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Ameritron AL811 / AL811H

Those 'Input Tuning' controls adjust the input impedance of the amplifier to (or closer to) the 50 ohms that your radio want's to see. They should be marked for which band that they apply to. Depending on the particular amplifier, there may be more than one band per adjustment place. 10 and 12 meters are usually 'lumped' on the same control for instance. It's typically either a compromise between the two, or an "either/or" thingy, one or the other.
- 'Doc
 
Shockwave said:
The only real difference between the 811A tube and the Taylor 572B tube is the graphite plate. That takes it from 65 watts to 160 watts anode dissipation. Using them in place of an 811A in AM service will extend tube life without any negative effects on the amp so long as you don't expect it to make any more power. The idea is not to get more power and then stress the tank and power supply components, it's to get the tubes to run cooler.

The AL-572 amplifier certainly has differences from the AL-811 series but those differences are only there to use the full potential of the 572B tubes graphite plate. Because the 572B has virtually the same inter element capacitances, the circuit requires no change to go from 811A tubes to 572B. Just don't exceed the amplifiers published ratings.
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A lot of guys replacing 811A's with 572B's have reported slightly worse IMD due to the tubes not being run with optimum plate voltage. The 572's thrive with about 24-2500 volts on them. I have not done this myself but merely passing on what I have seen written and heard discussed.
 
Captain Kilowatt said:
A lot of guys replacing 811A's with 572B's have reported slightly worse IMD due to the tubes not being run with optimum plate voltage. The 572's thrive with about 24-2500 volts on them. I have not done this myself but merely passing on what I have seen written and heard discussed.
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That would require another variable within the 572B tube other then just the anode being graphite. As I recall all older USA and new Taylor brand 572B tubes had the same element dimensions and spacings as the 811A. This is not the case with the popular Svetlana 572B. I see differences in it's anode and possibly the grid structure that could effect IMD. It was designed with audio amplification in mind and probably has different IMD characteristics.

In the example of the Taylor 572B I'm fairly confident the only difference it has with an 811A is the thicker graphite anode. The inside dimensions of the anode were kept the same to maintain the same inter-element capacitance. I'm sure the 572B will have improved IMD at the higher plate voltage for a given power level. I question if it would perform worse then an 811A at the same voltage.
 
SharpShooter,
No, when tuning those inputs, what's on the output of that amplifier isn't going to make any difference. (That assumes you've tuned the amplifier for whatever it's feeding though.) You adjust those input coils till the radio/SWR meter show a good match, then leave them alone. They are a 'one time' adjustment. It certainly doesn't hurt to check that input SWR occasionally, but it's not typically something that has to be done on a daily/weekly/monthly basis. Every once in a while ought'a do just fine, unless you notice some fairly drastic change in the radio's ability to 'play nice' with the amplifier. Then it may not hurt to check the input SWR again. How often do you check your clocks to see if they are set right for 'daylight saving' time? That's a pretty good time to check input impedances on that amplifier.
- 'Doc
 
I've got an 811 (3 hole) sitting right here. I bought it used and ran it with the brand new Chinese tubes that came in it for a time. I observed that the Chinese 811s got cherry red when run in continuous duty (SSTV) even when limiting Ip to 400 mA and Ig to <120 mA. The manuals claims 400 watts ICAS but then tells you to limit plate current to 400 mills. You can't always get 400 watts out at those current levels. 300-350 watts (continuous) is more realistic above 20 meters. I can get 400 out on 10 meters with a plate current of 425 mA. You will not need anywhere near 100 watts to drive it. Mine will reach full output with 60 watts input. In order to prevent damage due to brief flashes of retardation, I built a 2 db pad for the input. The pad allows me to run the rig at 100 watts output without the ALC engaging so that I can ensure that I am linear in the digital modes. I strongly recommend a set of 572s from RF Parts. They will set you back about $50 each but will never get more than a dull glow at 400 watts for 2 minutes +/-. I noticed just the slightest increase in output and difference in tuning with the 572s certainly not worth the $150 if power was your justification for buying them.
 

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