Thanks for ideas from Ron, Curt, Jim, Garey, John and amphone. Garey, thanks for clearing up my misunderstanding: The QRO DX2500 amp SENDS the voltage limiting signal TO the T4XC transmitter.
We hope this post helps you fellows using the Drake Transmitters with amps, including the excellent amps from Ray Connin at QRO technologies (no affiliation). Since our first post, have had a chance to experiment and operate on 80M SSB with the Drake T4XC and the amp. More careful reading of the instructions written by Ray Connin of QRO, and experimentation, have probably solved the problem, and it looks like neither some kind of load between the T4XC and the amp, or a cathode limiting resister in the Drake 12BY7A driver, will be necessary after all. Here's what we found: Ray's DX 2500 amp is a different animal (actually, we're trying to hold down this tiger)! than most other amps. "A grid driven circuit using a 50 ohm 100 watt non-inductive shunt resistor feeds each 4CX800A tetrode tube which is connected in a grounded cathode configuration". The tubes are Russian military surplus with very high transconductance, and therefore, need very little drive compared to other amps. Most amps (including the Drake transmitter) can be tuned basis maximum smoke (RF output). This one requires tuning by the book, at least at first. You have to rotate tune clockwise until you get 50 Ma of screen, then counter clockwise the load back to 25 Ma on the screen. Then rotate tune until 50 Ma again, and counter rotate load, etc. You do this until tune no longer increases Ma on the screen. After you have found this resonance, you are allowed to slightly adjust tune and load for maximum smoke, but only slightly. We found the maximum smoke setting was only slightly different from Ray's "by the book" tune up method, and screen current usually dropped a bit, a desirable result. We can now feel confident we have the right resonance tuning for our amp going on into our tuner for the 600 foot perimeter loop (ladder line feed). Now the other important thing we learned is that drive makes a huge difference in Tune and Load resonance settings for this amp. This is the key. We were too timid and we used tune up drive of only 8 to 12 watts in our first post. We were afraid of angry neighbors running into the driveway with arms raised over their heads. Ray says use 30 watts of drive, so one morning after the New Year, when neighbors had gone back to work, we did it. This time we got Tune and Load logging settings that were quite different that the ones we were using with low drive. And, now with the QRO ALC setting between 3 and 4, there were no more amp double faults (yep, we have trouble with our tennis serve, too) on SSB voice peaks. The problem we had in our first post, was that we had set the amp for resonance with 8 to 12 watts of drive, where in reality, when the T4XC was set for SSB, with gain at 11:00 AM, it was pumping in considerably more drive. The amp was not resonant for that higher level of drive, and so it faulted (amp screen current greater than 150 MA on voice peaks). To quote Ray: "The ALC circuit of the map supplies negative voltage to the transmitter which reduces the amount of drive coming into the amp. Only use this feature after your amplifier has been properly tuned [in italics]." Another variable that may have been important was the replacement in our Dentron 3000A antenna tuner, of the balun. The original one inside overheated during a false tune on 20M, and was oozing grey stinky stuff. We bought and wound a much sturdier one from Palomar Engineers and mounted it between two blocks of 1/8 inch teflon. We glued two TA200 cores together with 500 degree F. JB Weld. Its important to have a wattmeter right at the output of the amp, and to be sure that reflected power is very low (or non-measurable) when you use higher levels of power (we've been mostly QRP CW for years). We are getting a Coaxial Dynamics Wattmeter into the shack, with an SSB peak and hold measuring module from lnatechnologies.com in the next couple of weeks. Ray Connin says that the wattmeter in our Dentron tuner, that uses a toroid, will not provide accurate power measurements at 600 to 1500 watts. Tentative measures show he is right. QRO is a whole new world, and requires better instruments and sturdier baluns. This instrument will really tell us how the ALC limiter on the front panel of the QRO is limiting Drake T4XC drive, and just exactly how much SSB PEP we are getting for a given ALC logging setting. We hope to post these results soon at this thread. We also hope that we can "throttle down" drive from the Drake T4XC with the ALC knob on the QRO amp to as little as 500 watts out SSB. And yet still set the T4XC gain setting to the 11:00 AM setting, for high quality audio during SSB. We'll see. More data to be posted soon.
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