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Tube amp help...

Discussion in 'Amplifiers' started by T23, May 15, 2011.

  1. T23

    T23 Active Member

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    I am new to tube amps but after following a guidde on how to tune one. my amp gets less than half is rated output. It is a Lafayette ha-260 I believe its a one output tube type for VHF. There is a second tube that's smaller. Don't know if its a driver tube or for the relay cirxut. I have a 19 amp supply connected to it.
    The amp is suppose to put out 60 Watts and only puts out less than 30.
    Could the tube be weak?
    Any thoughts?



    T23
     

  2. RatsoW8

    RatsoW8 Supporting Member, W9WDX ARC Member - WD8T

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    What 2 meter radio and input power are you driving it with? If your driving and tuning it correctly and still showing low power the most likely problem with a 2 meter amp that old is the tube.
     
  3. T23

    T23 Active Member

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    I am driving it with 5 Watts out of a Kenwood tm-241a. My radio can also be set to 10 Watts out or even 50. The eBay auction said 60 Watts out tested works as it should.

    What's weird is a local regular I talk to almost everyday said my audio was different with the tube amp and I didnt even ask about my audio quality. He said it sounded more warm and smooth.


    T23
     
  4. RatsoW8

    RatsoW8 Supporting Member, W9WDX ARC Member - WD8T

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    I'm thinkng that amp doesn't need more than a couple watts of drive. I had a friend way back when I was in the Navy who had one but I just can't remember enough about it to help you. Darned if I can find a manual (free one anyway) online either.
     
  5. T23

    T23 Active Member

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    Everywhere I read it stated 5 in 60 out. Can't find a lick of info on this amp though just 2 older sale threads with info. The PRF meter only reads about half too. The tube is an amperex 8637 twin tettrode.
    T23
     
  6. HiDef

    HiDef Active Member

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    The guy is not really regular.
     
  7. T23

    T23 Active Member

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    Everyday isn't regular? I know him in person. I know he said it sounded different atleast. But that's not the point. The point is or atleast was the power output is low on this amp.

    T23
    Lets keep on subject...
     
  8. Kamikaze

    Kamikaze Active Member

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    Kinda sounds like that mudduck HiDef is trying to say the guy is F.O.S. because of the "tube sound" statement. No way FM should ever sound one iota different through an amplifier unless it was not full quieting to begin with.

    One second of overdrive could kill the grid in your tube. Just a thought.

    Are you tweeking back and fourth between plate and load? Sometimes it's a little strange on VHF because tuning can be extremely sharp.

    Try advancing the load just a little and retune. See if you can find a little more output like that. If not go the other direction.

    Is the SWR between radio and amp really high? That could be throwing the drive power measurement off.
     
  9. T23

    T23 Active Member

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    My amp has an antenna load a plate tune and a grid tune. What order do I tune these in?

    Thanks

    T23
     
  10. Kamikaze

    Kamikaze Active Member

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    With low drive power and the load set about halfway, tune the plate for peak output on a sensitive wattmeter on a low scale. Try the load next. Alternate back and fourth between these taking a break after every 20 seconds to let the tube cool off.

    Tune the grid tune control for max out of the amp. This should roughly coincide with low SWR at the driver radio's output.

    Increase drive and repeat the process.

    After you get the thing tuned it should not need retuning unless you move frequency.

    VHF amps sometimes have lousy neutralization by design and are not tuned for a dip in plate or cathode current. This differs from the average HF RF amp.

    If the manual differs from the above use the directions in the manual.
     
  11. MrSuburban

    MrSuburban Guest

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    I take Metamucil if I am not regular:whistle:
     
  12. T23

    T23 Active Member

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    I am not sure what's going on here but my prf meter at peak tune reads half. The VHF Watt meter reads about 20 or a hair less on high wattage meter setting. On low it reads more than 25 Watts and with an hf Watt meter it reads correctly on 60 Watts on the 2kw setting and reads 150 or more on the 200 setting. My VHF Watt meter also seems to add 10 Watts on the low wattage setting when just the radio is connected. While testing I can't even key up three times without the plate overheating, that's just under normal operation..

    T23

    " Won't eat prunes again!"
     
  13. Kamikaze

    Kamikaze Active Member

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    Obviously some of those meters are not going to work at VHF. Use your 50 watt radio by itself to qualify them.

    If the plate isn't designed to glow and it does glow due to problems it may have been damaged.

    If the tube is blushing it is not loaded properly or maybe the bias voltage is too low.

    Does the amp go into meltdown mode keyed up with very low drive?
     
  14. T23

    T23 Active Member

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    I am not sure about the tube blushing. But I am driving it with 5 Watts and have peaked the output on the desired frequency. The tube is a amperex 8637. The VHF Watt meter reads the correct high output of just the radio at 50 Watts. Everywhere I have read it says five watts input for this amp. What prf meter supose to read on the amp is it RF power output?

    T23
     
  15. Kamikaze

    Kamikaze Active Member

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    Is there a manual and or schematic anywhere on the www?

    What tube is that? 5894? The smaller one is for the RF sniffer as you suspected.

    If the thing is rated for 50 watts AM carrier it ought to do pretty well on FM but might benefit from a bias change.
     

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