2 Meter Homebrew Amp. Thinking of Tube Substitute

Discussion in 'Amateur Radio Amplifiers' started by K2WH, Oct 18, 2011.

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  1. K2WH

    K2WH Premium Subscriber QRZ Page

    Just want to pass this by this learned crowd.

    I am currently building a 500 watt 2 meter amp using a 4cx250 vacuum tube. However upon reading the specs on a 4cx400, aside from some small physical dimension and small interelectrode capacitance differences, I believe I can use the 4cx400 instead of the 4cx250 in the same circuit. According to the reviews of the tube, the output is cleaner and it will provide about twice the output power as the 250.

    What do you think?

  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Should work, if you have a big enough power supply for the added power.

    Is the 4CX400 still in production? I thought this was a Russian tube that may no longer be in production...

    I'd use a pair of 4CX250s only because they work really well in p-p with a link coupled output circuit and I have a lot of them!
  3. K2WH

    K2WH Premium Subscriber QRZ Page

    Yes they are Russian. Their name for them is GS-36B.

    RF Parts apparently has them also NOS.

  4. KM3F

    KM3F Ham Member QRZ Page

    Single 4cx250 type is only good for about 300 watts +/- on 2m run flat out.
    A single Russion GS36B is good for about 500+/- on 2m.
    I have experience on 6m with both these types.
    Retofitted a 250F amp with the 36B and raised the output from 325 to 600 PEP rated power out on that band.
    All the 150, 250, 350, 400 tubes are interchangable in the same socket as well as the 36B as long as the power supply can keep up current wise.
    Differences is mainly idle current bias.
    The 36B will take 2400 DC on the anode but do limit the drive if grid driven to well less than 5 watts.
    These tubes will keep taking drive until they distroy themselves by overheating and arc over if the DC limits are not adhered to.
    Once they arc over, you will see the power down and slumping, however they most often can be coaxed back to full output if you play with them idling on heater only then pulsing them current wise to get the emmission back up again.
    Awhile back I got the last pair of military select 36B tubes from my Russion supplier. He may have found more by now.
    Do pay attention to the air tube cover vs anode diameter.
    If a 250 size Ceramic air cover is machined just right it can be used on all the sizes if it air seals around the top and bottom of the anode.
    Also for cost the 250F is cheap if you can provide 26 volts AC for the heater.
    In my 6m amp I can use any version without big changes.
    I assume you know the cooling is by forced air.
    Graingers now has the blower line that used to be called Dayton, but under other part numbers.
    The 2009 ARRL handbook gives airflow info as well as the Dayton blower numbers for these tubes.
    Research on these tube type versions yields a lot of info.
    It appears the 350F verson is the best for IMD.
    If you can put some resistance in the tube kathode to ground, it help IMD even more.
    The screen supply needs to be pretty well regulated by design.
    Good luck.
  5. K2WH

    K2WH Premium Subscriber QRZ Page

    Thanks for the good info. I was waiting to see if someone has interchanged the 250 for the 400. Your reply has given me the incentive to "Just do it".

    G4CBZ likes this.
  6. WA7KKP

    WA7KKP Ham Member QRZ Page

    I'd stick to 250's . . . they're a very common IPA tube in FM transmitters (read this as cheap useable pulls).

    Gary WA7KKP
  7. KM1H

    KM1H Ham Member QRZ Page

    The 250 pulls from FM use in particular make poor SSB tubes. They are beat to hell, have screen emission and sound like a CB rig. And yes they are all over 6 and 2M and should be banned.

    There are a few in the 250 family that arent too bad for IMD but the Svetlana 4CX400A is better and a sweet tube to work with. Ive retrofitted into NCL-2000's with 2800-2900V standby and 2500V full load, Dentron Clipperton V, and several other 250 based amps where there is room for the wider anode cooler which is almost identical to the 8930 and DX-393 (Amperex). They also run an easy and clean 350W PEP in the AM-6154/55 (144/222/432) and over 400W on CW with about 3W of drive.

  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Yeah, but...but...I have a drawer full of brand new, never used, 4CX250Bs.

    IMD on 2m isn't that important unless you happen to live quite close to other weak signal enthusiasts. At a few hundred miles, it doesn't matter much. On CW or off the moon, it doesn't matter at all.

    I used P-P 4CX250Bs in my 2m PAs for a very long time. I'd keep them in AB1 with no grid current at all and they could still make 600-800W output and seemed to be pretty clean. In Class C on FM, they'd make 1 kW output and back in the old days when a lot of contest contacts were on 146.52 (in the 70s, before the rules change) I made hundreds of contacts that way. 1 kW to a Stationmaster at 70 feet drew a lot of answers back then.:eek:
  9. KM1H

    KM1H Ham Member QRZ Page

    In a high activity area the IMD can result in wars.

    Ages ago (mid 90's) I worked with G3SEK in his development of the "Tetrode Board" specifically for 250B's and a 5-10dB IMD improvement is obtained when starting with new tubes. The Johnson 6&2 T-Bolt was close to 10dB but it started at only -22dB. With the 8122's in a NCL-2000 its 5db (-33dB) and its 6dB (-38dB) with the already excellent 4CX400A/GS-36B in the same amp.

  10. WA9SVD

    WA9SVD Ham Member QRZ Page

    Well, if you're willing to toss a few 4CX400A's my way I'd appreciate it , as I'm starting to build a 6 Meter and then a 2 Meter linear, but I have a supply of 4CX250's and a couple of 4CX300A's (for 2Meters,) so that's what I'm inclined to use. I'll be careful to run the amps in AB1, and have already included stripline filters in the design. Is that adequate?
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