Alpha 99 is back up...... BUT ?

Discussion in 'Amateur Radio Amplifiers' started by N6DCN, Mar 1, 2011.

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  1. N6DCN

    N6DCN Ham Member QRZ Page

    I have a Alpha 99 that I recently got going again. Had to put in a new blower, new tubes, and allot of repairs to the screen voltage circuit. It seems to be doing well except for the exciter (Drive) power seems off.

    Plate voltage is 3000v. Screen voltage is 325V

    According to the manual...."15w of CW carrier should put out approximately 300w to 500w"
    If I drive it with 15W it puts 800w out.

    At 30w drive I get 1500w out.

    I thought It should have 45w to 60w Drive to do 1500w ?????

    Hoping to get some input from QRZ.

    73,Thanks
    Ron
     
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    What are you measuring the power with? Is it into a dummy load (zero reflected power) or into an antenna that has some reflected power?
     
  3. N6DCN

    N6DCN Ham Member QRZ Page

    Well hi Steve been quite a while. Hope all is well......... Anyway 1.0 to 1.1:1 before and after the amp and the swamping resistors are good

    Ron
     
  4. W8JI

    W8JI Ham Member QRZ Page


    You almost certainly want to correct that. The last thing you want is an amp that drives that easy on a 100 watt radio, especially a tetrode amp!!! Tetrodes are enough trouble without having gain like that.

    What is the grid bias? Can you measure it?

    What is the grid swamping resistance?

    What is the normal screen voltage? Is it normal now at 325?? Excessive screen voltage can cause excessive gain. If screen is normal and you want to reduce gain, you'll have to change the swamping resistance or add an attenutator. Changing swamping resistance would be better for stability.

    73 Tom
     
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Hi Ron,

    Did you confirm the output power with a known calibrated wattmeter? 17 dB is a lot of gain, I'd be afraid of spiking it to destruction.
     
  6. N6DCN

    N6DCN Ham Member QRZ Page

    Hi Tom,

    What is the grid bias? Can you measure it?

    I'm not sure what it should be. Been trying to find out from Alpha with no luck. I would be guessing that the measurement would be taken @ pin 1 in tube deck. I just don't know.

    What is the grid swamping resistance?

    The measurement is 50.2 ohm. It is two 100 ohm 50w resistors in parallel to ground

    What is the normal screen voltage?

    Yes it is normal and stable @ 325v

    Hi Steve,

    Yes I did.

    I have been running it this way between 800w - 1400w being real careful with tuning and drive power for about 3 weeks. It is real stable and consistent.

    The bias has been my suspect but I can't seem to find out what it should be and where it should be checked at

    Thanks guys
    Ron
     
  7. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Interesting that plate voltage is 3000V: The 4CX800A data sheet I have (Svetlana) states 2500V maximum.

    Same data sheet shows typical AB2 operation with:

    Screen voltage = 350V
    Grid voltage = -47V (operate), -150V (cutoff/standby)
    Idling current per tube = 70 mA

    Yes, the control grid is Pin 1 on the socket.
     
  8. N6DCN

    N6DCN Ham Member QRZ Page

    The PV is about 2700v key down

    The alpha 99 did have a screen voltage of 350v on most of them. then they came out with a mod to lower it. Mine is a 2006 model that had this mod done by the factory.

    I don't know why I didn't check the tube specs. Good idea Steve. So pin one should be -47v @ key down and -150v when idling?
     
  9. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Not -150 when "idling," -150 when in standby (not keyed to transmit). And that's never very critical: As long as the tubes are cut off and not generating heat and noise, whatever voltage does that is fine. The "operating" bias should provide about 140mA idling current for two tubes. That's usually not very critical, either. I'd guess 100mA is okay, and 200mA will generate more heat but is also okay. With 2500V Ep and 350V Eg2, it should take about -47V to achieve this "operating" level, according to the Svetlana data sheet.

    Operating bias for tetrode amps is usually adjustable because gain varies from brand to brand and even tube to tube. No mention of setting idling current in the manual? Or even what it should be (idling current, amp keyed but not driven with RF)?

    I don't have an Alpha 99 manual and a quick search didn't find one for me.
     
  10. N6DCN

    N6DCN Ham Member QRZ Page

    There are 3 manuals because there is 3 or 4 different schematics of the 99. Mine is half 99 and half 8100 depending on the circuit board. So I am having a heck of a time going back and forth through the two schematics
     
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