Unusual feedback in a home-brew receiver

Hi all

I have a very unusual receiver on my bench. It is a two-valve regenerative set made with two valves. The RF part is a 38 pentode and the audio amplifier is the pentode part of a 12 A 7.

I have reverse engineered the circuit and enclose it below. I assume that the receiver has been built in late 40's, although the schematic may have been published even in early 30's.

There are two questions:

1. The regenerative circuit is a series connected LC with about 10-400 pF variable cap. Is this common and what are the advantages / disadvantages of this design.

2. The screen grid of the 38 valve is connected to a voltage divider on the cathode of the 12A7. Is this some kind of AVC design? Without proper screen grid voltage the 38 operates more like a triode, doesn't it?

Can this receiver ever have worked??

Awaiting your comments,

Harri

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• Schematics (102 KB)

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Dear Mr. Lumme,

the schematic in general makes sense to me. The method of regeneration using a variable condenser in series with the tickler coil is known in German as "Leithäuser circuit". It was used, for example, in the Volksempfänger VE301W, where the coil is grounded and the condenser is floating at plate potential.

Even though the circuit has its drawbacks, it can be considered operational.

The voltage divider at the screen grid of the 38 provides a positive bias. I see no sign for AVC, as this voltage is only dependent on the dc bias of the output tube, not on the signal strength. 

If the tube type 38 is actually the power pentode of that name, it would have to be biased fairly low to be used as an audio detector, for not to break into constant oscillation. The 12A7, on the other hand, has a typical plate current in the order of 10mA, so the voltage at the screen grid of the 38 would be around 7V. The suppressor grid, which you are not showing to be connected, would be internally tied to cathode.

The arrangement of the circuit appears functional, even if the tube choice is a little strange, with a power tube for audion, and a low-power tube section for speaker output. If the set really was operated at 220V, the 12A7 (with 135V typical plate and screen voltage) would likely be overloaded.

 

Yours,

Nikolaus Löwe

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Vielen Dank Herr Löwe,

 

Thanks for your thorough response.

Regarding the 38 tube, I believe that it has been there from the beginning. From what I have heard of the history of this receiver, the builder was a radio hobbyist, but also a merchant ship radio operator.

The build quality is better than average, but there are some strange component choices like the tubes with 0,3 A heaters.

The receiver had a original European 220 V power cord, so I assume that it has been plugged in at some time. The dropper resistor in the heater string is a massive ceramic body wound with asbestos supported resistor wire.

I have not measured the dropper resistor since it was open, but I calculated that it must have run very hot as long as it worked. 

I don't plan to restore it functionally, but cosmetically I try to get the best appearance. The wooden case is in good condition.

What is your expert opinion, would it deserve to make a model page from this radio to Radiomuseum.org?

Warmest regards,

Harri Lumme

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Dear Mr. Lumme,

if your set has a story to tell, I believe radiomuseum.org is the right place to tell it. And indeed, you seem to know the story!

The manufacturer to choose would likely be "unknown - custom built (Finland)", in short FIN_unknown (for naming the pictures). Please add all the information that you know, but leave the fields open where you are uncertain. The "remarks" field is the place for the background story!

Regarding the operating voltage: You have indicated an incandescent lamp in the heater chain. That means, the dropping resistor didin't have to burn the excess voltage alone. Is the lamp present, has it any markings, e.g. power rating or voltage? Would it be helpful in identifying the operating voltage?

Yours,

Nikolaus Löwe

 

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Hi Harri,

the circuit of the 12A7 diode section is dangerously wrong designed !
The full 220V line voltage is applied to the diode anode without a protection resistor !

First fault: this may destroy the 12A7- diode !

Second fault: the 12A7 penthode section should never operated on a supply exceeding 135 V DC.
In this case, it will be even more then 220 V !

A dropping resistor should be connected in series to the 12A7 diode anode to achieve around 110 V DC plate voltage.

I have severe doubts whether the 38 valve will work properly with its screen grid is connected to that voltage divider. I think this a mistake !

I consider this circuit was designed by someone who has little knowledge about tube radio design.

Best Regards,
Jacob

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Dear Jacob,

Thanks for your comments,

I totally share your doubts on the functionality of the receiver, and as already mentioned I have no intention to bring it to life again.

As a detective story this is still somewhat interesting, as I find it hard to believe that the builder chose to design and build this nice looking set (see the enclosed photo) without a fair conviction of it eventually working. Therefore I assume that he copied bits and pieces of circuits from contemporary sources but used no pre-existing complete radio design as a basis.

Especially I'm intrigued by the screen grid voltage circuit of the 38 tube because I never encountered any similar designs. Practically everything about the 38 tube is out of the ordinary: Plate resistor is huge and no grid bias.

Take care,

Harri

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• Case_front (214 KB)

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