Peter, I have had the same problem here, but have never actually "fixed" it. I changed antennas and never really looked back, although it is till on my list of someday items. Here are two ideas that may work.
Use your station dummy load as the primary power absorber. Do a little bit of math to calculate a pickoff resistor value to allow 10ish watts to the amp. Basically this an L attenuator with most of the power heading for your 50 ohm dummy load. Off the top of my head, a 50 ohm dummy to ground and 50 ohm series, feed the 50 ohm amp input at about 14 watts (assuming 85 watts output of the T4X). There is also 14 watts dissipated in the series resistor, and 55 watts in the dummy load. Also the SWR at the TX will differ slightly as the load is now 33 ohms, not 50. This should be tunable for the t4X as it should be totally resistive. Big downside is that you need to switch around the antenna on RX, or loose a large amount of RX signal to the dummy load too. Or mod the amp relay for this kind of arrangement. Look at the manuals for the Drake 2 meter transverter. I believe that in there they show a Drake designed mod to allow the finals (6JB6s) to be turned off/on for driving the tranverters. I believe that your driving with just the 12BY7. I think that might be about the power level you are looking for. Let us know how you proceed. 73, Ron WD8SBB --- On Tue, 12/29/09, Peter Hoon <[email protected]> wrote: > From: Peter Hoon <[email protected]> > Subject: [Drakelist] How to Lower T4XC Drive To Linear Amp During SSB? > To: [email protected] > Date: Tuesday, December 29, 2009, 1:06 AM > 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. > PeterVE1CHS/0Nederland, > Colorado > > -----Inline Attachment Follows----- > > _______________________________________________ > Drakelist mailing list > [email protected] > http://mailman.zerobeat.net/mailman/listinfo/drakelist > _______________________________________________ Drakelist mailing list [email protected] http://mailman.zerobeat.net/mailman/listinfo/drakelist
