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ARRL handbook 4cx250 amp for HF?

Discussion in 'Homebrew and Kit Projects' started by VA3DXV, May 15, 2018.

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  1. VA3DXV

    VA3DXV Ham Member QRZ Page

    Hey folks,

    I have someones unfinished amplifier project... It's a dual 4cx250 (or 150 or 350?) for HF. Looks like taps on the big inductor for 80, 40, 20, 15 and 10.

    It's well built, so instead of tearing it down for parts I'd like to maybe finish it. The guy I got it from said it was a design from the ARRL handbook but I'm not sure what year. I've searched a bunch of them and haven't found any HF amp using those tubes. Most of the amp designs for these tubes were for 6m/2m and 70cm.

    The amp chassis has the filament transformer and a 24v DC supply for the amphenol relays, I have a separate unrelated power supply I can use for the plate and screen.

    I was hoping an amp guru might be familiar with it and could tell me what year handbook to look for. If anyone has any ideas what year ARRL handbook might have the schematic and description I'd love to hear! Thanks!
  2. W1VT

    W1VT Ham Member QRZ Page

  3. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    I just looked at that article and was astonished to find some things I'd never do, having used 4CX250-series tubes for many years in many amplifiers.

    VA3, if you're looking to complete the amp, I'd recommend changing a few things.

    The author didn't regulate the grid bias supply. If Ig is always zero, then of course this wouldn't be needed; but it won't be. It looks like Eg is through a 5000 Ohm potentiometer; if that's set mid-way and grid current is even a measly 10 mA, the grid current can wiggle by 25V. That seems like a poor idea. I always electronically regulate the grid bias supply.

    He went to great lengths to regulate the screen supply, which is great, but in the article comments that Ig2 can run up to 60 mA (!) -- which I've never seen happen in my life with sufficient B+ and proper loading. It's usually close to zero if the HV/Eg2 ratio is at least 5:1. Regulation's a great idea, but the Ig2 should never run up to 60 mA, that's just shy of destruction.

    Also, it looks like his screen and plate supplies are completely independent of each other, so if the B+ fails for some reason, screen voltage could remain active. Bad idea, that leads to absolute destruction very quickly. I'd always link these, so if B+ is not there, Eg2 cannot be, either.

    Looks like he did a beautiful job, though!
    K2XT, WA7PRC and KA9JLM like this.
  4. N2EY

    N2EY Premium Subscriber QRZ Page


    He says it gets that high in Class C. Seems a bit much...

    An easy way to do this is to have a DC relay with a high resistance coil which operates from the B+ through a suitable string of dropping resistors, whose contacts open the screen supply. That way, if the B+ fails, the screen will not get any voltage.

    One other point....The time-delay relay is energized whenever the heaters are lit. I prefer to have a stick relay repeat the TDR, so that a power failure causes the entire delay to run again. See QST for May, 1995, page 84, for more info on Amperite thermal time delay relays.


    In those days, you didn't see a lot of amateur HF amplifiers using tubes like the 4CX250. I think this was a cost thing - with 3-500Zs and 572Bs available from American tubemakers at relatively low prices, external-anode tubes were mostly used for VHF/UHF. One notable exception was the NCL-2000, with a pair of 8122s. The development of tubes such as the 8873 changed the trend.

    73 de Jim, N2EY
    K2XT likes this.
  5. KM1H

    KM1H Ham Member QRZ Page

    The 4CX250B is a poor SSB IMD performer by current standards.

    Here is a late Eimac spec sheet that covers linear and Class C where high grid current is the norm.
    There are a couple of good circuit boards available to take care of the grid and screen voltage issues. Here is one and another I forget the call out in W7 land

    The 4CX350FJ is an excellent SSB tube and not that all hard to find still NIB

    Last edited: May 16, 2018
    K2XT likes this.
  6. VA3DXV

    VA3DXV Ham Member QRZ Page

    Thanks for your replies all. Yes I'm not sold on the 4CX250. I'm not convinced the LKA-1 is what my amp was based off either.

    I may be mistaken as to what tubes it was intended for, it could be 8321's (4CX350A) as there was some of those in the box with it as well. I assume those aren't the same as the 4CX350FJ though. There's definitely some not insignificant differences in the datasheets.
  7. N2EY

    N2EY Premium Subscriber QRZ Page

    Could you post some pictures?


    73 es GL de Jim, N2EY
  8. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    4X150A/4CX250B family tubes were very popular in VHF (and even some UHF) amps for a long time because they were cheap, very available, powerful for their size, and work up to about 400 MHz. Good VHF tubes are usually very "stable" when used on HF. Of course Collins used them in the late 50s in the KWS-1.

    But they aren't great for IMD. Collins tried some tricks to improve that, but the KWS-1 came out when AM was still the predominant phone mode, signals were wide, and maybe nobody noticed.:p

    Tubes with serious grid dissipation capability, especially the Eimac power grid triodes, are way better for simple GG designs and of course really took over. The NCL-2000 and SR-2000 both used 8122s, which at least are still available new.:)

    But 4CX250-series tubes are still widely available as NOS (surplus) pretty cheaply. If not overdriven or used in circuits where the screen supply can be active without the plate supply, the damned things seem to last forever. I have original 4CX250Bs in my homebrew 2m P-P amplifier -- they were "new" in 1979 and will be 39 years old this year. They've been "heavily used" and show no signs of deterioration.
    N2EY and WA7PRC like this.
  9. W8EJO

    W8EJO Ham Member QRZ Page

    Check the Orr handbooks. he had several designs using tetrodes.

    See here for download:
  10. WA9SVD

    WA9SVD Ham Member QRZ Page

    One other thing pointed out by Bill Orr: many of the ceramic tetrodes (including the 4CX250 and like) should NEVER be connected in usual grounded grid fashion, with BOTH control and screen grids tied to ground. Keying such an amp for more than a second or two WILL result in immediate destruction of the tubes. It's been tried, and even was a QST construction article in "daze of yore," but that was when military surplus tubes could be found for $10.00 !
    There are still Bill Orr "Amateur Application Notes" available from the EIMAC site, but it may have a different name now, but GOOGLE "EIMAC" and you should find the site. Read the notes on tetrodes.

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