Single control RX TX

[VE3LYX]

Hi, I am a newbie.
I am deep into homebrew and tubes. I just finished a homebrew rig for AM phone. I am partial to 50L6 tubes and have a good working CW single tube set. I am also a transformerless Ps nut. Anyway I built this rig which might be loosley described as a transceiver. It uses my version of the powerhouse two tuber.(Aug Pop elctronics 1966) but I used octal tubes. 12sl7 and a 50l6. The tx section uses the same tubes. The 12sl7 is1/2 hartley osc and 1/2 modulation amp. The mic circuit uses a WW2 handheld carbon mic , a battery and a normal table radio audio transformer. The 50l6 output tube is screen modulated. It works but power output is low. Part of that is because it is screen modulated and the balance of carrier to modulation must be such that it can be totally modulated otherwise at full carrier you have a Cw tx and not am. That I understand but I use the 50l6 a lot and like it. (poor mans 6l6) I normally get over 10 watts on Cw so was hoping for 4 or 5 on AM. I have about a 1/4 watt. I know I have messed up somewhere ever though voltages are good and it tunes up nicely and at mid control of modulation control produces good audio in my monitor rx nearby. The basic idea for the circuit came from a Am broadcaster  design but I have made it my own now. I am building a second 50l6 output amp. (just waiting for a couple of power resistors to come) still I wonder if I am missing something. I do run my other 50l6 on 300 volts and this one is on 150 but still i had hoped for more than a 1/4 watt output.
The PA I am building is on 300 volts as well. If anyone has some ideas I am open. I am new to AM but old to the hobby. Just tired of quacking and want to speak like a human again.
Don VE3LYX

[WU2D]

Hi Don,

Welcome to The Amfone Forum. The 50L6 is rated at 200V but probably can take 300-400V without much trouble. Off the line stuff is fun and at the same time, challenging. I assume that you are running a voltage doubler off the line for a  little over 300V on the plate. You should be doing a little better for power but screen modulation when set up right does reduce your carrier. And you may still be shy of the kind of voltage on the plate that it will take to reach your goal. Even so you should be doing 3-5 Watts. My 50C5 Heising rig does a couple of Watts out with 300V on the plate and simply a 12BA6 ECO as the drive. I would suggest that you convert to Heising. Do you have a small choke?

Mike WU2D

[VE3LYX]

Yes I do. I believe the heising "werde besser werken"  but I am experimenting with screen for now. That was the whole purpose of the build. What i dont quite get is how I can get 10 watts cw from a crystal controlled Power Osc and yet using the same tube come up with only 1/4 watt with a hartley osc in 1/2 a12sl7 driving it. I guess I hadnt realized a Crystal controlled single tube or even a tnt style power osc would be better power wise than a low power osc driving the same tube. I played with screen voltage ignoring for the moment the modulation idea just to see if the power rose. I think the hartley is not driving the tube hard enuf. But I am not sure. I am in new territory here. I decided that since I have an Rx and tx tracking within a few kcs and the osc in both section are stable , the mdulation works properly and I have the abilty to turn it up and also key for CW maybe I should just build a final for it. I have another 50L6 already done up for that. (was short a 400 ohm power resistor but it is in the mail!)I have a parrallel tuned tank in it almost indentical to this one but it has all the sreen voltage of my regular Cw 50L6 transmitter as well as 300 volts no load well filtered voltage doubler. (Centre fed double cap non series doubler. )  I am hoping the 1/4 watt from the tx wil drive the tube hard enuf to get me at least 3 or 4 watts output. I should though as you touched on probably try this 50L6 at 300 too. The audio Pa in the reciever is also a 50L6 and the biasing is not that diffferent. It has enuf poop I never have the volume up full. However whose to say how much Af power that is. I just checked it for a point of reference. The modulator in the tx works well. I can even just drop the carrier out and it will come in instantly on speaking and stay without dropping out till after i stop speaking. I keep it just above that level and can drive the output while speaking to the same level as with full screen voltage so I am happy with that and its control. However, Heising modulation . I would be inclinded to add that feature to my 50L6 Cw rig. I would need another  50L6 as an audio amp to drive it, right? I use a carbon mic electromechanical circuit with a battery for a preamp replacement. I have become very familiar with this type of mic circuit and love it. Speaking of 50L6 and the miniture 50C5 it cousin, I am using 50C6 to replace my 50L6s in some of my stuff. It seems to like the higher voltage better and gives an all around better peformance. I am going to try your sugggestion of heising mod but on my Cw rig. This rx tx is somewhat of a sacred cow now. I never thought it would work but it does. Any difference in the rx tx freq and it is very minor can be pulled with a quick tune of the PA tank with no decernable powerloss.   I still though am a bit mystified at why the power out is so low. I guess I should take my other 50L6 tx and drop the B+ to 150 and see how much I lose. Thanks for the suggestions. I will be rereading it several times to make sure I didnt miss anything. Here it is from the front.
Don VE3LYX