Golborne Vintage Radio

Try the 2nd diode. They are crazy cheap.

Hi.
I've loads of OA47 and some RS Gold bonded Germanium diodes and some GEX32 unused, also others with no markings. The GEX32 works very well with the OA47 being the worst.
I tried the EPAD ALD10090 fed from a home wound toroid with no output at all.

It's against the spirit of the project somewhat, but what about a 6AL5 / EB91 / D77 double diode, just as a trial?

Obviously it'll need a heater supply. But as thermionic diodes have an absolutely fantastic ratio of forward:reverse resistance, better than any Ge or Si device, it would be interesting to try. With, of course, the Mike Watterson grid leak!

Mike Watterson Wrote:

Amie Wrote:....I've seen diodes used as in Trevor's diagram occasionally....

No, I've never seen a single diode in an official circuit like that....

Just to answer the contradiction....

Well, I have, the Radio Shack/Tandy Globe Patrol transistor radio is an example. I have one.

Reading on the internet the consensus is that the circuit is known as a floating detector that relies on the inherent leakage of the diode to work, it isn't good practice obviously, but does ensure that the only load to the signal seen by the preceding circuit is whatever the detector is driving, probably the reason for Trevor's excellent results.

The Radio shack version was sold as a kit and a promisingly comprehensive manual was provided, but unfortunately the circuit description of the detector doesn't explain exactly how it works.

Also in this particular example because it's a regenerative stage preceding the detector and inherently non linear, the diode may be completely redundant anyway.

A web page that has the circuit and a link to a PDF of the manual can be found at:
https://people.ohio.edu/postr/bapix/RSglobeP.htm

Trevor:
As a suggested method if you wanted to construct a sensitive infinite impedance detector for the radio using those valves, could you use a floating supply for the filament? A separate LT cell for each valve wouldn't be a hardship....but then why fix what isn't broken?

Very interesting. Thanks for sharing!

Amie.

Hi.
Thanks Amie.
One other circuit that comes to mind with a floating diode is the radio circuit in the Philips EE8/20, it has no DC path.

(06-07-2020, 07:34 AM)Amie Wrote: [ -> ]As a suggested method if you wanted to construct a sensitive infinite impedance detector for the radio using those valves, could you use a floating supply for the filament? A separate LT cell for each valve wouldn't be a hardship....but then why fix what isn't broken?

I can't see how that does anything much as the filaments are the cathodes and the input impedance is much higher than regular valves.
They were used as the input stage in an Electrometer using also 1j24b due to very high input impedance which may be enhanced by the low current, thus low space charge.
https://www.radiomuseum.org/forum/1j42a_1j42a.html#2
The g1a and g1b tied together.

@Trevor I can post a couple of modern 1N60 letter post if you don't want to wait for China or wherever. PM, or better email address. You have mine, I think I got the plotter off you that I've only recently fixed. A tale for another thread?

Well, Philips and Radioshack/Tandy/Realistic had some nice innovative stuff, up to and including the FM Superhet IC with only 70kHZ IF & Op-Amp based RC IF filter (It uses the PLL detector as a tracking filter to reduce the FM deviation). Occasionally bonkers like the floating diode, or the Philips Valve set Unicontrol, an ergonomic disaster. Also Philps persevered with complex TRF using tunable Q/bandwidth (a TRF unlike superhet has a bandwidth varying across the band) and glass coil formers to get superhet performance.

Thanks very much Michael.
I have ordered them from a UK seller and they should be here Wednesday or Thursday.
I've now got a Pye B16T to restore and that'll take up my time for a good few weeks. I want to try another idea later with the radio using a 1N4148 with some forward bias. I agree that these valves have very high input impedance so any damping massively reduces performance.

The 1N4148 even with bias doesn't come anywhere close to performance of the 1N60 for a low level signal. We tried that and the signal type HP Schottky diodes, (also with and without bias). I'd maybe prefer the 1N4148 for an FM discriminator / ratio detector, low power SMPSU up to 100mA @ 100V PIV, ordinary low power rectifiers for bridges and charge pumps for AC linear PSUs, clamps, meter protection (though I might use 1N400x), AM Video detectors, RF switches where the DC biases are much higher than RF signal (PIN diodes are better as the RF can exceed DC) DTL logic, diode programming matrixes etc.

Mike Watterson Wrote:

Amie Wrote:As a suggested method if you wanted to construct a sensitive infinite impedance detector for the radio using those valves, could you use a floating supply for the filament? A separate LT cell for each valve wouldn't be a hardship...

I can't see how that does anything much as the filaments are the cathodes and the input impedance is much higher than regular valves.

Well, as an infinite impedance detector, you'd get good linearity and low distortion. I think it's a good suggestion, probably not one often seen because of the extra, floating, battery. But on the experimenter's bench, anything is possible!

(06-07-2020, 12:40 PM)Kalee20 Wrote: [ -> ]

Mike Watterson Wrote:

Amie Wrote:As a suggested method if you wanted to construct a sensitive infinite impedance detector for the radio using those valves, could you use a floating supply for the filament? A separate LT cell for each valve wouldn't be a hardship...

I can't see how that does anything much as the filaments are the cathodes and the input impedance is much higher than regular valves.

Well, as an infinite impedance detector, you'd get good linearity and low distortion. I think it's a good suggestion, probably not one often seen because of the extra, floating, battery. But on the experimenter's bench, anything is possible!

I'd try anything to improve the overall performance, to be fair it actually works well and is very much a usable radio even as it stands. It works far far better than any other regen TRF I've ever made, to receive what it does with a ferrite rod aerial is amazing even with no reaction it pulls in the BBC feeds from Westerglen that is 28 crow flying miles from my QTH. Even BBC Ulster can be heard without reaction, not useable though.