99's substitutes





Manufacturing electronic tubes replacements
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14. Testing the first ersatz: LO verification

As I have made only one substitute, I decided to test it in the local oscillator stage.
The operation is easy: plug in the power supplies, and a oscilloscope on the oscillator coils.
However, it should be noted an important point: as the test is done with only one lamp, you either limit the heating voltage ( "A"), either limit the current with a 22 ohm resistor in the circuit.
Without this precaution, there is a risk of burning the filament of the lamp!
Here's how to connect the power supplies (no need to connect the +45 V):

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The oscilloscope can be connected to pin #5 of Catacomb:

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The lamp is plugged in slot #2:

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Here waveform when the varcap is maxi:

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The frequency is 239kHz .... very strange ...

On the other hand, when the varcap is mini, the oscillation does not occur; If we want the OL works well, varcap tuning must be at least 10%, in this case, the maximum frequency is 543kHz.
Is there any problem?




15. Wiring another ersatz

During ersatz tests on RCA radiola III, I tried a 1L4 with success...
Maybe this lamp works too?
Wire it in triode mode (G2 = A) and adjust the heating voltage (1.4 V)
I decided to quickly wire an adapter, which is to say that I use a 7-pins miniature socket wired with a 4-nubs base, in this way, the lamp becomes replaceable.
Here the lamp, ready to be tested:

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16. Testing this second lamp on the LO stage

The results are identical: same frequencies. Just a small difference: this lamp does not allow -10V for -C bias, it requires minimum -7V. Otherwise, the oscillation does not start ever.

So, this proves that whatever the lamp, the oscillation occurs at really low frequencies .
Pondering a little, could we not imagine that the use of second harmonic of the LO produces the beat?

Consider the numbers:
varcap mini: Fmax = 587kHz (with the 1L4) multiplied by 2 = 1174 kHz
varcap maxi: Fmin = 239kHz multiplied by 2 = 478kHz

Assuming that we take the lower beating, it means that the wavelengths that can be received will be fixed between:
300000 / (1174 +42) equals 247 and 300000 / (478 +42) equals 577 meters.
Values that I think are quite acceptable.
To make sure it is the #2 harmonic which is used, I can measure the inductance of the coils of the local oscillator ...

To do this, simply unplug the 3 wires of the coils from the Catacomb, and the wire on the varcap too.
Then input a signal on the non-tuned coils, and visualize the signal on the other coils, tuned with fixed known value capacitor, then tuned with 2 identical caps (or one with double value).
Thus, through this little spreadsheet file, with 2 measures, I can determine inductance and capacitance of the coil.

Fichier xls  The coil inductance

The inductance is approximately 550 ÁH. The capacitance is around 30pF, that gives, with varcap tunde on max, a frequency about 250kHz, which is consistent with what I found.

Thus, by design, it is truly the second harmonic beating with the incident wave to produce the 42kHz IF.




17. Constructing other substitutes

I tried several other lamps tu substitute 99.

First, a 3S4 that has not yielded good results in LO stage,
then a russian 2SH27L which gives good results.

I made a 2░ ersatz of the first type (with a 3Q5), and then, surprise: no more oscillation! I had to remove the 180k resistor, i.e. wire it in truly triode mode.

Then I get a 5░ version that works well.

Summarize:

So I built a total of 6 ersatz:

Here is a picture of the lamps set:

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And the outlines of the 3 versions that I keep now:

Fichier pdf  99's substitutes





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