3-400 / 3-500

Discussion in 'Amateur Radio Amplifiers' started by WA8UEG, Dec 12, 2014.

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  1. W1GCI

    W1GCI Ham Member QRZ Page

    This is an interesting thread, but I am curious how it was determined that one of the two 3-500z tubes has no output? Since the tubes operate in parallel, lifting the plate connection from one tube and then the other would show what each is contributing to the output. If this is the case, I'd suggest swapping the tubes to see if the problem is related to the HV connection through the parasitic suppressors or something related to the tube socket rather than the tube itself. My experience is that a 3-500z with filaments that light will have some amount of output - not zero - unless there's another problem.
     
  2. WA8UEG

    WA8UEG XML Subscriber QRZ Page

    It is strange. The first thing I did was to swap the tubes as I felt for sure it was a connection problem either at the socket or the HV connection but sure enough the one that was working was fine when swapped and the one that was not working still did not. The watt meter indicates what I would expect driving it with 100 watts from one 3-500 (600 watts which is what my 3-400 MKII does with one tube) and the real indication of course is that the anode has no signs of life at all (not glowing red). I am going to put the 3-400 in shortly and will post what happens. I am not familiar with the construction of these but seems that the connection from the HV cap inside the tube is no good.
     
  3. W2VW

    W2VW Ham Member QRZ Page

    Lack of connection from plate cap to plate would be very difficult to have on this tube.
     
  4. WA8UEG

    WA8UEG XML Subscriber QRZ Page

    Well it works but not well. As a couple have mentioned the miss match is just to great.

    In the tune/CW position the 3-400 is a nice normal red but the 3-500 is a very very warm red. In the SSB position the 3-400 is cherry red and the 3-500 is about where the 3-400 was in the tune position. Output was not bad considering the miss match and the fact that the 3-400 is north of 30 years old and has a ton of hours on it. The combo put out 1050 watts with a hundred watts of drive in the SSB position. I only keyed them long enough to observe, less than 5 seconds. I decided to go a step further and put a matching 3-400 in place of the 3-500 and you guessed it, everything worked as it should so I pulled the 3-400's and re attached the HV caps for the 3-500's and will wait for the new tubes to arrive before turning it on again.

    I still am bewildered as to how the one tube has filament but 0 HV, how on earth does the cap to plate connection go bad????

    At least I have confirmed it is the tube and not a problem elsewhere. I did notice that the tube working has nice crisp lettering like it has had little use while the one not working is faded to the point that you need to look very close to read it.

    Thanks for your input guys.
     
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