FL2100Z - using 811A

Discussion in 'Amateur Radio Amplifiers' started by VA3QY, Jul 7, 2020.

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  1. VA3QY

    VA3QY Ham Member QRZ Page


    I have been wondering if I can replace 572b with 811a? It is a question as I can drop 572s into 811 why not vice versa - yes power output will be lower.

    What is required to make FL2100Z to work with 811A?
  2. W1QJ

    W1QJ Ham Member QRZ Page

    No because an 811 doesn’t want to see 2500 volts but a 572b is ok with 1700 volts. So it’s only a one way swap.
    N2OTG, WA9SVD, WD4IGX and 3 others like this.
  3. VA3QY

    VA3QY Ham Member QRZ Page

    Yes, but cant one drop he HT voltage using the lower voltage transformer taps.
    K0UO likes this.
  4. W1QJ

    W1QJ Ham Member QRZ Page

    That is possible.
  5. VE7DQ

    VE7DQ Ham Member QRZ Page

    What does that do to the filament voltage?
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I didn't just look at the FL-2100Z schematic, but if it has only one power transformer for everything, I don't see how that could really work.

    First, "lower voltage taps" are usually on the primary, which means using them would raise the high voltage at the secondary, not lower it.

    Second, if the filament winding is shared by the same transformer, changing primary taps would screw up the filament voltage which should be maintained within 5%.

    Using 811As at anything over 1700V can be quickly destructive. Ameritron (and others, in the past) push the hell out of these tubes which were originally designed for 1250V B+ and then redesigned over time to handle 1500V or something.

    572B fits the same socket and has the same heater rating, so it's a great drop-in for an 811A but its construction is quite different, as are materials used to manufacture it. One 572B can handle as much power as 2-3 811As.
  7. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I just checked the FL-2100Z manual and there's only one power transformer.

    If you use "lower voltage taps" that will only increase both the high voltage and also the filament voltage.

  8. G0JUR

    G0JUR Ham Member QRZ Page

    there is a 425v tap on the secondry winding give half the ht wouldnt it? circa 1200v?
  9. W1QJ

    W1QJ Ham Member QRZ Page

    Yes indeed there is.
  10. N8VIL

    N8VIL Ham Member QRZ Page

    There is also a 620v tap on the secondary. It would give you 1750 volts roughly. Might make an otherwise dead amplifier operational, at reduced power, if all you had were a couple of 811 tubes available. I find this rather interesting if it would work. I wonder what the safe output power would be for 2 811's.

    Last edited: Jul 9, 2020

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