AL-811 Not Loading Correctly on 80/160

Discussion in 'Amateur Radio Amplifiers' started by AD7I, Nov 20, 2019.

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  1. KC1KWX

    KC1KWX Ham Member QRZ Page

    I found exactly this with my SB220. Took me a bit to find. I thought I was being good by tuning at 25 watts in but as the manual says Do it with 100 watts in.
  2. AD7I

    AD7I Ham Member QRZ Page

    Thank you all for your insights. Now that I know what to look for, I can see that this phenomenon of the load control needing more C as the drive is reduced on my AL-811 amp. Is this more-loading-C-needed-at-less-drive phenomenon particular to the AL-811 amplifier? Any amp using 811 tubes? Or most any grounded grid amplifier regardless of the type of tubes used?
  3. W1QJ

    W1QJ Ham Member QRZ Page

    All grounded grid amps basically act the same way. The lower the drive level the more load C you need. Adding additional load C that is not needed when the amp is run at normal output power would cost extra money to do. When the fact is the extra C is not needed at normal power output levels they do not included it. They write the manual in such a way that if you follow it, the end result is that the extra C is not needed. With the onset of many hams using amps on digital modes at lower power levels the lack of sufficient load C at low power is an issue. I don't think that issue would be addressed any time soon I don't think. My guess is if you called Ameritron and discussed the issue they would probably say as long as the plate current dips you would be OK. Efficiency is going to be low anyway when operating an amp out of the power band range.
    K7TRF likes this.
  4. AD7I

    AD7I Ham Member QRZ Page

    Thank you Louis W1QJ for the info you provided. That’s very helpful.

    When I use the AL-811 amp with digital modes (usually RTTY, Domino, but never FT-8 of course) on 40M and 20M I’m getting about 350 to 400 watts output (about 28 watts input) when I hit the 400 mA plate current limit for continuous operation (that’s the Ip limit as stated in the Ameriton AL-811 user manual for long term continuous transmittion). I find that the grid current at this power output level is about 50mA, and goes down to 40 mA at 21 and 28 MHz (but the power output is also down to 200 watts at those higher frequencies).

    I note that as I move the LOAD control away from what provides max power output it affects grid current. If I move the LOAD control so as to provide more loading capacitance the grid current goes up, and goes up quickly as I add more loading capacitance via the LOAD control. And that as I move the load capacitor toward less capacitance from what was needed for full output power that the grid current goes down, but not as quickly as grid current rose on the other side of the matched condition. Is that why I sometimes I see tuning procedures that recommend that the LOAD control should be adjusted for max output power and then turn it just a tad more toward less loading capacitance? To lessen he dissipation of the grid?

    Other than reduced amp efficiency, are there any other technical downsides (arc over the internals, cause on the air spatter) to using less loading capacitance than the amount that would cause max output power?

    Thank you.
  5. W1QJ

    W1QJ Ham Member QRZ Page

    The amount you advance the load beyond the peak output point is very little. You do not want to advance it too much more, maybe one line the most on an 811 amp. Don't over think this.
    AD7I likes this.

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