G.E. UPX-003B phono pre-amp - half wave to full have rectifier

Discussion in 'Tube Audio' started by Piper Blvd, Sep 14, 2018 at 4:49 PM.

  1. Piper Blvd

    Piper Blvd New Member

    Messages:
    9
    Hello!!

    I just acquired a General Electric UPX-003B phono pre-amp (to be used with a GE VR II & a GE RPX pick up).

    I plan on doing a complete overhaul, which would include replacing the (half wave) selenium rectifier. I have read that some people suggest changing the half wave rectifier over to a full wave rectifier, but I am not 100% sure on how to do this.....I usually avoid sets with selenium rectifiers, just because I don't like dealing with them, call it seleniphobia…

    I did replace a selenium bridge with a silicon bridge on my Philips AG9016. But that was just about a "drop in" as it gets. The only difference was adding a voltage dropping resistor between the rectifier and first filter cap.

    So here is my question (which I hope makes sense). The current rectifier in the UPX-003B has the lead from the power transformer connected to the (+) section, and then the (-) section is connected to one of the sections of the filter can cap......And that's it....

    A rectifier bridge (I would buy a rectifier bridge because of space constraints) has 4 lugs: 2 = AC 1= (+) 1 = (-)……..

    Would I...…..Attach the lead from the transformer to 1 of the AC lugs of the rectifier, then the (+) to the filter cap and the (-) to the chassis (and just let the other AC lug unused)???

    Thanks for reading!!!

    RC.
     

     

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  2. cademan

    cademan Addicted Member

    Messages:
    8,939
    I think maybe just replace the original with a new one. Something like a 1N4007 should do the trick.
     
  3. gadget73

    gadget73 junk junkie Subscriber

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    Location:
    Southern NJ
    Got a schematic?

    In theory the full wave should be able to be filtered to a cleaner supply. In reality, phono preamps typically use such a small amount of power that it may not actually matter any.

    The two leads from the transformer would go to the ~ terminals on the bridge rectifier. - on the bridge goes to chassis. + on the bridge goes to the first filter cap. Voltage output will be higher, so you'll need to add drop resistors to get voltage back down. If you want to improve filtering, cap right off the rectifier, then a drop resistor from that point to the original filter cap, adjusting value of the drop resistor as needed to get voltages back to stock.

    or, leave it be. If it doesn't hum now, making it not hum more is not really of any value. I'm also not really all that worried about selenium rectifiers personally so I'd probably leave that in place too if its good.
     
  4. Tom Bavis

    Tom Bavis Audiophool Subscriber

    Messages:
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    Location:
    Macedon NY
    Some small transformers have a common lead for heater and HV ground - with one of those, you're stuck with a half-wave rectifier.
     
  5. tubeactive

    tubeactive AK Subscriber Subscriber

    Messages:
    173
    Location:
    NJ
    Deja Vu time again...We have covered this numerous times on many forums. While the technical reasoning for justifying using a wimpy half-wave rectifier seems to have interfered with economic marketing, bean counter reasoning, let us also consider some practical reasoning, shall we ?

    For the technical minded, how many amps have you seen or worked with which have half-wave rectified fixed bias supplies ? While certainly not the best choice from an electronic theory standpoint, the successful application of half-wave rectified power amplifiers and/or simple bias supplies within amps has been widespread since the mid-1920s.

    For the practical minded, "quality of listening" concerned, some of the best sounding (YMMV) early stereo vintage preamps used half-wave rectification. Marantz 7C and Lafayette KT-600, among a long list of other preamps, used the lowly, half-wave simplicity method. The importance is simple. These classics definitely have sound quality merits and increasing monetary values.

    Now, to the OP: Decisions, decisions and decisions.... The GE UPX mono phono preamps were and can still be considered "staples" among record collectors. Their simplicity, cute size and ease of signal improvements underneath continue to be desirable. How about simply restoring reliable functionality first ? Coupling caps with wax coating will have to go, sooner or later. Increasing capacitance in the power supply will yield tremendous sound quality gains. If the silly selenium rect. still supplies proper voltage, why worry ? If you believe they are dangerous, like many aerosol products in our houses would be, then simply replace with the modern silicon diode UF-4007. The UF series has a much lower, residual hash noise floor than the earlier 1N-4007.

    However, consider that more modern diodes have lower voltage drop, thus possibly requiring more resistance "along the line" of the power supply to get voltages right-on. 6SC7 tubes draw about 1-2 mA per section. So, 4 mA max. for the little sel. stack is not a hard life. That is why these phono stages seem to last the test of time; true survivors. I have written quantities about the full UPX series within the lencoheaven dot net site's Mono and 78 threads. so when you feel like researching, that site has a huge wealth of mono collector input.

    GE RPX and VR-II phono carts were designed with the UPX preamps. The 003B uses active feedback EQ for the necessary Bass Turnover/Bass Boost functions. Uniquely, GE chose to retain the (then) time proven method of "input loading" for the necessary Treble Rolloff EQ function. That meant simply installing a lower value resistor across the input jack could provide the treble de-emphasis needed with phono playback EQ. 6K to 6800 Ohms was their choice, at first. A simple input potentiometer (linear) installed across the input, of say 50k or 100K, could serve a user of both types of GE carts very well.

    VR-II carts sound "hotter" up top and typically seem to yield a more forward sound than the mellower RPX variety carts. Thus, a variable input resistor can easily and quickly serve up delicious sounding mono disc listening. Down the road, these preamps can be further exploited with more precise phono EQ networks. Installing the classic, two R-C "networks in series" feedback EQ can step it up a notch in flavor. The Phono-Mic switch can then provide function for switchable treble rollof which can also affect turnover frequency, as the two networks interact to produce the Turnover. In some of my resto mods with these fun, little preamps, this can easily become a precise RIAA EQ +/or a pre-RIAA phono EQ preamp. ENJOY....
     

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