GM Peter:
IMHO, you need a pad between the Tx and amp. You're asking the T4 to
really throttle back and in doing so with the gain pot, other things
begin to change, not just the output power.
I run a similar setup here with a grid driven AMP from a T4x.. I have a
three dB Tee attenuator in a small box mounted in the line going into
the amp. You need to make sure this gets bypassed when in receive, so
either building it into the amp or using an external t/r relay is
required.
Use non-reactive resistors and remeber that these need to dissipate all
the power you are losing.
Another way is to use one of the monolithic attenuators available. they
are easily found on epay and are very reasonable. they need to be
mounted on a heat sink but are reliable and no-fuss and come in a
variety of losses.
Hope this helps,
Curt
KU8L
Peter Hoon wrote:
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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