Am having difficulty running our Drake T4XC into a QRO Technologies DX 2500 Linear Amp.
Here is a description of the problem, things we've tried so far, and an appeal for your ideas and input. The DX 2500 amp requires very little drive. Preferably, we want to run 500 to 800 watts PEP out of the amp to our loop at this 9,000 foot above sea level Rocky Mountain ridge QTH. Have neighbors on both sides, and so far we all get along. That would mean about 9 to 18 watts SSB PEP (40 and 80 meters) out from the T4XC into the DX 2500 amp. Listeners say our best T4XC speech in SSB running barefoot is with T4XC gain set at the 11:00 AM position (with a Drake desk mike). But then, when we turn on our amp, we have way too much drive, the amp tubes' screen current is exceeded on voice peaks, and the DX 2500 protection circuit puts it into the fault mode, thus turning off the amp. Here are solutions we have tried: Yes, we have looked at the SWR the amp sees going into the tuner for the loop. We are able to get virtually a perfect match, with no reactance, and zero reflected watts. Have tried injecting ALC from the T4XC into the DX 2500 (from the jack on the AC4). With the T4XC gain set at the 11:00 AM position, the T4XC sends about -8 volts max on SSB voice peaks. The DX 2500 can handle up to -20 volts of ALC. We still cannot get the ALC pot on the front panel of the DX 2500 to sufficiently limit drive on voice peaks. Perhaps with more practice and tweaking, we can find the correct ALC pot setting. But so far no cigar. Tried completely closing the load control (full counter clockwise) on the T4XC to reduce drive. But it only reduces drive to 60 watts CW in tune mode, still far too much drive PEP in SSB. Even with plate re-dipping with the changed load control, am concerned about possible spurious products when the load capacitor in the T4XC is not at resonance. Tried reducing the T4XC gain control to the 9:00 AM position. But fellows say our speech lacks the "presence" it has when running barefoot with gain at 11:00 AM position. Suspect the basic problem is that the T4XC gain control does limit CW power output in Tune and CW, but controls speech amplifier gain (but not PEP output power) in the SSB mode. Please suggest ideas to try. One idea that could be worth trying is to mount a separate pot at the rear of the T4XC (in an empty hole) that would limit cathode current to the 12BY7A driver, when in the SSB mode. A SPST switch could "short" out this pot when not wanted. There may also be a way of placing a rear chassis mounted pot at the proper place on one of the 12AX7A speech amp sections during SSB that would limit T4XC PEP output. Thanks. Peter VE1CHS/0 Nederland, Colorado
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