12-07-2020, 04:44 PM
(This post was last modified: 12-07-2020, 04:58 PM by Mike Watterson.)
Something else is wrong!
1) A DC path works better.
2) A single Diode ACROSS the RF fed with a capacitor needs a series resistor before RF decoupling cap.
3) You need RF decoupling, or else the next valve is more RF detector than audio amp
4) The only reason the dual diode peak detector isn't always used for signal level or AM demodulation is the cost of a 2nd diode! It's always better.
The 1j42a was
a) A very late design
b) Hardest to make
c) Very specialist application.
So unless you really need 9V HT or the special difficult to use mixer action (which needs two valves for mixer and oscillator) the 1j24b is a better choice.
While a one valve mixer/oscillator does work (using same principle as VHF valve mixer/oscillator), using two valves is dramatically better, still less than 25mA (22mA typical) and it's close to Hexode-triode performance, same circuit works. See the DK96 -> 2 x 1j24b thread.
The 1j29b is superior for frequency response, transmitter PA, audio PA and RF front end with big overload margin, but 2 x 1j24b in Class AB about 1/2 filament current of one 1j29b and similar or more audio or RF power than a DL94. An entire superhet radio with 1j24b and 1N60 is possible. I know the 1j29b is about as good as EF80 for VHF, but I don't know yet if the 1j24b works.
For headphones or lower speaker volume using 3 x PP3 (18V end of life, approx 28V fresh) works and depending on the part of circuit then you use the full HT or the 2x PP3 point as HT for the g2, to solve the issue that a single series grid resistor isn't great, you need a potential divider (wastes HT) or an HT tap.
I've also considered 2 x 12V alkaline minicells for the g2 supply.
For serious volume you want 45V to 90V HT. The 45V with a push-pull AB dual 1j24b should be good enough. Also PP has the advantage any centre tapped primary mains transformer works as the standing currrents cancel. Use a low pass filter from the oscillator grid to get the -Ve bias for the output stage.
Edit:
And people were looking for the 1j42a for years before I found one supplier! It's rare. Though there are other rod tubes (maybe mostly from late 50s early 60s) that have not been seen.
1) A DC path works better.
2) A single Diode ACROSS the RF fed with a capacitor needs a series resistor before RF decoupling cap.
3) You need RF decoupling, or else the next valve is more RF detector than audio amp
4) The only reason the dual diode peak detector isn't always used for signal level or AM demodulation is the cost of a 2nd diode! It's always better.
The 1j42a was
a) A very late design
b) Hardest to make
c) Very specialist application.
So unless you really need 9V HT or the special difficult to use mixer action (which needs two valves for mixer and oscillator) the 1j24b is a better choice.
While a one valve mixer/oscillator does work (using same principle as VHF valve mixer/oscillator), using two valves is dramatically better, still less than 25mA (22mA typical) and it's close to Hexode-triode performance, same circuit works. See the DK96 -> 2 x 1j24b thread.
The 1j29b is superior for frequency response, transmitter PA, audio PA and RF front end with big overload margin, but 2 x 1j24b in Class AB about 1/2 filament current of one 1j29b and similar or more audio or RF power than a DL94. An entire superhet radio with 1j24b and 1N60 is possible. I know the 1j29b is about as good as EF80 for VHF, but I don't know yet if the 1j24b works.
For headphones or lower speaker volume using 3 x PP3 (18V end of life, approx 28V fresh) works and depending on the part of circuit then you use the full HT or the 2x PP3 point as HT for the g2, to solve the issue that a single series grid resistor isn't great, you need a potential divider (wastes HT) or an HT tap.
I've also considered 2 x 12V alkaline minicells for the g2 supply.
For serious volume you want 45V to 90V HT. The 45V with a push-pull AB dual 1j24b should be good enough. Also PP has the advantage any centre tapped primary mains transformer works as the standing currrents cancel. Use a low pass filter from the oscillator grid to get the -Ve bias for the output stage.
Edit:
And people were looking for the 1j42a for years before I found one supplier! It's rare. Though there are other rod tubes (maybe mostly from late 50s early 60s) that have not been seen.
