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3-400 / 3-500

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

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  1. WA8UEG

    WA8UEG XML Subscriber QRZ Page

    I was given a very nice late model Swan MKII, I have an early 3-400 one that I have been using since the 80's. I built a PC board and replaced the caps, bleeders, diodes in the power supply. Although the Eimac 3-500's look great one has no output, filaments are fine but no high voltage. My question is I have an extra 3-400 that is good, of course the 3-400's are 0 bias and the later Swan has bias added for the 3-500's. Rather than purchase a new 3-500 what ill effects would happen if I ran the 3-400 with bias added with the 3-500? :confused:
  2. K5RT

    K5RT Ham Member QRZ Page

    I'm not following you.

    You have two amplifiers, one with HV and one with no HV?

    The one with HV has a bad 3-500 and you are asking if you can use a 3-400 in its place?

  3. W8GP

    W8GP Subscriber QRZ Page

    Your post was a little confusing, but to answer your question about running a 3-400 with a 3-500, don't do it. The gain of the tubes is much different and would result in a gross imbalance of grid and plate current.
  4. WA8UEG

    WA8UEG XML Subscriber QRZ Page


    Question is can I run (1) 3-400 and (1) 3-500 in the MKII. The MKII that I am asking about has zener diode bias for operating 3-500's. My older MKII which was designed to run only 3-400's has no zener diode bias.

    If I replace the bad 3-500 in the newer amp which has zener diode bias with a 3-400 (no bias needed) will it cause any problems.

    I'm going to replace the bad 3-500 with a new one but would like to have an amp for the 10 meter contest this weekend and will not have a new tube till next week. I have a good 3-400 I can pop in.
  5. WA8UEG

    WA8UEG XML Subscriber QRZ Page

    Thanks, I'll hold off till the new tubes arrive.
  6. W1QJ

    W1QJ Ham Member QRZ Page

    You could try it. It will probably be OK but I would watch the anode color and see if one tube is being overworked. It's a mismatch but I think it will be OK but I would not leave it like that for a great length of time. The biasing is not a big deal.
  7. WA8UEG

    WA8UEG XML Subscriber QRZ Page

    Thanks, it's easy enough to pop it in and observe the anodes. My concern was more with the bias, just not familiar enough to know if that would hurt anything. Really will be no big deal if I lose the 3-400 or 3-500 as I have a pair of 3-500 on their way. I'm adding bias to the older amp and converting it to 3-500's also. 3-400's are now just about impossible to find NOS and even then their a crap shoot.
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Steve, in a pinch you could use the amp with just one tube in it (if the filaments aren't wired in series -- I forgot if they are or not).

    I've used 2x3-500Z amps with just one tube lots of times. The tuning is a bit different, but should be within range of the controls, and of course you get "half the power," but it's still a lot more than barefoot.
  9. WA8UEG

    WA8UEG XML Subscriber QRZ Page

    No there not in series. It does work fine with just the 1 tube, about 600 watts. The loading is a little funky but works.

    I'm going to pop the 3-400 in it just to see how it works, if it loads OK I'll keep a close eye on the anodes with key down.
  10. W2VW

    W2VW XML Subscriber QRZ Page

    It will work better than you might think.

    Once I ran a triode connected 4-400 in parallel with a 3-500z.

    The tubes are not a whole lot different.

    I took a pair of 3-400s for a ride in a TL-922 once and it just acted as if it had worn 3-500zs until I increased the drive a bit. Conduction angle will be down so drive requirement goes up along with distortion.
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