Premier Radio 3 valve TRF receiver.

[Murphyv310]

Something odd bout the Midgetronic circuit.
The slider of VR1 goes to earth so when its at the earthy end it is effectively shorting out the aerial coil and damping the whole of the rf coils thus no signals.
I'd suspect VR1 to be low resistance anyway, around 1k so there will be damping of the aerial coil which inevitably will reduce signal and selectivity.
VR1 should surely be grounded not retuned to the top of the aerial coil?

[Geordie McBoyne]

Hi Trevor, It's not a particularly nice arrangement having the volume control progressively bypassing signals to ground when turning the volume down.
Anyway, the Premier Radio uses a vari-mu RF amplifier valve so there should be no necessity to employ such an arrangement, things might be different when a sharp cut-off valve like the SP61 is employed. I've disconnected the aerial input coil from the volume control and now it works in a much smoother manner as well as a slight improvement in sensitivity.

Geordie McBoyne.

[Terry]

Same with the three valve version, Trevor.

[Geordie McBoyne]

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

Hi Tracy, I guess the Midgetronic radio gets it's performance by using hi-slope pentodes. The crystal diode detector will introduce damping of the tuned circuit, something better is needed and I'd suggest the infinite impedance detector. A triode-pentode can replace the SP61, an ECF80 perhaps. For the RF amplifier the vari-mu EF85 can be considered, or even the EF183.

The cabinet of Midgetronic radio resembles the Mighty Gem battery radio made by the Majestic Radio Co. This set was marketed in the UK. Uses the 1D8GT diode-triode-pentode tube.

From the Radiomuseum: https://www.radiomuseum.org/r/majestic_mighty_gem.html

Geordie McBoyne.

[BusyBee]

I have just taken another look at the Midgetronic. Thanks for your helpful hints Geordie. From the diode onward I have no issues, but I find the RF stage confusing too. The volume control looks like a combination of input attenuator and cathode RF gain. In my view the coils used in this set may well make all the difference. In my circuit so far I have used manual RF gain on the cathode, but I don't think I would try to combine with anything else. Currently with my work I have now constructed a variable frequency RF stage and coupled it to one section of a 3 gang variable capacitor. I have tried it with my RF stage and detector and got useable results over most of the medium wave with several good listenable signals. It currently tunes between about 590 and 950 kHz. My next step is to see if I can get input and amplifier tracking together.

Tracy

[BusyBee]

For interest, I went back to basics and did a readup on TRF recievers in the Radiotron Designer's Handbook (4th edition). The volume control arrangement, as in the Midgetronic, is shown, with a small explanation. The emphasis seems to be minimal cost, with control of valve biasing and low minimum volume for strong local signals. See AC Operated Receivers, page 1256.

Tracy

[Geordie McBoyne]

Hi Tracy,
You'll no doubt find many TRF construction projects in the 1950s Practical Wireless magazines. The type of volume control we are discussing was often used in contributors designs.
Getting back to the Premier Radio, the vari-mu RF amplifier valve's screen grid is now supplied from a potential divider. The upper resistor to HT is 27Kohms and the lower resistor is 40Kohms, (actually 18K and 22K resistors in series) The screen grid is decoupled by a 0.1mfd capacitor.
Doesn't seem to be any improvement in performance but at least the valve is operating according the specifications outlined in the Brimar valve book.

The anode bend detector valve is a 6J7. The slope of this valve is only 1.25mA/V. Any high slope pentode would be better but I don't know of any high slope valve in the international octal range with a top cap control grid connection. The Mazda SP61 has a high gm but has the Mazda octal base.

Geordie McBoyne.

[Geordie McBoyne]

Found a high slope octal pentode for the anode bend detector function, the Marconi-Osram Z66. The example I've fitted in the Premier Radio is actually a Mazda SP61 on an international octal base. Gives much better performance than the 6J7G.

Geordie McBoyne.

[Ed_Dinning]

Hi David, I don't think that Premier were that concerned about the valves or bases used in their sets.
That is not judging by the variants that appeared on the market.

Ed

[Geordie McBoyne]

I don't think that Premier were that concerned about the valves or bases used in their sets.
That is not judging by the variants that appeared on the market.

Ed

Hi Ed,
I reckon it'll be quite in order to fit a Mazda Octal base and use an SP61 for the detector.
The SP61 has a slope of 8.4mA/V, good figures for what is a 1938 valve.

Geordie McBoyne.