Advice with an 811H amp

Discussion in 'Amateur Radio Amplifiers' started by K1OIK, Dec 29, 2017.

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  1. W1BR

    W1BR Premium Subscriber QRZ Page

    You can't test a transmitting tube adequately in a tube tester. Tester voltages are not high enough to test for gas--used tubes should be high pot tested and then have their output power verified in an amp. NOS Russian 811a tubes are cheap.
    K1OIK likes this.
  2. K1OIK

    K1OIK Ham Member QRZ Page

    I am a bit slow so I don't know where you are coming from unless its don't buy used no name, unlabeled tubes on eBay. I stayed at 12 Crest which is off Field RD off Clapboard Ridge very close to the high school, there was a lake at the bottom of the hill, stopped by there in 1998 and 2007, looked very much the same 50 years later, downtown Danbury much different, no hat factory. Used to play at Candlewood Lake. Then I could see the prison from Field rd. It was a miracle I found the house in 1998 as I was a small child when last there.
  3. KR2C

    KR2C Platinum Subscriber Platinum Subscriber QRZ Page

    Just to clear my concious, Burt. If we're you, I would replace all those HV caps, bleeder resistors, D16, and the 51 ohm resistors if you have them.

    I would consider that routine maintanence.
  4. K1OIK

    K1OIK Ham Member QRZ Page

    I know you are extra cautious but to me it is pointless to replace what works, just as I do not allow my water heater, roof, microwave to be replaced even though they are living near the end of their life.
  5. K1OIK

    K1OIK Ham Member QRZ Page

    Here are photo of the tubes, of the three you can see the filaments are slightly displaced in these tubes.

    811 filament top.JPG 811 side.JPG
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Remember the 811A is a 80 year-old design. It has long, fragile internal filament wires that aren't all that well supported. After use it becomes more fragile. Many 811As on the market today (and for several years) are intended for audio amplifier use (although they're still labeled "811A") and don't work well in HF applications: Testing with a tube tester won't reveal any of that.

    The AL-811/H pushes the tubes to beyond their rated limit (plate voltage) by design and it takes "good" 811As to withstand that: Good meaning hipot tested at ~3kV, not tested in a tube tester at 1250V (the original 811A rating per the data sheet).

    I agree with Lou, an 811A isn't one of the tubes I'd consider buying "used."
    K1OIK likes this.
  7. K7MH

    K7MH XML Subscriber QRZ Page

    When it comes to the roof and the water heater, you may in the end wish you had!!

    They are not all that expensive new. You may have to buy a pretty big handful of them used to get a few good ones.
    Last edited: Jan 14, 2018
    K1OIK likes this.
  8. W1BR

    W1BR Premium Subscriber QRZ Page

    The spring supports for the filaments are loose in one tube-- filament is no longer intact. May have been caused by excessive G forces by the shipper.
  9. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I agree about the roof, not so much the water heater.

    I've owned many houses and replaced many water heaters: None ever exploded, they just "leak" when it's time to replace, and as long as they don't leak on anything important, this usually isn't a big deal.

    Roofs are different. You don't want one to leak, even once.:p

    RF Parts (and maybe some others) offer a good warranty on their 811As and the other tubes they sell. They also claim to test them in a real "ham radio" type environment, where they may be operated at 1700-1800V (which exceeds the tube ratings), although I've never seen that. But I do know they honor their warranty just fine and will replace any bad ones for free, and they'll pay the shipping to do that.
    K1OIK likes this.
  10. KA9JLM

    KA9JLM Ham Member QRZ Page

    Why does the glass looked cracked on those tubes ?

    Looks like you lost suction.

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