Driving An Amplifier

Discussion in 'Amateur Radio Amplifiers' started by N9CUC, Nov 5, 2016.

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  1. AA7QQ

    AA7QQ Ham Member QRZ Page

    W8JI (The designer of the amp) states on his page that when you have high input SWR, it is usually due to a bad tube.

    KC8VWM likes this.
  2. WZ7U

    WZ7U Ham Member QRZ Page

    You must've seen something the rest of us (or at least a few of us) didn't, because the sentences otherwise makes no sense. What gives? Where is #172 & 180, or did the ignore feature kick in?
  3. KM1H

    KM1H Ham Member QRZ Page

    That post was quickly pulled, it was a diatribe full of F bombs and similar directed at a wide swath of forum members
  4. WZ7U

    WZ7U Ham Member QRZ Page

    Ah, someone spooled up F bombs. Never a problem till they get dropped, huh. That happens from time to time. Thanks for the 'splainin. I hope I was one of the targeted :eek:

    I keep several spooled up, on the ready, just in case. The case hasn't been made yet to deploy one so all is good in Whoville.

    so far............o_O
  5. N9CUC

    N9CUC Ham Member QRZ Page

    That seems to be what I am finding out. And what my electrical engineer seemed to think as well. Hopefully this will get sorted out next week and i'll be back online with the 811H.
  6. KS9Q

    KS9Q Ham Member QRZ Page

    Hi Mike,

    An amplifier is a tuned circuit that has to function over a fairly wide range of frequencies. Each position of the bandswitch engages a specific set of tuning elements whose purpose is to match the input of the tubes to the source and the output of the tubes to the load on a given band. Any change in the operational characteristic in even one of the four tubes (as they are in parallel, not unlike 4 horses yoked together) will impact the circuit as a whole, and most likely on all bands, because now the impedance of the tube(s) becomes something other than what it's expected to be. Since you've indicated higher than acceptable SWR on all bands (and I would say anything greater than 1.5:1 is cause for concern. 2:1 is... m'ehhhh - okay but could be better. Any higher than 2:1 and something is definitely not right), that has me thinking maybe a tube is out of whack. The amp is also neutralized and if there's an adjustment and it isn't right, it can impact the higher bands. Neutralization is not a job for beginners, BTW.

    If just a single band is giving problems then I'd be questioning the mechanical switching. The higher bands will also be a little more tricky by nature to tune because their peak points are narrower. Beyond that I'd have to see the amp and do some diagnostics to figure out exactly what's going on.
    2E0NCY likes this.
  7. WA9SVD

    WA9SVD Ham Member QRZ Page

    Just let the clutch out slowly, while applying increased "gas."
  8. N9CUC

    N9CUC Ham Member QRZ Page

    Nope, its not just 15M. Have the same problems on 10M and 20M. It tunes okay on 40M and 80M.
  9. KS9Q

    KS9Q Ham Member QRZ Page

    I have an SB-200 amp which is an old school linear using a couple of 572B tubes (beefy and more powerful version of the 811A). I've been chasing an issue with it in which the amp oscillates at 20, 15 and 10 meters, but only when I have my tuner switched in and only with zero drive. Select the dummy load or directly feeding a well matched antenna - no problems. I suppose you could be seeing something similar in your unit. If one of the tubes is, in fact, bad, it might throw off the neutralization circuit which is intended to keep the amp stable. This phenomenon is difficult to see if you don't have a monitor scope but I also see a substantial indication on my reflected power meter while the amp is oscillating. Might be worth looking into. Without seeing the unit directly I'm taking a shot in the dark.


    Jim K. - KS9Q
  10. N9CUC

    N9CUC Ham Member QRZ Page

    Hopefully this issue will be resolved this week. The folks at the Ham Station in Evansville will be back in the office after the Thanksgiving holiday starting tomorrow I believe. They are good to work with so hopefully my 811H will be back on the air later in the week.

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