I just can't seem to get away from these 807s. Although I am to the point where I might replace them with 6146s.
I built this transmitter with a 6AG7 oscillator, and 2x 807 final amplifier. The schematic is a well known diagram and many people have built it. The oscillator works great. No problems there. The issue is when I measure the plates of the 807s, they are on a totally different frequency than the fundamental. I have experimented with different plate chokes, no change. On the 807 control grids, the frequency going in is dead on, 1000kHz. On the plates of the 807s, it's somewhere around 4000kHz (roughly.) I know 807s are prone to parasitics, so I shortened my leads from the 6AG7 plate. I shortened and re-routed the 807 plate B+ supply. No change. It's almost like the 807s are self-resonating. I've checked my high and low B+, they are clean. There is filtering on both. I have the parasitic suppressors on my 807 plates as shown in the diagram.
There are a few things I have done differently than the diagram.
I have my 807 plate current meter in the B+ line with a .01uF capacitor across the meter. (The diagram has the meter in the 807 cathodes.)
I do not have a 750v transformer. Mine is 650v for the high B+.
The crystal I am using is a HC-6 crystal. The diagram uses a FT-243.
I do not have the grid current meter, 6800ohm, 270ohm, or 10k pot installed.
I am doing all of my testing into a dummy load.
Any ideas? Things I could test?
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1\$\begingroup\$ No grid stoppers? How are you measuring the plate signal? \$\endgroup\$– Tim WilliamsMar 7 at 4:51
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1\$\begingroup\$ OP's schematic apparently differs considerably from the 1 MHz circuit...no tuned-circuit component values are provided. It is also possible that overall feedback overpowers the crystal's attempt to oscillate at 1MHz: construction and wiring matter. \$\endgroup\$– glen_geekMar 7 at 14:08
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\$\begingroup\$ As mentioned in the original question, the oscillator works great. I calculated the value of L1 for 1mHz. I have a 160uH coil, with a 25-300pF air variable capacitor. The plate of the 6AG7 is easily tuned to resonance. \$\endgroup\$– McMurdoMar 8 at 4:49
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1 Answer
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It's years since I've played with valve amps, but if you have parasitic oscillation, I'd look at Grid Stopper resistors.
Obviously the values referenced in the link are for an audio amp, but the analysis remains valid, and should allow you to select a size that suits your cut-off 1MHz frequency.
For example, here is a link to an 807 transmitter with grid resistors at 47R.
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\$\begingroup\$ Thanks for the reply. I have considered low value grid resistors, but I didn't want to stray too far from the original diagram since many people have built this circuit and it works fine. Unfortunately, I think I will have to add them and see what happens. I have also read that adding low value resistors to the 807 screen grids can help. \$\endgroup\$– McMurdoMar 8 at 4:52
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1\$\begingroup\$ Understand desire to stick to something that works for others, but the thing with the parasitic resonance is that it can be very sensitive to details of build and tubes. The low value resistors on the grid/screens, if correctly sized, don't reduce gain by much, yet add significant margin against resonance. Well worth a try. Do let us know if it helps, to make the answer more useful to others. \$\endgroup\$– colintdMar 8 at 5:00
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1\$\begingroup\$ Well that was it. I installed two 20ohm resistors right at the 807 grid sockets (as close to the tube as possible) and the 807s are now right on the fundamental frequency. My plate current is respectable (150mA vs 45mA when the 807s were self-oscillating.) One of the 807s was red-plating just in the corner of the bends, but one I'll check out the bias next. Appreciate the help! \$\endgroup\$– McMurdoMar 9 at 2:05
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1\$\begingroup\$ No problem. Always happy to help others save the time it took to find out the answers the hard way! \$\endgroup\$– colintdMar 9 at 17:55