[Boatanchors] Re: [The WS No.19 Group] VC radio tube questions

[J. Forster]

Ed Lyon wrote:

> Likely that the converter stage might use 1AC6 rather than 1R5 or 1L6,
> according to the pinout given,

The tube in the socket is a 1R5, but there was an error in tracing out the
wiring, and pin 5 went to what appeared to be a screen supply. It turns out that
supply was, in fact, connected to pin 3 not pin 5, so the 1R5 is a reasonable
choice.

> and the IF stages might rather use 1T4 vice
> 1L4, since the 1T4 is remote cutoff, and would behave better if there is an
> AGC circuit. (Look for a heavily-filtered lead running from the last IF
> secondary back to the control grid returns of those IF stages.)

There is no AGC. The front panel control appears to be an RF gain control and is
a pot connected to B+ and to a 100K to Gnd. The wiper is connected to the G2 of
the 1st and 2nd IF tubes through a 10K series R and  a 0.05 to Gnd. Again, the
1L4s appear to be reasonable choices.

> In the below message, the question marks regarding pin 6 of the output stage
> have
> an unclear meaning.  But otherwise, a 1S4 seems to work, as it is an output
> pentode, and pin 6 is tied to the plate, same as pin 2.

The ?? were because that pin is connected to B+ through a small capacitor. Since
the tube was unknown, I could not ID the pin function, but guessed it might be a
grid. It turns out to be an alternate plate, and the small cap is across the
primary of the output transformer. The 1S4 is also a reasonable choice
therefore.

> Pin 4 is the screen, and pin 3 is the control grid.

Thanks very much for the suggestions,
-John

> Ed Lyon
>
>
> Some time ago Bill Howard was asking some questions about reproducing a
> captured VC radio. Since then, we've been working together on reverse
> engineering the
> set he has. There is some question about whether the set has the correct tubes
>
> in it now.
>
> A brief set description:  The set is basically a 5 tube superhet. The first
> tube is a pentagrid converter. There is no RF amplifier preceding it. The LO
> is
> uses a three winding air core coil. The primary is bifilar wound in the
> filament
> circuit, the secondary is parallel tuned tank (tuned by a section of the
> main tuning capacitor). The RF input is from a 1:4 step-up air core
> transformer,
> the secondary of which is another parallel tank, tuned by the other section of
>
> the main tuning capacitor.
>
> The IF is composed of two 1L4 tubes and three double tuned circuits. Each IF
> tuned circuit is composed of two cored coils, each resonated by an external
> capacitor. Coupling is via a small capacitor, bridging the input tank to the
> output tank. The result is an IF coupling network of LC - C - LC.
>
> The output of the last IF goes to the diode plate of a 1S5, the other part
> of which acts as a gain stage. The audio output is capacitively coupled to a
> pentode output stage and a filament (most likely) transformer used as an
> output transformer. The final tube is either a BFO or some variety of noise
> blanker, but the circuit is unclear at the moment.
>
> The questions:
>
> In the first stage, the tube appears to be a 1R5, except that a 1R5 has pin
> 5 internally tied to pin 1. In the circuit, pin 5 is tied to a decoupled b+
> through a 15K resistor and a 0.2 MFD bypass capacitor to ground. Does anyone
> know of another tube, similar in most respects to a 1R5, but which would
> likely use a decoupled B+ to pin 5?
>
> The next question is similar, but concerns the output stage. It is clearly
> supposed to be a pentode, with the screen on pin 4, connected to B+ through
> a 22 K resistor. So, does anyone know of a tube like a 1L4, but with the
> following pinout:
>
> 1 & 5 tied, Filament
> 2  Plate
> 3  G1
> 4  Screen
> 6  G2 ??
> 7  Filament
>
> It IS possible that the correct tubes may NOT be US types. They could be
> Chinese or Russian  or their variants of US types.
>
> Thanks,
> -John