30-06-2020, 10:07 AM
(This post was last modified: 30-06-2020, 10:08 AM by Mike Watterson.)
A diode doesn't load. It's the resistor after the diode that loads.
We did extensive tests in a lab. Best RF detector is a 1N60. Better than a valve detector in a DAF96 or EABC80 etc. Note the valve sets often used about 500 K load, the 1M pot in parallel with AGC filter / line. Certainly you can use 100K.
I think Joe put up some circuits on the radio museum for the 1j37b from Russian schematics. They work for a 1j42s. But it's really simple!
Add a second winding of about 100 turns (you can try more later) using two wires at the same time to the ferrite rod tuned aerial on a separate piece of card. Put it at the middle if it's got a MW coil one end and LW at the other. Two colours of enamel is handy, that's bifilar winding. Wire start of one wire to the end of the other: That is the centre tap. The free ends go to g1a and g1b.
Put a 1 Megaohm "grid leak" from the centre tap to either f- or f+ depending on your supply volts.
Connect 100pF from centre tap to the Local oscillator output (just copy the part on g1 & g2 on any battery valve heptode/octode oscillator using a second 1j42a or a 1j24b if you have 45V Ht). The LO is triodised, but g3 to 0V / f-. If using a 2nd 1j42a as an oscillator short g1a and g1b.
Then on the mixer the g2 is HT (if only 9V) or a potential divider, g3 is f- and the anode drives an IFT. You can use any double tuned valve IF or two single tuned transistor IFTs with the secondarys back to back.
For SW, add 5t to 50t on the bifilar coil (really an accurate centre tapped coil), aerial, earth and tune that coil instead of MW or LW. Leave unused band coils open on a ferrite rod and separated as much as possible.
Verify that on MW, the LO leakage is less than a transistor set.
Always put earthed (f-) cut up coffee tin (or similar) sleeves on every Rod Tube, RF, IF or AF.
If you are going for small, also use 2 x transistor IFTs back-to-back on the IF amp anode to drive the 1N60. Try a 100 K to 250 K ohm pot to drive audio amp. Use it +ve like a transistor set not valve. Then if you use a DM70 or the Russian version as a Magic eye, the junction of anode and HT resistor can drive the g2 pins of the mixer and the IF amp for AGC, max volts = max gain, min volts = min gain on g2. Regular negative AGC on the g1 as per normal valves hardly works, because the rod tubes are linear sharp cut off. The variable mu / remote cut off valves use a progressively wound grid, i.e. the spacing varies. Impossible to do on a Rod tube design. But the 1j24b with 85V HT certainly do decent gain variation varying g2 from 20V (min, strong signal) to 55V (max gain weak signal). The 1j42 will probably do something similar, but at a lower voltage.
We did extensive tests in a lab. Best RF detector is a 1N60. Better than a valve detector in a DAF96 or EABC80 etc. Note the valve sets often used about 500 K load, the 1M pot in parallel with AGC filter / line. Certainly you can use 100K.
I think Joe put up some circuits on the radio museum for the 1j37b from Russian schematics. They work for a 1j42s. But it's really simple!
Add a second winding of about 100 turns (you can try more later) using two wires at the same time to the ferrite rod tuned aerial on a separate piece of card. Put it at the middle if it's got a MW coil one end and LW at the other. Two colours of enamel is handy, that's bifilar winding. Wire start of one wire to the end of the other: That is the centre tap. The free ends go to g1a and g1b.
Put a 1 Megaohm "grid leak" from the centre tap to either f- or f+ depending on your supply volts.
Connect 100pF from centre tap to the Local oscillator output (just copy the part on g1 & g2 on any battery valve heptode/octode oscillator using a second 1j42a or a 1j24b if you have 45V Ht). The LO is triodised, but g3 to 0V / f-. If using a 2nd 1j42a as an oscillator short g1a and g1b.
Then on the mixer the g2 is HT (if only 9V) or a potential divider, g3 is f- and the anode drives an IFT. You can use any double tuned valve IF or two single tuned transistor IFTs with the secondarys back to back.
For SW, add 5t to 50t on the bifilar coil (really an accurate centre tapped coil), aerial, earth and tune that coil instead of MW or LW. Leave unused band coils open on a ferrite rod and separated as much as possible.
Verify that on MW, the LO leakage is less than a transistor set.
Always put earthed (f-) cut up coffee tin (or similar) sleeves on every Rod Tube, RF, IF or AF.
If you are going for small, also use 2 x transistor IFTs back-to-back on the IF amp anode to drive the 1N60. Try a 100 K to 250 K ohm pot to drive audio amp. Use it +ve like a transistor set not valve. Then if you use a DM70 or the Russian version as a Magic eye, the junction of anode and HT resistor can drive the g2 pins of the mixer and the IF amp for AGC, max volts = max gain, min volts = min gain on g2. Regular negative AGC on the g1 as per normal valves hardly works, because the rod tubes are linear sharp cut off. The variable mu / remote cut off valves use a progressively wound grid, i.e. the spacing varies. Impossible to do on a Rod tube design. But the 1j24b with 85V HT certainly do decent gain variation varying g2 from 20V (min, strong signal) to 55V (max gain weak signal). The 1j42 will probably do something similar, but at a lower voltage.
