Need a HV power transformer

Discussion in '"Boat Anchor" & Classic Equipment' started by K5DH, Jun 21, 2018.

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  1. K5DH

    K5DH Ham Member QRZ Page

    Hey y'all,

    I recently picked up a restorable Heathkit DX-40 with a bad power transformer. I need a power transformer that does the following:
    • 120 VAC primary
    • 400 to 425 VAC or 800 to 850 VCT, at 125 to 150 mA or so
    • 6.3 VAC at 3 to 5 A or so
    • maximum physical size 4-1/2" by 4-1/2" by 5" tall
    Obviously I'm not rebuilding the power supply to stock specs. I'm ditching the tube rectifier in favor of silicon diodes. I don't care if I have to modify the chassis to fit a non-original transformer. It's okay if the exterior of the transformer is dirty/cruddy/rusty as long as it works right. Of course, the leads need to be of decent usable length.

    Not interested in buying an original Heath transformer. Not interested in spending a lot of money.

    Luckily the old transformer didn't melt down and gum up the chassis with tar, or fail catastrophically and scorch the chassis. I've seen 'em fail both ways. . .
  2. W2WDX

    W2WDX Subscriber QRZ Page

    You could try a Hammond 375x, should do the job nicely. Even has 50v bias sec and a 5V sec if you need em. Is $250 to pricey? Maybe ... but it will work. And you know it will work. Any junker piece may work ... maybe ... for a few minutes ... possibly ... for 20 years ... maybe. Here's the spec sheet:

    Attached Files:

    • 375X.pdf
      File size:
      284.9 KB
    Last edited: Jun 21, 2018
  3. W4KJG

    W4KJG Subscriber QRZ Page

    Take a look at Here is a link to their transformers:

    His prices are very reasonable for boat anchor power transformers. You can probably find the right power levels, but the transformer probably won't look like the original DX-40 transformer.

    Good luck,
  4. W9GB

    W9GB Ham Member QRZ Page

    Original HeathKit DX-40 Transformer (Peter Dahl database)

    CORE) EI-l25 X 1.75 29GA M6X
    PR) 117 VAC 60 HZ 1 PH
    S1) 750-0-750 VAC @ 150MA ICAS (FWCT)
    S2) 6.3 VAC @ 3.25A CCS
    S3) 5.0 VAC @ 3A CCS
    S4) 6.3 VAC @ 0.65A CCS
    DM) HT = 4.290 WT = 3.600 DT = 3.200 MD = 3.890 MW = 2.750 WEIGHT) 12 LBS
    When Scrounging, or Looking for Cheap ....

    The power transformer from any D&A Mfg. sweep-tube CB amplifier should work.
    In the late 1960s, E.J Delaney of D&A Mfg. specified used “off-shelf” Stancor transformers in his sweep tube designs. A transformer of this size should work well in your restoration project.
    Maverick 250
    Hornet (redrawn schematic)

    These 50+ year old CB amplifiers appear at Garage sales, flea markets or Craigslist.
    Sweep Tubes worth more than rest of the parts.

    D&A Phantom 500 (CB sweep tube amplifier) used the Stancor P-8339 Power Transformer (discontinued model). In fact, there were three (3) Stancor transformers in that HV power supply.

    Stancor Transformer Catalog (circa 1966)
    from the Bunker of Doom
    P-8339 found on PAGE 14, (page 15 of PDF)
    The Stancor P-8339 transformer was used in vacuum tube audio amplifiers and
    some entry level AM/CW transmitters or receivers from the 1950s and 1960s.

    Stancor Transformer “Style” Letters

    Peter Dahl database of Transformers, manufacturers with “S” listing.

    STANCOR P-8339
    STYLE) C, with a copper shorting band to reduce external magnetic field
    PR) 117 VAC 60 HZ 1 PH
    S1) 650 VCT @ 300MA CCS
    S2) 5.0 VAC @ 3A CCS
    S3) 12.6 VCT @ 5.25A CCS
    DM) 3.75 x 4.00 x 4.875 x 3.00 x 3.75 x 4.75 x (3/16 x 1/2)
    WEIGHT) 8.5 LBS
    So, STRIP these old 1960s/1970s CB amplifiers for the Sweep Tubes and the power transformer
    Plenty of Vacuum tube audiophiles have CASH $$ to burn.

    IF you had to rewind (bad) or build a new transformer,
    Heyboer Transformer (Grand Haven, MI) does quailty work.
    Last edited: Jun 21, 2018
    N2EY likes this.
  5. KE4OH

    KE4OH Ham Member QRZ Page

    You may not like my suggestion: Look for beat up junk DX-40 that has a good transformer. In the long run, it will be cheaper that way. DX-40 is not a rare item. I can't imagine paying more for a transformer than an entire working DX-40 is worth. But that's just me. And I love keeping this old gear running.

    You probably know this, but in case you don't, the DX-40 has 2 6.3v windings due to their power supply scheme that puts the driver tube at a higher potential than the others. If you don't have an original transformer or a replacement that has 2 windings, you will also need to buy a separate 6.3v filament transformer to power the driver tube.

    I do feel your some of your pain, though. I have a DX-35, which has the same basic design and a similar weak power transformer. In my case, I got very lucky and my transformer failure was only the beefier of the two 6.3v windings. The hard part for me was finding a place to mount the oversized filament transformer I bought.
  6. AC0OB

    AC0OB Subscriber QRZ Page

    I highly recommend revamping the power supply with a new Power Transformer wit a Full Wave Voltage doubler which they used later in the DX-60.

    Changing the B+ power feed to the OSC and Buffer/Driver is an upgrade that will stabilize the RF section.


    Attached Files:

  7. N2EY

    N2EY Premium Subscriber QRZ Page

    Only one problem with that approach: Finding a junker DX-40 with a good power transformer. The power iron is the weak spot in both the DX-35 and DX-40.

    Plus, if @K5DH does find an exact replacement.....he'll still have that weak point. If he uses a more-rugged transformer, that problem will be eliminated.

    That issue is easily resolved by eliminating the "series feed" system used in those rigs. This can be done with either the voltage doubler or "economy" rectifier circuits.

    If it were my rig, I'd hunt up a suitable power transformer and use the full-wave-bridge/economy circuit, and get two B+ levels. I'd run all the low-level stuff off the low-B+ and the final from the high-B+. Would require a bunch of electrolytics and some silicon but that's par for the course. The result would be a much more rugged power supply.

    73 de Jim, N2EY
  8. K5DH

    K5DH Ham Member QRZ Page

    At W4KJG's suggestion, I checked out the Old Radio Parts web site. They have several transformers listed that will fill the bill, at a price I can swallow. I'll give them a call this week and procure one. Thanks for all of the suggestions and advice. It may be a while, but I'll try to remember to post some pictures after I get the DX-40 working, and a copy of the new power supply schematic.
    N2EY and KE4OH like this.
  9. K5DH

    K5DH Ham Member QRZ Page

    I purchased a suitable transformer from Old Radio Parts. Thanks again to W4KJG for the lead! The replacement transformer is a big honkin' sealed military unit. Compared to the original wimpy Heathkit transformer, the replacement is a behemoth! I prototyped the supply on the bench. I'm using a capacitor-input filter with 23.5 uF (two 47's in series), the original Heath choke, and another 23.5 uF, with the original Heathkit 20 K, 10 W bleeder resistors across the output caps. The output voltage is quite a bit higher than the original supply. My supply puts out 633 VDC, whereas the original was supposed to put out 580 VDC. The 6146 will have no problem with the increased voltage, but I'll probably have to add some plate dropping resistors and increase the value of the screen grid resistors on the other tubes. I'm wondering if a current limiting resistor will be necessary to deal with the inrush as the filter capacitors charge. I don't want to over-stress that ancient filter choke. Should I include a current limiting resistor? If so, what resistance and wattage? As I recall, a few Ohms ought to be sufficient. I thought about doing away with the choke, but I figured I might as well go ahead and use it since there's nothing wrong with it.

    I have modified the DX-40's chassis to fit the new transformer, and I'm ready to start building up the power supply section. More to come.

    Here's the DX-40, as it looked when I got it home from the hamfest.


    And here's the chassis with the new transformer installed. Like I said, it's a big 'un!



    More to come. . .
    KD2ACO and K1OIK like this.
  10. N2EY

    N2EY Premium Subscriber QRZ Page

    I take it that you'll be using silicon rectifiers in the classic full-wave center-tap configuration. (I'd go with at least 2, and preferably 3, 1N5408s in series).

    If so:

    1) Don't worry about capacitor-charging surge. The choke does that for you. It won't be strained.

    2) All the concerns about not using the accessory socket to power a VFO or anything else are based on the delicacy of the power transformer. You don't need to worry about that any more!

    3) Don't worry too much about the higher plate voltage. In particular, do NOT put a dropping resistor in the lead to the 6146 plate; there's absolutely no need for it. Plus that 580 volt measurement is key-down, IIRC, and you were probably measuring it with just the bleeder load.

    Also see 4) below.

    4) I can't read the transformer specifications from the picture, but I do note that there are three secondaries besides the HV. One of these is almost certainly a 5 volt rectifier filament winding, which you don't need with silicon rectifiers. If so, you might consider wiring it in series with the primary.

    Here's why:

    Many older transformers were designed to be operated from 110 -115 volts or so. In the bad old days, line voltages in homes could be as low as 105 volts, so the transformers were designed for a lower voltage range. Today, it's common to get more than 125 volts at the outlet in many homes, which can stress some older gear.

    By wiring the heater filament winding in series with the primary (and phasing it correctly), you'll get a few more primary turns and slightly lower output voltages, which is all to the good in many cases. And all it takes is one piece of wire and a few added connections. Adding a 5 volt winding results in about a 4% voltage reduction, max.

    OTOH, you might consider using a tube rectifier, in which case the 5 volt winding is needed for the filament. The resulting voltage will be lower so you'll be all set.

    What are the transformer ratings?

    73 de Jim, N2EY
    KC8VWM likes this.

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