coax isn't too long. Put simply the Z is not matched and the C of the coax would attenuate the signal and could load the Anode cct.
(Pls don't jump on me for this simplification)
The other one I made was for 50 Ohm loads so that would be ideal. A matching circuit could also be made for this one .

boiss · 13-09-2013, 08:08 PM

Ok, I traced the Cct out but I found that it is running on 50V HT not 25 because ithe rectifier is connected across the 2 15V windings. Thinking back this extended the range of the crystals that would work, however it did still work OK on 25V at 1MHz.
Try it on the lower voltage and if necessary increase it (Voltage doubler). I think the L in the anode circuit is around 1mH. The Xtal coupling cap is not necessary but I prefer not to have volts on the Xtal.
Mike

Skywave · 13-09-2013, 08:27 PM

Mike - you need to amend your cct. diagram: you've omitted the cathode of the PL36: an indirectly heated pentode.

Al. / Sept. 13, 2013 //

pwdrive · 13-09-2013, 08:43 PM

And the supressor grid.

Lawrence.

boiss · 13-09-2013, 08:59 PM

Thanks Al, Lawrence,
Well spotted ......................
I've corrected it but i haven't got any tippex.
here tis again, gonna have to start with one of the cct drawing tools.
Mike

pwdrive · 13-09-2013, 09:08 PM

Your writing is neater than mine....

Lawrence.

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