Amp Settings Mic Gain and Compression

Discussion in 'Amateur Radio Amplifiers' started by KF5NAJ, Nov 8, 2019.

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  1. KF5NAJ

    KF5NAJ Ham Member QRZ Page

    I have a Dentron MLA-1200 600 w Amp that works pretty good. To save the tube I only run 50 watts
    into it. I do not tune amp for max power out (500 to 600 w) but tune at 50 watts for power out which is about 350 to 400 watts. I notice on almost all tube amp instructions they say to tune for max power, I believe this may be unnecessary wear on the tube since I never drive the amp with more than 50 watts and once tuned I do not change power into the amp. I tune using CW pulser.

    Does changing mic gain or adding compression create a problem when using the amp.
    I can see changing mic gain might change power into the amp and affect amp tuning, the reason I ask is that I have a radio internal memory that calls CQ and the mic gain drops below when I'm using the mic.

    Thank you
     
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    The reason to tune up at "full power," generally, is to allow the amp to run at maximum linearity when driven by a varying power source like SSB. This method can create a cleaner transmitted signal with less chance of increasing amplifier-created distortion on voice peaks.

    Some transceivers (quite a few, actually) create "overshoot" of output power in some conditions, usually the leading edge of CW characters or voice peaks, and if the amp is tuned at reduced power those narrow, sharp and very brief peaks can damage the amplifier -- much less likely to damage anything if tuned up at maximum power first.

    IMO, it's usually best to tune up at "full power," then reduce the drive to lower power and set it wherever you want without re-tuning the amp. Exceptions might be using the amp on FM or RTTY where the drive is continuous and steady, or when using an exciter that cannot produce enough power to tune the amp at "max."
     
    AG5CK likes this.
  3. AG5CK

    AG5CK Ham Member QRZ Page

    Tuning up should take no more than a few seconds. I generally adjust the plate/tune at low power and then a quick load adjustment at full power takes only a second. If you can tune up quickly you aren't hurting anything.
     
    KW6LA likes this.
  4. KF5NAJ

    KF5NAJ Ham Member QRZ Page

    Thank you both for answering, I will in the future tune at full power

    Is this correct after max tune:
    When changing power into the amp should I fine tune the load
    When I change frequency I should fine tune the Tune adjustment

    Thank you
     
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I'd say no to the first one (if it's loaded up, it's loaded up and if this is done at "full power" that's the right setting for any power); but yes to the second one (both PLATE TUNE and LOAD may require some adjustment when changing frequencies, and surely will when changing bands). Changing frequency within one band, this "rule of thumb" seems to work pretty well: If output power drops 10% when you change frequencies, give the tuning a touch-up. If it doesn't, leave it alone.

    The MLA-1200 has two co-related weaknesses. It has an 8875 tube which today is virtually unobtanium and hasn't been manufactured in decades, so a replacement, if one can even be found, is likely to be very expensive; and related to that is that the MLA-1200 has no GRID CURRENT metering. Grid current is the most important thing to watch and tune by with such amps, and they didn't bother metering it!

    The 8875, if the fan works properly, can handle an enormous amount of plate and cathode current but can only handle 30 mA maximum grid current before you can literally destroy the tube from excessive grid dissipation. That would be the most important parameter to measure. LOADing without being able to monitor that is a tricky exercise, as grid current is the best indicator of proper loading.

    If I had one, and I don't, I'd eliminate the HV metering and re-wire the amp to use that switch position to monitor grid current. That's not a simple job, but it's possible if one knows what they're doing.
     
    WA7PRC likes this.
  6. KF5NAJ

    KF5NAJ Ham Member QRZ Page

    I was sent the instructions to change the meter to read Grid Current they were beyond my skill level
    My final tuning step is to add a little more load past final tune, I believe this keeps the grid current a little lower. Not sure how much to increase the load, I dip it approx 10w past max power
    Once that is done I do not need to change load adjustment again unless I change frequency
    .
    One more question the amp does not have a band selection for 17m, I have been tuning it up on the 15m band with no problem and I run approx 350w out, it will do close to 500w out.

    The amp tunes quick and easy, the extra 300 to 400 watts out makes a real difference

    Thank you all for the help, now I know why to tune max load
     
  7. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Yes, for 17m you would use the 15m bandswitch position.

    Increasing LOADing a bit to reduce power slightly, which usually will reduce grid current, is a great idea but of course without actually monitoring grid current it's impossible to know what effect this really has.

    The 8875 can produce more like 600W output, but I would avoid driving it that hard without being able to monitor grid current.

    Keep the cooling fan clean and in good condition, it's the only thing saving that tube.
     
  8. AI3V

    AI3V Ham Member QRZ Page

    Once again Steve runs on about grid current while completely ignoring the metering the op does have.

    Yes it does.

    More power= more heat

    While tuning up for maximum power is great for replacement tube salesmen,

    Tuning up for rated plate current at the resonance dip will be a whole lot easier on your wallet.

    DC input watts = plate volts times plate amps

    Power turned into heat in the tubes = DC input watts minus RF output watts

    If the Power turned into heat number is bigger than the manufacturer OF THE TUBE "plate dissapation rating" you are overloading the tube.

    Rege
     
  9. KF5NAJ

    KF5NAJ Ham Member QRZ Page

    Will the "overshoot" of output power caused by voice peaks on SSB be greater than the output power produced by tuning up on CW. If I final tune at 55w after slightly advancing load and then reduce/operate at 45w or 50w into the amp will that work?

    Is there a way to add an external grid meter, seems that would be easier than changing the meter switch to read grid current plus you would always know the grid current

    I am very concerned about tube life and tuning for max power does make me nervous.

    I'm going to check the fan to day

    Thank you
     
  10. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    The only way to really check for "overshoot" (if there is any) is with an oscilloscope observing the output envelope. Many popular "rig" models have been reviewed by the ARRL Lab with results published in QST, and if a rig has an overshoot problem they usually mention that; so you might look up your rig to see if it's been reviewed, and if so, read to see if there's any known problem. This is "usually" a design problem so it would not be unique to just one particular rig and not the others of the same model.

    Circuit to add grid current metering has been published before (I didn't just look, but I've seen it on amps reflectors) and if it's beyond your capability, maybe you have a local ham friend who is very experienced with amp designs, or just a good electrical engineer who may be able to help you with this?

    With an 8875 tube, tuning for a plate current dip and carefully monitoring plate dissipation does zero for extending tube life. The only fragile element is the grid, and much like monitoring heartbeat and respiration with a human patient, is the important thing to monitor: You can use a grid current "peak" as the precise indication of plate tank resonance, and the grid current quantity as an indicator of proper loading. It's really a "tell-all" indicator with such tubes that cannot handle any real grid dissipation. Tuning for plate current dip corresponding to output power and measuring dissipation without knowing what the grid current is -- is what made tube retailers rich.

    And Dentron was just about the only manufacturer to ever use the 8875 in a commercial product; so when Eimac pulled the plug on manufacturing this series of tubes (8873/8874/8875) that one was the first to be discontinued. It's a rare entity today; hams with a lot of hands-on amp experience have been known to substitute other tube types in these Dentron amps, often involving a socket and cooling system change and maybe a different filament transformer -- but it has been successfully done, mostly because the owners couldn't find an 8875!
     

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