Newbie here : My first AM Tube TX Build

[rudirhino]

Hi people of the HF Underground! So glad that I have found this site. I have also been taking a lot of inspiration from the StationQRP site. Here is a rough schematic of how my TX is now, and still in development. I would really appreciate any feedback/suggestions from you people regarding ways in which I might be doing things wrong, or ways which I can improve things.

Google Drive link:
https://drive.google.com/file/d/0B2ejtzhIa9tFNjQ3ZC1BaVJMRnc/view?usp=sharing

I was using a PLL driver but when that blew up I had to resort to just a VFO to create my frequency. Currently running 6960KHz for testing into a dummy load. I have used a 12v version of a 6146 as the oscillator, a 6883B. This then drives two PL504 sweep tubes with 400V on the plates. There are a few things which I am unsure about, such as the way in which I am biasing the PL504's. I am using a pot for each 504 as a voltage reference with -120V going in. this then feeds negative DC to the grid via an RF choke of about 2.5mH. Things still work if I remove the -120V and just use the pot as a variable resistor to GND, but then if the oscillator fails to start the 504's saturate with current :-/. Currently I am setting them to both have 65mA Ik when being driven, which gives a safe dissipation I think. Datasheet states 16W and if I take away my ~6W per tube in RF output that leaves 11W in heat dissipation... yes? :-)

Right now the total power input on HT rail is about 40-50W and I am getting 12-14W RF out when everything is tuned up. I have calculated all values for the output pi filter and all seems correct there. This power output seems quite low considering that the 504s should be operating in Class C. Any thoughts? I have also yet to try different tubes in case these ones are past their best. All the tubes and parts I am using are from the junk pile, so I am unsure of some parts.

I don't have a scope capable of these frequencies so I have been using a DIY RF probe to detect "RMS" equivalent voltages in the TX. Currently I am getting about 25 volts coming out of the 6883. With a calculated plate impedance of 4500 ohms this would translate to about 140mW in RF. Or am I wrong? I am wondering whether this voltage is enough to drive the 504s into Class C region or not.

Then comes my modulation method; I'm using plate modulation but using a 100V line Public Address amp. This seems ideal as I can get unto 450Vrms out of it and it has a transformer isolated output, which should be enough for 100% modulation of the 400V output stage. When I test it though as per the schematic I am only getting proper undistorted audio when the 504s are biased close to off. Once they start conducting the audio becomes totally choked and distorted. I am thinking it is maybe the choke (7.5H) which is too small in value, as I have seen designs which use unto 80H choke in this position... any ideas?

Anyway I look forward to hear your thoughts :-)

73,
Rudi

[davemartin]

It looks like you are attempting to use a form of shunt fed modulation. But if that is the case then the choke is too small in value and the line transformer is not going to shunt the voltage.
I'm confused as to which side of the transformer you are applying the audio to.
   

[staticlistener]

Things still work if I remove the -120V and just use the pot as a variable resistor to GND, but then if the oscillator fails to start the 504's saturate with current :-/.

In this case, the pot is acting like a variable grid leak resistor, which needs the drive from the exciter to create the bias voltage on the grids. No drive and the plate current will more or less run away with no control.

As far as the modulation arrangement goes, your blocking cap on your modulation transformer is on the wrong side from every arrangement I've seen, it would be on the side that is connected to the choke, not on the grounded side.

Your screen dropping resistor is also in the completely wrong place, as the schematic is drawn, it appears that you would be feeding RF back into the tube on the screen, you want to have that screen dropping resistor connected between the power supply side of the plate choke and the screen of the PL504s, not the tube side of the plate choke.

[staticlistener]

Also, I make sure you have enough bias to bring the tube into class C (well below the plate current cutoff), in addition drive from the oscillator. 140 mW just doesn't sound like enough. As far as the side of the modulation choke, calculate the impedance of the finals (not the RF impedance which I believe is (I/E)/2, but the modulation impedance which is just I/E) then take that value times .0045 to get the minimum value you need for the choke. Another way is just simply 10H for every 1000 ohms of modulation impedance of the final, so if your impedance is 4000 ohms, you'll need a 40H choke.