Tube AM transmiter amplifier question

Hi, I would like to build this little transmitter http://electronbunker.ca/eb/OneTubeXMTR_2.html
I like the fact that it is screen modulated and uses no anode transformer.
I wonder if I use a linear output amplifier, would I need to perform the modulation on it, or at the tube oscillator as is?
My HF commercial transceiver (that has linear amplifier inside, is AM modulated from the early stages prior to the power amplifier, that is why I am asking.

I like the comment about the 'grid clamping effect'. Translated that means it presents a diode across the audio and bias is achieved by the charge created on the input capacitor. Unfortunately, that results in maximum anode current when there is no input and you have to be careful what voltage you present to the audio in socket.

In general, you can control the current in a tetrode or pentode stage by varying the screen grid voltage but try to keep it from going too low or the oscillator may stop running.

There is a compromise between modulating an early stage and modulating the PA. The earlier you modulate the signal the less modulating power you need, the big 'but' is that all the stages following it have to be linear which tends to make them less efficient. If your transceiver has SSB capability, the core sideband signal is probably generated by a balanced mixer and filter circuit which inherently produces low level output. That means the amplifying stages have to be linear and so it makes sense to generate low level AM and use the same amplifiers rather than use another high power audio stage to modulate the PA.

Brian.

betwixt said:

I like the comment about the 'grid clamping effect'. Translated that means it presents a diode across the audio and bias is achieved by the charge created on the input capacitor. Unfortunately, that results in maximum anode current when there is no input and you have to be careful what voltage you present to the audio in socket.

In general, you can control the current in a tetrode or pentode stage by varying the screen grid voltage but try to keep it from going too low or the oscillator may stop running.

There is a compromise between modulating an early stage and modulating the PA. The earlier you modulate the signal the less modulating power you need, the big 'but' is that all the stages following it have to be linear which tends to make them less efficient. If your transceiver has SSB capability, the core sideband signal is probably generated by a balanced mixer and filter circuit which inherently produces low level output. That means the amplifying stages have to be linear and so it makes sense to generate low level AM and use the same amplifiers rather than use another high power audio stage to modulate the PA.

Brian.

Click to expand...

Ok, so it can be done. I guessed that. Also note my commercial transceiver generates 40% of it's full SSB power in AM mode, probably to keep the transistors linear, as they have to amplify the carrier as well.

There is a nice article that shows a detailed broadband linear for HF (switched) here https://www.robkalmeijer.nl/technie...k/hambladen/radcom/1986/03/page168/index.html and here https://www.robkalmeijer.nl/technie...k/hambladen/radcom/1986/04/page256/index.html
This amplifier is relatively easy to build and it takes 1W of input to output 100W, but that is in SSB, I guess the power is 25% at AM or so. Not all this band-switching is needed for 160m (and below) AM too.
Also I do not know if the oscillator posted earlier can provide 1W, so maybe a small buffer would be needed.
About 40W of AM power generated from it, without any rare modulation transformers and a whole circuit of a power amplifier seems a great deal to me. The other advantage is that you can build it with one tube and then if you need more power, add more in parallel, provided that the PSU can handle the extra current needed.
Your advice will be usefull!

The design looks OK although I would be a little worried about using an LED in series with the screen supply. With ~800V DC on the anodes and possibly twice that at peak signal, I think an LED would be at risk if it flashed over. You also have to be very careful to stabilize a reliable control grid voltage (the -49V rail) as the 6146s would be toast within seconds if it failed. I have used similar amplifiers in the past and seen the glass envelopes of the tubes melt and sink into the vacuum!

The main reason for derating for AM use is it works in class AB mode, in other words is part DC controlled and part signal controlled. There will be some small anode current all the time ('A' mode) but the incoming signal increases the conduction ('B' mode) and lets far more anode current flow. For SSB, the duty cycle is fairly low, it gets time to cool down between words and quiet audio but when driven from a continuous signal, the heat is generated all the time. You should still be able to get full power out in AM mode but it will almost certainly overheat quickly.

You might squeeze 1W out of the 6CQ8 design but I would think 0.25W is more likely. It is only a small signal device and you would be using it as a power oscillator rather than as an amplifier stage. As an oscillator it will be self biasing to keep taking itself in and out of conduction so the anode current will be quite low.

Brian.

Here is the same transmitter circuit as the OP was looking at, but with a linear amp on the output.
https://www.xminusone.com/am/power/power.htm

betwixt said:

I like the comment about the 'grid clamping effect'. Translated that means it presents a diode across the audio and bias is achieved by the charge created on the input capacitor. Unfortunately, that results in maximum anode current when there is no input...

Click to expand...

But note that the anode resistor is 820k with a 165 volt anode supply, so there's no risk of drawing too much anode current.

Remember that the original circuit was intended as a Part 15 transmitter delivering about 100 mW. If you want megawatts, it's probably best to start from scratch.