I finally got started on the RCA BTA-1S project

[VE1IDX]

As of 2230Z, eight hundred watts was caused to be emitted at a frequency of 720 KHz for several minutes from this QTH. All emissions were into a dummy load. Then the oil soaked load resistors began to stink so I shut things down. LOL. The 500 watt load is not in an oil bath yet and still had some oil left on/in them from a previous time. My tubes may not have handles but the dummy load will. Picture a 20 US quart stainless steel stock pot with a coax connector in the lid. Don’t laugh, I already have it started and have a nice supply of silicone load oil for it too. Now on to the good stuff.

 

  I wired up a 220v  50 amp socket and a regular 120v outlet right below my electrical panel and directly behind where the RCA TX will reside. The first attempt at powering up failed. I forgot to install a jumper for the control voltage on TB-4. The second attempt to power up failed as well. I forgot to install the plate interlock jumper on the remote control terminal strip. The third attempt failed as well, almost. It seems that the plate ON button will not engage the plates. The plate contactor pulls in and then drops out. At first I thought it was an overload condition. Everything seemed fine however. I found that holding the plate ON button for a half second allowing the plates to turn ON-OFF-ON again and things would hold fine. The power output was only around 260 watts, or 2.3 amps through an RF ammeter inline with the dummy load. My first reaction was it is on nighttime cutback but the plate voltage was 3100 v which is the 1000 level. The grid drive was about one half spec so I installed a new (I think) 6146B and the meter readings came up to spec and I gained some power. Now I was at close to 400 watts. I really hate the tuning system that tx has. There is only one variable in the output stage and that is an inductor. I like tuning and loading controls. It will have them when I am finished. I swapped the RF and modulator tubes to see if it would make a difference in the power output and it did. Power was now around 500 watts or 3.2 amps of RF. After reading the manual a bit I began to change the taps on L5 in the output network and after moving the tap three or four times at one turn each I had a good 800 watts with 3100 volts and 390 mA current or 66% eff. I left it there for the night. I am not going to push for everything I can get out of it with the tubes as they are. I believe spec calls for 3100 v at 465 mA for 1000 watts so I am happy as it is. Beside things will change when the freq conversion takes place. I did not put any audio through it yet. That is the next stage and if all goes well with that it is time to gut the output network and install a real tune/load Pi network and change the driver circuit to accept external RF input. Actually I may change the driver first and see if I can at least make RF with the TX being driven with the output of a good signal generator. I’ll try my oldie URM –something or another-generator.  I also have to look at the nighttime power reduction circuit which was an option. I am sure the components are there for it but it does not work. I just need a little more time to trace things out but it appears at least two relays that are optional are indeed installed.

 

  So that is where I stand on the big 80m AM project. It has taken some time to even get started but at least it has been started and shows signs of success. Wish me luck!

 

[k4kyv]

Will be looking forward to working you somewhere down below 3700 kc/s, 3685 plus or minus QRM, or anywhere between 3600 and 3775.

Actually, I'd tear out all that RCA rf circuitry and redesign the whole thing more like (high quality) conventional amateur radio type construction.  It was not uncommon for RCA to string together rf components with 2-feet long leads.  I suppose that is no big deal on the broadcast band, but by the time you get to 75/80m, it can generate all kinds of feedback and parasitics.

Nor do I care for those 1 kw slug tuned coils.  I suspect the efficiency drops way down when you go from below 1600 kc/s to nearly 4000.

Despite their JS circuit layouts,  RCA built their transmitters with top grade components.  It's hard to find better quality stuff.

[VE1IDX]

My plan is to strip out all the RF output components between the plate blocker and the antenna connection and install a pair of 1000 pF 10 Kv Jennings vacuum variables I have on hand as tune and load controls. I can reuse one of the large inductors to make a standard Pi type output network.The driver stage will need to be changed as well due to the "wide band" tuning that RCA did. The driver stage is "tuned" by adding/subtracting turns to the plate choke of the 6146B driver tube.With any luck I will test the audio stages later today and hopefully all is well. I do know there was no hummmmmm or any noise on the carrier so that is a good start. I really need to find out what is with the nighttime power cutback control however. I would much rather mess around with it at 250 watts with about 1550 volts than the whole shebang.It's such a pain to hardwire everything over to 250 watts and then back again.

[VE1IDX]

Well I started on the audio section today.The audio was weak and crappy sounding to say the least.I noticed the bias was way off on the left mod tube .I set the right bias with no problem but when I tried to set the left bias to 70 mA it would not move unless I really cranked the pot.Then the thing took off and the overload kicked it out.I turned the bias pot all the way down and tried again.When I got to about 30 mA I stopped but the meter kept climbing higher until it just spiked and BANG! The overload relays kicked it out again.I changed out the left mod 4-400 and was able to get the proper bias levels but the tx still sounds crappy,no highs and distortion.I had to give up as I was frying my dummyload with 800 watts carrier and about 75% mod.I worked on the dummyload project after that and all is ready to go in the morning.The dummy load consists of 10 50 watt Globar resistors bronz welded to circular end plates and came from some military surpluss equipment.I have now completed the 12 quart stock pot (erroneously stated earlier as 20 quart) conversion to a dummyload container.All that remains is to fit the load onto the lid and fill the pot with the silicone load oil I have. I figure a load speced at 500 watt dry and placed in about 11.5 quarts of high quality silicone load oil should do the trick for testing.Oh yeah,it does have a safety vent.I really want to make sure this thing is 100% before I even attempt to move it to 80m.Oh well more troubleshooting tomorrow.