Rod Pentode TRF

[BusyBee]

the translated Russian documents mention that there can be massive variation of the characteristics of individual valves of each type. They point out that this is the reason for using a potential divider for g2. This may well explain the variation of results obtained with circuitry.

Not only a potential divider, but an adjustable one at that!

I've got an AF circuit using two 1j24b's, from same filament supply and same HT. Each has a 270k anode load. Each has grid-leak returned to 0V so zero bias (and measuring with a DVM it really is zero.

But to get the same 45V quiescent anode voltage, one needs 23.6V on g2 and the other  36.4V.

I'm sure I could run them from the same potential divider but then I'd need to provision adjustable bias for g1.

I haven't tried any other specimens, unfortunately it's a soldered-together circuit!

I'm just accepting that I'll have to have several trimpots.

I have conducted a few test over the past couple of days and my results seem to compare similarly with yours.

Please see the attached information.

Initially I got a single 1j24b and tested it at three different voltages. Please see the diagram. The nominal 100V HT is in pink, 60V in black and 20V in green. Only the anode and g2 resistors were changed, for optimum gain. Please note that these resistors may not be optimum as my switched range boxes have limited values but general coverage is Ok. The other components were fixed in value (generally part of my test gear). Input signal was set to 50mV p to p 1kHz sine wave. The output signal was measured on the output, both being taken from the oscilloscope screen so of limited (but acceptable in this context) precision. The input probe was x1 and output x10.

Below the diagram the three lines give the results of initial tests. Note – The distortion was that of the output waveform noticed by increasing the input signal and was very subjective, just really and indication that the amplifier was behaving sensibly and, as such, is not particularly useful for these tests.

From these initial tests I went on to test four further examples of the 1j24b at nominal 60V with the resistors determined for maximum gain from  the previous tests (100k Ohm for anode and g2 resistors). As a further test I tested each again with the anode load increased to 270 kOhm.

The results indicated a wild variance with the 270 kOhm anode load (no change of g2 resistors) so I then went on to do a set of further tests with a 47 kOhm anode load. This did give sensible results although lower gain as would be expected. For the last numbered sample (#4) I did afterward try a different g2 resistor and found increased gain.

As regards valve run in and warm-up, I found some variance in the first few minutes of running but after that, little change. That said I have not run things for hours. These valves have not been used since I have had them, and perhaps not for years but they are all NOS. I practice they seem similar to the western NOS I have as regards initial use.

Conclusions:

I have found great differences as previously reported and the results do indicate that individual setting up may well be beneficial. My samples were 1j24b's. I do wonder whether 1j24b-v's would have closer characteristics but I do not have any to compare.

If anyone has suggestions I have put the test valves to one side. If I think of any other tests that would be useful I will try but, at the moment, I'll let things sink in. If anyone has any question please ask as I may obviously have overlooked something.

Tracy