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  • #1379
    cvrs
    Forum Participant

    I am re-posting this proceedure from the newsgroup rec.antique.radio+phono with Roger’s Permission.
    Phil

    Hi, Vacuumlanders,
    Warning: not rocket science, just an easy modification (but see
    disclaimer, below.)

    While working on a 1938 Rogers Majestic chassis, similar to models
    13/75 and 875 (per the RCC CD), with two 2×3 rectifier tubes, it
    crossed my mind that I only had the two of them (testing "good+", no
    spares) but nearly a dozen 5Y3’s. Since the 2X3 is about the same as
    1/2 a 5Y3, but somewhat less common, it seemed like a good idea to
    substitute the 5Y3 in one of the sockets and preserve the 2X3’s, at
    least for all the on/off testing. It turns out that it is very easy
    to do this while still preserving the ability to plug back in the
    2X3’s whevever one wants to be "authentic". Here’s how…

    Call the rectifier sockets A and B. The 5Y3 will go into socket B.
    Socket A will have a modified tube base inserted (see below.) The 2X3
    has a 2.5 volt filament (pins 2 & 8) and they are in series across the
    5 VAC rectifier heater winding. For some reason the B+ take-off in the
    original is from one side of this – socket A, pin 8. I had already
    changed that to a mid-point take-off to minimize the AC component from
    the 5 VAC supply (small as it compared to the full-wave ripple
    voltage), i.e. from pin 8 of socket B. Most likely you’ll have to
    change the tube socket B for one with a full set of pins since the
    originals will likely only have pins sockets 1, 2, 4 & 8 for the 2X3.
    I took out the rivets carefully by grinding with a Dremel tool and
    then punching out with finishing nail punch.

    Connect a well insulated link from socket A, pin 4, to socket B, pin
    6, to put HV on a 5Y3 plate. The other 5Y3 plate is on the same pin
    as the 2X3 plate (pin 4), already connected when socket B was
    replaced.

    Take a cleaned-out tube base and short-out pins 2 and 8, the 2X3
    filament pins. For safety, insulate the top with a glued-in piece of
    cardboard and mark it "2 to 8 shorted". Plug it into socket A. Plug
    a 5Y3 into socket B.

    Check all wiring, heater and HV, VERY carefully, i.e. measure each
    side of the HV winding to ground (should be in the 300 ohm range);
    between the two 5Y3 plates should show 600 ohms, or so; 5Y3 filament
    to ground should show just the smoothing caps (R rising to several
    megohms or up to any circuit shunts across the B+.)

    Bring the set up very slowly on a variac watching the initial CURRENT
    – any HV AC short will show immediately as a primary current spike
    over 0.3 amp, or so – but since you’ve double checked the wiring this
    won’t happen, will it?! Finally, watch the B+ go up. At 115 VAC in
    you will have the original B+ and the job is done.

    Save the 2X3’s for whatever purpose you may have… mine are now in
    the "collectible tubes" box as this particular Roger-Majestic chassis
    is destined to be a workshop "daily driver", at least for a while.
    I’ve also replaced the 41M (sent to "collectibles") with a 6V6 plus
    cathode bias (6K6 is the exact substitute but I wanted a bit more B+
    current draw on the modified p/s); added a new OPT (the one I found
    there was a very wrong "replacement" – 1/2 a battery set P-P unit!);
    redesigned the power supply and B+ filter to allow the use of a PM
    workshop speaker rather than the original field coil winding (which
    works well, original LS also now in "collectibles")… but that’s
    another story.

    Disclaimer: Don’t do any of the above if you are not experienced in
    high-voltage tube work. Voltages are lethally high.

    A final question: why did they use two 2X3’s instead of the, I think,
    then available 5Y3 or 5Y4? Was it just to push up the tube count?

    Cheers,

    Roger

    #1486
    ferris
    Forum Participant

    Hello,

    Here is a link to photobucket. It shows a Rogers replacement tube manual that includes info on replacing the two 2X3’s with a 5Y3G, 80, or 5Y4G.
    https://s169.photobucket.com/albums/u205 … _Tube-Sub/

    Thanks to Jim Frey and John Bartley at the Antique Radio Forum for sharing the information.

    -ferris

    #1487
    Gerry O’Hara
    Keymaster

    This is a fairly simple re-wiring mod. I did this on my DeForest Crosley Model 870 (Rogers Chassis 8M-721) as a stop-gap until I found another 2X3 (I only had one) – see my posting on this set under ‘Show and Tell’. I share Phils puzzlement as to why Rogers ever used two half wave rectifier tubes to act as a full wave rectifier (they did in many of their sets in this period – well after the widespread use of dual rectifier tubes). Like Phil, I can only think of cynical reasons like more tube sales and ‘tube count boasting rights’ by Rogers marketing folks. Anyone have a good technical reason?

    Gerry

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