Question: AL-80B/3-500Z Driving Power

Discussion in 'Amateur Radio Amplifiers' started by KJ4BJ, May 16, 2019.

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  1. KJ4BJ

    KJ4BJ Ham Member QRZ Page

    Can anyone tell me exactly WHY the AL-80b amp is rated for a maximum 100 watts driving power? Is it because the manual states that, at 100 watts maximum drive, the amp should show a max of 250 milliamps grid current and 600 milliamps plate current which cause the 3-500Z tube to dissipate it's maximum rated 20 watts of power on the grid? These are maximum ratings as I understand it. Anything above these ratings are certain to cause damage.

    The manual states that USUAL max output occurs with 175 milliamps of grid current and about 475 -500 milliamps of plate current with a carrier such as CW. So, assuming grid and plate current as well as tube power dissipation are most important to watch, is it ok to drive with more than 100 watts if needed to get the amp to produce the rated full output at 175 milliamps grid current? If so, is there an absolute maximum drive level?

    My amp was bought used. I have no idea how old the tube is. At 20 meters and above, I can get full output with 175 milliamps grid current, or less, at about 80- 100 watts drive. On 40 and 80 meters I have to drive with about 110 watts. On 160 meters I have to drive with about 125 watts. Is this just simply an indication of the tube beginning to show it's age and is getting weaker, and a new tube will allow me to drive with the rated 85 watts or so for full output?

    Just trying to understand more. It would be most helpful if the manual was more specific and explained things in more detail. But than again, I suppose it's easier to just have the user manual explain the DO's and DON'Ts and the maximum readings, instead of providing more technical information, for most of us.
     
  2. N8FVJ

    N8FVJ Ham Member QRZ Page

    I would not worry about some extra drive power as long as you stay within maximum 3-500Z specifications. You did not state the 3-500Z power output so I cannot really comment on the 3-500Z tube condition. Also, the amplifier input SWR match may be a little high so a little more power is required for full output. Additionally, the 3-500Z tube may be getting a little soft. Usually 100 watts input produces 800+ PEP watts output. Some report 1000 PEP watts, but their watt meter may be generous.
     
  3. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    I have an AL-80B (bought it "new" in late 2000, so it's 18 years old now!) and it does not require any extra drive on 160m, or really on any band.

    65-70W drive will run it at "full power" on each band (no go on 30m, but we're not allowed such power on 30m anyway).

    What the grid current is is largely based on LOADing. The sweet spot for tuning is to peak PLATE and LOAD for max power, then increase LOADing (turn clockwise to reduce load capacitance) more while watching the grid current to keep the Ig in the 150-175 mA range. This means "over-LOADing" the amp, purposely -- and is strongly suggested by the amp's designer, W8JI on his website which provides improved tuning instructions for exactly that amplifier.

    Using excessive drive power can risk damaging the input tuning (pi) network components, and although they aren't that difficult to replace -- why do that?

    It is true that as the tube ages and loses emission, more drive will be required for normal output power; that's an early indicator the tube is getting old, however it's not "dead," just old.:) A new tube may spruce it up. I wouldn't use >100W on any band, and have found that ~70W should be "enough" to achieve 175mA Ig and at least 800W CW output power, which is what the amp is good for.
     
  4. KJ4BJ

    KJ4BJ Ham Member QRZ Page

    Thanks for the input. On 160 meters, and using the wattmeter on my 1000MP Mark5, I have to drive the amp with about 125 W to achieve 800-850 W CW carrier output. The grid current is about 160 milliamps as I recall. At these readings, I get about 900 watts PEP on ssb according to the amps meter. The SWR on the rig shows 1.4 to 1. SWR . On the output of my ATR-30 tuner the SWR is 1.2 to 1. The antenna used is a dipole cut for 1.8 MHz and is 450 ohm ladder line fed from feedpoint to shack window, then a 4:1 Comtek dual core current balun to 10 feet of RG8/U to the tuner. The tuned input on the amp is set for a 1:1 SWR at 1.9 MHz. My educated guess is that the tube may be getting a little soft, and may produce a little more output with only 85-100 W drive if the SWR on the rig was flat, assuming the rig's watt meter, and the amp watt meter, is accurate enough within about 20% or less. I am the third owner of the amp, so I have no idea how hard the amp was driven in the past, or how much the amp was used.
     
  5. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Not sure how to reconcile those two statements. Is the input tuned for 1:1 as measured by the rig or tuned for 1.4:1 as measured by the rig or is the 1.4:1 perhaps at a different frequency than 1.9 MHz? Alternatively did you temporarily place an SWR meter between the rig and the amp and use that to tune the amp for 1:1 SWR at 1.9 MHz. If you used a low level drive source like an antenna analyzer to adjust and measure that 1:1 SWR looking into the active amp then you'll get differences as the amp's input impedance changes with drive level and should be adjusted at nominal drive levels as in 75 to 100 watts if that's how you'll drive the amp.

    The likely bet is your tubes are getting a bit soft, but since you're the third owner could someone have added a swamping (aka feedback) resistor between the tuned input networks and the tube cathodes? It seems to be a common mod found on a lot of web sites and if so, that could impact required drive levels as well.
     
  6. KJ4BJ

    KJ4BJ Ham Member QRZ Page

    Ok thanks. I drive the amp with no more than about 110 watts according to the watt meter on my FT1000MP Mark 5. I had wondered about the ratings of the tuned input on the amp, but just wasn't sure what those ratings were. Having an adjustable tuned input is very nice to have to get the SWR down on the transmitter. I much prefer NOT have to make jumpers of different lengths just to find out what length provides good SWR on the input side of the amp for all the bands. 100W drive gives me about 700W CW carrier on 160 meters, which yields about 850W on ssb peaks. I think the tube may be getting a little soft, but I can live with 850W on 160 meters. More power wouldn't make any difference anyway.
     
  7. KJ4BJ

    KJ4BJ Ham Member QRZ Page

    The tuned input was tuned on 1.900.00 KHz with amp in XMIT mode, using a plastic tuning tool, with amplifier keyed and producing 850W CW carrier. Using the rig's SWR meter (built in tuner OFF) the best I could get the input was 1.4 to 1 with amp in XMIT mode. In standby, the SWR is 1.1 - 1.3 to 1, depending how wet or dry the antenna and feedline are. And yes, I inserted an outboard SWR meter between the rig and amp. Both SWR meters were at 1.5 to 1 or less with the amp in XMIT.
     
    K7TRF likes this.
  8. K7TRF

    K7TRF Premium Subscriber QRZ Page

    That makes sense. The standby SWR isn't really relevant to the drive power question as you won't be in standby and won't be seeing the actual antenna system impedance when driving the active amp. But yeah, that makes sense.

    You're not losing a lot of power and shouldn't be folding back any transmit power with a 1.5:1 active mode SWR so that gets back to either softening tubes assuming no one added a swamping resistor to the amp's input stage.
     
  9. KJ4BJ

    KJ4BJ Ham Member QRZ Page

    I failed to mention the SWR on the amp in XMIT mode is 1.2 to 1. Not sure about the swamping resistor but I would think not. When I first got the amp it would constantly reach 1KW on 160 SSB...still does on some voice peaks but not as often as it did when I first got it. Judging by all other meter readings etc., I am leaning toward the tube getting a little soft.
     
  10. WL7PM

    WL7PM Ham Member QRZ Page

    I run up to 60 watts drive into my AL-80B, original EIMAC 3-500 tube from 1996 is still doing it's job.
     

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