Premier Radio 3 valve TRF receiver.

[boater sam]

Love the metal valve range, they all seem to perform better than the glass ones and can be driven harder, like 6V6, till they are red hot.

[Geordie McBoyne]

If fact the metal valves were first valves to have the international octal base, the glass ones followed shortly after.

According to the data in the Brimar valves and teletubes book No 6 the screen grid voltage applied to a 6K7 should not exceed 125V. In the TRF radio we're discussing the screen grid is supplied direct from the HT line. So it follows it might be a good idea to supply G2 from a potential divider across the HT supply.

Info about the 6K7 from the Radiomuseum: https://www.radiomuseum.org/tubes/tube_6k7.html

The diagram shows a 47Kohm resistor in series with feed to the screen grid. This is not good practise because the screen grid volts will rise when the grid bias is increased. Much better to use a potential divider.

The same rule about screen grid volts applies to the EF89.
https://www.radiomuseum.org/tubes/tube_ef89.html

Geordie McBoyne.

[pwdrive]

I built a regen receiver that used a 6K7, worked best with about 20 volts on the screen, that was a Hartley configuration.

Lawrence.

[Terry]

How many of these sets have you got?

The diagram you posted shows the screen grid of the 6K7 fed from a potential divider comprising 68k from the HT rail and R11 (value not stated) to ground.

No trace of the 47k resistor you mention in your last post.

Edit: Just re-read the thread and post #5 in particular.

[Geordie McBoyne]

Hi Terry,
I was referring to the diagram shown with the information provided for the 6K7 by the Radiomuseum. The 47Kohm screen feed resistor is fine for static conditions but as the 6K7 is usually an AGC controlled valve a more stable source of volts is better for this vari-mu valve.

There is a similar looking set in my collection which has been rewired as a superhet. Circuit is now similar to the first generation Little Maestro series.


Geordie McBoyne.

[BusyBee]

Thank you Geordie for posting the circuit diagram. I am currently looking into TRF receivers and like to refer to a lot of different circuitry, but this was one to look at as I have a couple of metal cased 6K7's. At some point in the future I would like to put together something more vintage looking. With my more modern valves I am less concerned, but like the look of more vintage valves (I suppose they all are really?) in a more period setting.

Thanks Ed, I will definitely bear in mind for when I get into doing what I mentioned above, or refurb if a set comes my way. I keep quite a few things, and have built up a stock of variable capacitors. I often try to wind my own coils, but I know some are complicated. I would not likely try a wave wound, but have done such things as basket weave.

My usual forte is use of slightly newer valves in things I put together myself. I don't intend to hijack the thread, but I have currently just tested yesterday a basic setup on single frequency (648 kHz, Radio Caroline). with tuned 80 cm x 80cm frame aerial to ferrite rod tuned input to untuned EF183 pentode with manual gain control. Next a Russian miniature pentode (not rod) 6j1b-v with anode tuned circuit, then to a 6C4 triode working as an infinite impedance detector. I output to my bench audio amplifier. The MW coil on the ferrite I wound myself (as I did the frame aerial).  To be honest I was amazed at the results. I think a lot of it was in the aerial and input matching, but audio seemed just as good as my usual comms receiver or transistor radio, which I use daily. There was not a lot else during the day (I only varied the tuning on ferrite rod capacitor) but as evening approached I had to back off the gain (seemed like appearance of cross mod with the higher signal level. It's seems well worth looking further into, which I will be doing. Anyway I will perhaps open a thread in the future when I have more to share.  

Tracy

   

[Murphyv310]

Brilliant work Tracy.
I made a single rod pentode TRF a few years ago and with the G2 fed with a series resistor and an HT of 50v its possible to get it always on the point of reaction without any controls. Performance was spectacular.

[BusyBee]

Thank you Trevor. You work wonders with these single stage circuits. Without your experience it is difficut for me at the moment. I think clearer in single functions per stage, and it make faultfinding easier too. In that way I think my designs will be somewhat larger than most, for a while at least.

[Geordie McBoyne]

Presenting a two valve TRF radio which was offered in kit form by Electronic Precision Components Ltd. It's the Midgetronic home build receiver. It uses two SP61 RF pentodes and for the HT rectifier an ex-radar valve, the VR135, a strange choice of valve but I reckon it's up to the job as the HT rectifier.
A germanium diode performs the function as the detector, if one wishes to keep the set all thermionic then the EA50 or VR92 could be considered.
The attachments show the circuit diagram and some constructional details.
Info about the VR135:   https://www.radiomuseum.org/tubes/tube_vr135.html

Geordie McBoyne.

[BusyBee]

An interesting looking circuit. It looks like it would need a bit of adaptation for modern tehniques and components.

Tracy