Found the issue. The amplifier tunes good enough for operation the problem is that i have two Vacuum tubes in it while the amplifier needs all 3 to work properly. How i found this out is i took out so there is only one tube left and checked the modulation. With one tube the audio down low before distortion was not even audible.
Rewriting my reply for clarity.
As others have noted, there are two types of distortion issues that could be at fault: 1) self-made RF interference creating audio distortion and/or 2) RF distortion.
For item 1) You probably don't realize this but your comments about taking out one tube reinforces exactly what many have been (rightly) pointing out that it sounds like a very, very routine case of RF interference; the RF available within the vicinity of the transmitter is interfering and probably creating the distortion that you hear. You took out one of the two tubes, this reduces the power output and because the amount of distortion is directly proportional to the power output, the distortion decreases with the power output decrease. If it you don't do anything to address the RF interference, when you install the third tube, the distortion will likely get worse. The solutions are, as was mentioned, ferrite chokes, a 1:1 isolation balun to attenuate RF on the shield of the coax, grounding, shielding, proper cable layout, etc.
For item 2) RF distortion is also very possible here. As pointed out, you have been running the amplifier pretty hard. It's a LINEAR amplifier and running it that hard is fundamentally going to cause it to be operating in a non-linear manner. You need to back off the drive and the output to give it "headroom" to be linear. It sounds like ThaDood and RobRich know the specifics for this amp but in general, the unmodulated carrier power output of a linear amplifier amplifying an AM signal shouldn't be more than 25% of the max fully-modulated output because AM max PEP output is fundamentally 4x the power of the carrier. The MFJ website says that the amp is 600 W PEP output, so don't try to run it with more than 600/4 = 150 W carrier output.
On top of that, that 600 W PEP assumes ICAS (
Intermediate
Commercial and
Amateur
Service) conditions, where the amp is amplifying full-gas somewhat less than 100% of the time. The last time I checked, you are on the air 24/7.
That amplifier was not made to do this, especially at the power level you have been running. This is stressful on not only the tubes but also the high-voltage power supply inside. For this reason, you should back it off much more than 150 W carrier maximum.
Finally, the amplifier is made to operate with - and designed for - three tubes. You need to understand that minimizing RF distortion in a linear amplifier is a balancing act and relies on a consistent amplifier configuration. When you take out one or two tubes, the conditions of balance are no longer there. You've done the right thing by acquiring a third tube.
Topic: Ameritron Al811 help (Read 1598 times)
