10-12-2016, 10:12 PM
Important to understand that the g1a and g1b are not like two gates on a FET. It's the same pair of grid plates that pinch the electron beams as on all the other rod tubes, but not tied together.
Thus if the signal is common mode, the characteristic (gain etc) is identical to a regular rod tube. Unless you have a modulator, special AGC, frequency doubler, detector or demodulator, tie g1a and g1b together.
1j42a and high power 1j37b
Frequency Doubler and Detector operation
The differential voltage is like the voltage on electrostatic deflection plates of a CRT. Actually the tube is totally Electrostatic. G2 and G3 are an Electron lens. Thus no matter what the beam current is from the regular common mode (g1a and g1b portion that matches), the differential voltage deflects the beam onto the g2 rods and g3 rods (or plates, the 1j37b has rods for g2 and g3, two for each on each beam = 8 rods, the pair of anodes are plates. The 1j42a has a slot for g2 or g3, I forget which, and a triangular pair of anodes). So the current on Anode drops and current on g2 & g3 rises, no matter which polarity the g1a and g1b difference is. So the main signal that is the same on g1a and g2a will be varied in gain. If there is no common mode signal, then actually the differential signal is full wave rectified. If you put a high pass filter, it's a frequency doubler, and if you put a low pass filter, it's a very good detector, sensitive and full wave!
Mixer Operation
You use a centre tapped transformer for the LO drive. Only low level is needed. the + and - phase drive the g1a and g1b. The RF in or Audio in (or IF in if it's part of a BFO detector) feeds between 0V and the centre tap, so both grids are fed equally. The other way of using a Rod pentode as a mixer is to to direct couple a triodised (g3 to f-) Rod Pentode oscillator anode to g2 of the RF amp/Mixer. No other bias needed. This might be preferable for RF in as the LO leakage is less, even though a higher level. A separate Osc and Mixer is always preferable to single valve solution (Hexode - Triode ECH series better than EK or EH etc Octodes, Heptodes, Nonodes. There is actually Triode Hexode and Triode Pentode 1.4V battery types as well as 2V types)
AGC or DC volume control
A remote cut off or variable mu Pentode is created by having a variable pitch grid helix instead of fixed pitch. But none of the grids exist in a rod pentode, it's not really a pentode at all, but a pair of back to back electron guns with electrostatic lenses. It can ONLY work with a filament, a indirect cathode is too fat! The pair of grid plates (the 1p29b and 1p24b are actually using two parallel beams from two filaments, hence an extra filament pin and three grid plates.) are very close either side of the filament to pinch the beam. Normally all the grid plates are tied together. On the 1j42a and high power 1j37b they are not tied together, those types have no more grids than any other rod pentode. The 1j37b has least number of rods!
So inherently there is no remote cut-off, a variable mu design isn't possible. No Rod Pentode has usable AGC via g1. However g2 will vary the gain (hence using g2 as mixer LO in), as indeed it does on EF80, or DF92 etc. My Hallicrafters uses sharp cut off IF amps and DC on g2 to vary gain. For the 1j24b, about 20V on g2 is almost no gain and about 50V or 55V is maximum gain.
So with 1j42 you can have two capacitors to couple signal to g1a and g1b, put one grid leak (1M or 2.2M ) to 0V and the other to the AGC line, or if only using 6V to 9V HT, use a positive AGC line too! The polarity of g1a and g1b doesn't matter, it's only the magnitude of the increasing difference that reduces the gain. Or you can vary g2 volts from +4V to +9V to vary gain (1j42a only).
The 1j37b takes much more filament current and designed for 60V to 90V HT.
Gammatron mode.
You earth (f-) anode, g3 and g2. You can pick g1a OR g1b as the grid. Then the other g1 is the anode. Even the 1j39b will work at 6V in this mode. I've not tried the 1j42a yet as a Gammatron. The Gammatron is a very old idea, but the early ones were not made well and it wasn't understood as to what was happening! The control grid affects the space charge and thus the available current to the output "grid". The g1 in ALL rod pentodes is a pair of plates on either side of the filament at right angles to beam. The 1j29b and 1p24b has two sets of beams so the middle g1, middle g2 rods and middle g3 rods are shared. They simply has a wider pair of anode plates for the four rather than two beams. Use one filament only on 1j29b or 1p24b and you get close to half power and half gain.
It's a shame they were told to stop developing these (due to silicon replacing germanium transistors). I can imagine a 7660 style beam switching tube for SSB etc ... or logic gates
Just a matter of not tying rods together internally, like the simple "snip" and extra pin that created the 1j37b and 1j42a.
Thus if the signal is common mode, the characteristic (gain etc) is identical to a regular rod tube. Unless you have a modulator, special AGC, frequency doubler, detector or demodulator, tie g1a and g1b together.
1j42a and high power 1j37b
Frequency Doubler and Detector operation
The differential voltage is like the voltage on electrostatic deflection plates of a CRT. Actually the tube is totally Electrostatic. G2 and G3 are an Electron lens. Thus no matter what the beam current is from the regular common mode (g1a and g1b portion that matches), the differential voltage deflects the beam onto the g2 rods and g3 rods (or plates, the 1j37b has rods for g2 and g3, two for each on each beam = 8 rods, the pair of anodes are plates. The 1j42a has a slot for g2 or g3, I forget which, and a triangular pair of anodes). So the current on Anode drops and current on g2 & g3 rises, no matter which polarity the g1a and g1b difference is. So the main signal that is the same on g1a and g2a will be varied in gain. If there is no common mode signal, then actually the differential signal is full wave rectified. If you put a high pass filter, it's a frequency doubler, and if you put a low pass filter, it's a very good detector, sensitive and full wave!
Mixer Operation
You use a centre tapped transformer for the LO drive. Only low level is needed. the + and - phase drive the g1a and g1b. The RF in or Audio in (or IF in if it's part of a BFO detector) feeds between 0V and the centre tap, so both grids are fed equally. The other way of using a Rod pentode as a mixer is to to direct couple a triodised (g3 to f-) Rod Pentode oscillator anode to g2 of the RF amp/Mixer. No other bias needed. This might be preferable for RF in as the LO leakage is less, even though a higher level. A separate Osc and Mixer is always preferable to single valve solution (Hexode - Triode ECH series better than EK or EH etc Octodes, Heptodes, Nonodes. There is actually Triode Hexode and Triode Pentode 1.4V battery types as well as 2V types)
AGC or DC volume control
A remote cut off or variable mu Pentode is created by having a variable pitch grid helix instead of fixed pitch. But none of the grids exist in a rod pentode, it's not really a pentode at all, but a pair of back to back electron guns with electrostatic lenses. It can ONLY work with a filament, a indirect cathode is too fat! The pair of grid plates (the 1p29b and 1p24b are actually using two parallel beams from two filaments, hence an extra filament pin and three grid plates.) are very close either side of the filament to pinch the beam. Normally all the grid plates are tied together. On the 1j42a and high power 1j37b they are not tied together, those types have no more grids than any other rod pentode. The 1j37b has least number of rods!
So inherently there is no remote cut-off, a variable mu design isn't possible. No Rod Pentode has usable AGC via g1. However g2 will vary the gain (hence using g2 as mixer LO in), as indeed it does on EF80, or DF92 etc. My Hallicrafters uses sharp cut off IF amps and DC on g2 to vary gain. For the 1j24b, about 20V on g2 is almost no gain and about 50V or 55V is maximum gain.
So with 1j42 you can have two capacitors to couple signal to g1a and g1b, put one grid leak (1M or 2.2M ) to 0V and the other to the AGC line, or if only using 6V to 9V HT, use a positive AGC line too! The polarity of g1a and g1b doesn't matter, it's only the magnitude of the increasing difference that reduces the gain. Or you can vary g2 volts from +4V to +9V to vary gain (1j42a only).
The 1j37b takes much more filament current and designed for 60V to 90V HT.
Gammatron mode.
You earth (f-) anode, g3 and g2. You can pick g1a OR g1b as the grid. Then the other g1 is the anode. Even the 1j39b will work at 6V in this mode. I've not tried the 1j42a yet as a Gammatron. The Gammatron is a very old idea, but the early ones were not made well and it wasn't understood as to what was happening! The control grid affects the space charge and thus the available current to the output "grid". The g1 in ALL rod pentodes is a pair of plates on either side of the filament at right angles to beam. The 1j29b and 1p24b has two sets of beams so the middle g1, middle g2 rods and middle g3 rods are shared. They simply has a wider pair of anode plates for the four rather than two beams. Use one filament only on 1j29b or 1p24b and you get close to half power and half gain.
It's a shame they were told to stop developing these (due to silicon replacing germanium transistors). I can imagine a 7660 style beam switching tube for SSB etc ... or logic gates
Just a matter of not tying rods together internally, like the simple "snip" and extra pin that created the 1j37b and 1j42a.
