HFU HF Underground
Technical Topics => Equipment => Topic started by: Radio Krasnoprudskoe on December 28, 2021, 0750 UTC
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Hello!
I am Radio Pushka ;D
Well, currently got 6920 khz back up with a 15 watt transmitter. I rebuilt my antenna to be more powerful and now it has an swr of 1.0 on 6920 and 15 watts carrier gets out quite well. When I plug in my Ameritron, It tunes perfectly and I get a solid 200 watt carrier which i have enough drive for. But as soon as I put in audio it sounds all distorted as if something is not tuned correctly. Also, when i set my load control to 0 the power dips a little and the distortion gets worse, when i set the load to 10 which is max, the audio sounds less distorted and i get more output. Its like there is not enough load capacitance or something. Anyone know what could be the issue and how to resolve?
Thank you!
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RF feedback.
Str.
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Going through this myself, with recommendation from Timtron, on working him on 3885kHz AM, keep the AL-811's carrier output to 90W, or less. plug-in 572's tubes and you can up that power out carrier to around 110W. Also, be sure that you have some type of decent audio limiting as well and maybe 4:1 compression. And, always listen to that signal with a receiver without an antenna. Hope that helps.
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Is that AM? 200w unmodulated carrier on three 811A tubes is hitting them hard IMO.
811A class B plate max dissipation rating is 45w/65w CCA/ICAS per tube. So 65w * 3 tubes = 195w.
As ThaDood noted, a 90w or less carrier should be a good place to start. I would think even more like 50w to 60w for improved tube life and hopefully good linearity for AM if using 811A tubes. Figure around 200w to 250w PEP AM modulated power.
Swapping to 572B tubes should give a little more headroom, though note then you are going to be (stock) transformer limited, so still do not expect more than maybe 300w to 400w PEP AM modulated power from the amp. Yeah you can push the AL-811 for every PEP watt possible, but RF compression alone is going to squash and trash your audio, less long the extra heat and high input drive increasing the likelihood of damage to the tubes.
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All good advice above, If your not using a balun get one.
Ferrite chokes on audio and supply voltage lines.
Good/proper grounding.
Make sure you are tuning the amp properly, you say it tunes fine then you say it doesn't.
Remember this amp is not rated for CCS.
http://amfone.net/Amforum/index.php?topic=27712.0
Good luck
rgg
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Well thank you very much for the advice guys!
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. With two tubes it was low but still quite audible. I bought the third tube but it still is taking a while. In the meantime i ordered a couple of irfp460s and i hope to get a 200 watt carrier with two of them or at least a 100 watt carrier with one of them should be enough. 15 watt carrier i have now is not satisfactory as 6925 is an issue and noise is high. And in the amplifier i had 572b tubes.
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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.