Tube transmitter tuning

Question

I am trying to tune/tweak/repair this transmitter for a friend of mine. It is oscillating, and it has rf power on the antenna, but it just doesn't seem as "strong" as it should be.

The diagram says 35-40 watt carrier. Now its a class A final amp, so I should be expecting half (or less) of that carrier wattage, correct? It came to me with a VFO, Hartley oscillator which was very unstable, so I replaced it with a Colpitts crystal for testing purposes. (I changed the diagram as well.) The only other thing I have added is a 40uH RF choke in the plate supply line to the 807s. I could not get a clean carrier without this choke.

As far as I can tell, everything is working. It oscillates, has RF at the buffer EL84 stage, and has RF at the 807s.

I tune for the "dip" in 807 plate current, which then has the highest RF on my diy field strength meter, and here are the 807 measurements: Plate voltage: 500-525v Plate current: 90mA No signal Screen voltage: 115v No signal <-- this seems low to me. Grid voltage: -14.7v No signal <-- also seems low. (not negative enough)

The antenna is a 1 wave long wire. 25AWG magnet wire, about 10ft off the ground, no counterpoise. RF ground is a foundation stake pounded into the earth about 4 ft. (and it's raining here. ground is wet.) I was using an antenna matching large variable inductor with multiple taps and fine tuning, but it seems to be "robbing" RF. It has a large neon bulb that glows brightly when it's hooked up though.

Is there anything fundamentally wrong with this circuit? Any red flags looking at the schematic? Lack of RF chokes? RF feeding back into the power transformer? Does the EL84 oscillator buffer need a tank circuit on the plate to tune to resonance?

Any help would be appreciated! Thanks.

Here's a link to the 807 class A specs I've been using:

Dwayne Reid: Thanks for your help. When I pull the audio EL84, the 807 plate current shoots up to 160mA, screen voltage increases to 204v, grid voltage stays the same, -14ish. The field strength of the antenna increases as well.

Answer 1

I think this is normal operation for a screen modulated AM linear amplifier. The output of the amp for an unmodulated carrier should be 1/2 power. Applying full amplitude audio should cause the 807s power to cover the range of zero power to full power. The average power is still half the maximum power.

By being a class-A amplifier, a resonant cutcuit on the output isn't needed.

Answer 2

I don't know why you think your parallel 807's are a class A amplifier. In my ham radio opinion, they are a class C amplifier. From your stated plate current and voltage, the power is about 45 watts, so you should be able to deliver about 35 watts to a well-matched load (because nobody is 100% efficient, not even me).

However, you also have screen grid modulation. That is a cheap form of modulation that does not require a modulation transformer. It works by cutting the power way down, not by adding sideband power the way plate modulation does and is probably not capable of 100% modulation. So the 35 watts is your PEAK power when modulating, not your average power. That's OK, that's what my Heath DX-35 had in 1958 for voice (but I only did CW in those days).

Another issue is your measurement of the 807 grid voltage. You need to be aware that this requires a very high impedance voltmeter. The value of the grid leak resistor is not given in the schematic, but I am guessing it is high. In fact, if it is dirty or has leakage paths, its lowered resistance could impair the performance of the final amp. Your grid voltage may be higher than you think. You need a high impedance voltmeter to correctly measure it.

With your rig on my workbench of 60 years ago, I would also check that the RF from the EL84 buffer is strong enough to switch the 807's on and off. Better transmitters of that era provided a way to measure grid current to verify that sufficient drive was there. You are not doing any frequency multiplication here, so I don't think there is any need for a resonant circuit or filter between the EL84 and the parallel 807's.

Good luck finding rocks and replacement jugs for that beast!

Answer 3

I am wondering how you ended up with this transmitter. I have some comments and questions to help you along if that is still useful. Excuse me for my English, it is not my native speak.

First, the author is silent key since October 2017. That is why you cannot get an answer from him.

This schematic is developed with the aim to put as much power as possible to the Medium Wave Band with minimal components.

As some others are also indicating, the 807’s are in Class C. The control grid resister should normally be 20K. But since there are 2 tubes in parallel it could be a bit less. 10 or 12k I would think is in general too little.

Also, the datasheet for the 807 that you are referring to is the wrong one. The one you showed is for a push-pull circuit. You are using two 807 tubes parallel. You are also referring to 12 K control grid resisters. Again, this is mend for use in AF circuits.

I would try 20k. To be save I would go no less than 15K. It all depends on the power the EL84 HF driving tube produces. An 807 with screen modulation produces max 16 Watt RF output power but needs 2-5 watt input from the driving stage.

The EL84 produces max 6 watt RF power. I has to be distributed over two tubes. So, I think this configuration produces at best 20 Watt in the final.

In the antenna you will have max 10 watt RF power. (many amateurs do not see the this but believe me. It is true. (Why is explained here: https://de-radio-amateur.nl/de-waarheid-over-zendbuis-vermogen-in-class-a ) This rig works in class C but the modulation does not work as effective ame as in class C with plate modulation)

Then, de High plus power is not 600V but should be 800 Volts according to this schematic. 2* 300V alternating current produces 840 V direct current. Knowing that means that the low power line to the EL84 end the screen grids of the 807 contains 400 Volts and that is way to much for as well the EL84 as for the screen grids.

But in the low plus power line are resisters placed (40kohmin total with a switch tot reduce this value) to manage this voltage. The 807 screen grid uses 250V at the max. And the EL84 plate uses 300V at the max.

Without audio the EL84 will heat up and would blow up itself but the 40k resisters limit the max voltage and so the max current. With a current of 10mA the voltage will be practical zero (maybe 25 Volt?) With the switch you can raise the voltage (lower the resistance) but without audio input. Be careful.

The best performance of this rig will be when you have 250V plate and grid voltage. With audio signal on it the voltage may swing from 50V tot 350V (is indeed 150V PP). If you have much distortion then lower the audio power at the entrance of the AF stage.

Over modulation is not possible with this schematic but overloading the screen grids is certainly possible.