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Info Needed: Silverstone Model 18801 Stereo (Tubes)

Discussion in 'Electronic Repair' started by Tim, May 16, 2007.

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  1. Tim

    Tim Guest

    I am trying to get the old guy back up and running so I give it to
    someone who has no stereo equipment, but wants to play some LPs. There
    is no audio output at all, not even the usual hum. I did some research
    on the output stage which uses the 50EH5 tubes. Most of the schematics I
    have seen use an isolation transformer to power the tubes. This one does
    not have a power transformer in it at all. All of tubes light up, and I
    have checked them with my (don't laugh) Radio Shack Tube Tester. Most of
    the tubes are fine, but 1 of the 50EH5 shows as gaseous and shorted. I
    would like a schematic if possible, or some direction from anyone who
    may recall how this company designed the stuff.

    Another strange thing is that most of the schematics have the 2 50EH5
    heaters wired in series with a high wattage resistor so they run off a
    125 v ac or dc source. If I pull one of the tubes out of the chassis,
    the other one stays lit.

    I love working on this old stuff, but I need some guidance here. I
    figured that if 1 of the tubes is bad, the other channel should be ok.

    Thanks,

    - Tim -
     
  2. hr(bob)

    hr(bob) Guest

    You don't say how many other tubes there are. There may be two
    parallel strings of tubes. Do any of the tubes go out when you [pull
    one of the 50 EH5's?


    H. R. Hofmann
     
  3. Guest

    Tim:
    This is a typical (of the era) hot chassis design that wired the
    heaters in series to add up to 120 VAC.... if there were not enough
    tubes to add up to the line voltage then a suitable high wattage
    resistor was placed in series. The only isolation available was with
    the speaker output transformer that isolated the speakers, speaker
    jacks and terminals. Everything else was "hot", the input jacks, the
    phono cartridge wiring and phono cartridge (probably a low quality
    ceramic or crystal) and the metal shafts of the chassis mounted
    switches and controls.... be very careful.
    Are one of the channels working? If the tubes are lit, you should
    get some kind of hum as long as you have rectified DC getting to the
    plates of the 50EH5 tubes..... the red & blue wires on the speaker
    ouutput transformer are from the Primary windings.... usually the red
    wire goes to the B+... and the blue wire goes to the plate of the
    output tube. You should have approximately 140 to 160 V DC at these
    wires, assuming that the selenium or tube or whatever rectifier is
    working and that there are no open resisitors, shorted capacitors (old
    electrolytics are immediately suspect and if bad would cause tremedous
    hum) or other faults causing the lack of B+.
    This is not a complicated circuit and you should be able to figure
    this out without a schematic.... many times a high out phono
    cartridge could be connected directly to the grids (through a
    resistor) of the 50EH5 and they could produce ample output without a
    preamp tube.
    50EH5
    pin 1 Cathode & G3, usually connected to ground through a
    resistor ... 150 ohm approx.
    pins 2 & 5 G1 signal input
    pins 3 & 4 heater (50V)
    pin 6 G2, usually connected to B+ through a dropping
    resistor, should measure at least 100 VDC
    pin 7 Plate

    Good luck and be careful with this hot chassis design.
    electricitym
    ..
    ..
    ..
     
  4. Guest

    Hi!
    You might wish to check the speaker system and its wiring. Sometimes
    the voice coil in a speaker opens up, either because of abuse or
    normal wear and tear.

    If the speakers in this set are a conventional permanent magnet type,
    you can disconnect the leads from them and use either an ohmmeter on
    the resistance range OR a nine volt battery across the contacts of
    each speaker. Clicking sounds or static from the speaker when you
    connect the meter or battery indicate that it is good. Don't leave the
    meter or battery connected for very long, or you may open the voice
    coil.
    This is a so-called "line connected" or "series string" set. The tubes
    are wired in series so their heater voltages will all add up to
    roughly 110/120 volts. If the number of tubes and their voltages don't
    add up to line voltage, then a large resistor may be used to drop the
    voltage to the point where it is needed.

    You might look into replacing this resistor (if your set has one) with
    something more modern, such as a diode. This will make the set run
    cooler and more efficiently.

    Be VERY careful when working on this chassis under power. Better yet--
    don't have the set on or plugged in when you're working on it. There
    is absolutely no isolation from the power line and you could really
    hurt or kill yourself by doing the wrong thing. If you want to test
    the set while it is apart, plug it in, make sure the (hopefully
    nonconductive) knobs are on, conduct your tests and then unplug the
    set when you are done.
    Give this tube a visual inspection. You may be able to see the
    "getter' inside it. This should normally be a silver color. If it has
    turned milky white (or *maybe* red), the tube's vacuum is gone.
    Maybe. It depends upon how the unit is designed. Chances are most of
    the tubes are good. You will find the most problems with supporting
    components, such as capacitors and resistors. The capacitors may be
    left alone if they work, but for reliability (and possibly safety)
    reasons you should replace them if it all possible. You should also
    give thought to reworking the power system if it turns out that the
    radio does in fact use a dropping resistor.

    For more information, you might also try posting to:
    rec.antiques.radio+phono

    William
     
  5. hr(bob)

    hr(bob) Guest

    Several of the previous posters ignored the fact that the original
    poster stated that when one of the tubes was removed, the other
    filaments stayed lit This is not quite as simple as they believe.

    H. R. Hofmann
     
  6. Tim

    Tim Guest

    Well I do see a huge wirewound resistor bolted to the chassis near the
    ac input, so that is probably is limiting the current to the heaters.
    There is no sound from either channel. I tried my little signal injector
    into pin 5 of the tubes, but got no output at all. If I inject the tone
    into the 12ax7, I get a very faint tone out of the speakers.

    I will check into the dc supply though, as it would take out both
    channels if it was absent. I have found an old schematic from a
    Fleetwood 50eh5 that looks similar to what I see in the chassis.

    I guess I did not mention that I have the specs for the tube, so I know
    the voltage range that should be present. My concern was that someone
    had removed the transformer and hard wired the set to AC. The AC
    Interlock was defeated, so I was a little suspicious that something else
    may have been modified too.

    This particular set seems to have a lot of extra components compared to
    most other circuits of type I have found.

    Thanks for the help,

    - Tim -
     
  7. Guest

     
  8. Tim

    Tim Guest

     
  9. Sofie

    Sofie Guest

    In your first reply post you mentioned that there could be 2 parallel heater
    strings
    connected to the AC line, which could be true....
    that fact was not ignored but may be irrelevant..... even if one of the
    tubes are bad,
    some kind of hum or noise should be heard out of the other channel. The
    best bet is
    probably a B+ problem or open speakers??
     
  10. hr(bob)

    hr(bob) Guest

    Is it a "SIlverstone" or "Silvertone" The latter was the name Sears
    Roebuck used for many of its products.

    H. R. Hofmann
     
  11. hr(bob)

    hr(bob) Guest

    What is/are the total list of tubes. We need to see what the line-up
    is, then we can start giving you much better advice.

    H. R. Hofmann
     
  12. Tim

    Tim Guest

    I have to put this project on hold for a bit. I have many, many things
    on the go right now, and the person I was fixing it up for is not
    interested in a tubed stereo system.

    I did check the 50EH5 tubes voltages and they are way off, so I guess
    the problem is somewhere in the power supply side. The chassis is a
    jumble of wires and most of 'em are brown, so it makes it difficult to
    trace the lines. I will fix it up though, just to add it to my list of
    done deeds.

    Oh, the 50EH5s *are* wired in series, with the rest of the tube
    filaments off another line. When I pulled one out, the other one went
    dark, but the rest of the tubes stayed lit.

    - Tim -
     
  13. Guest

    Was this US practice? I ask as I've never seen anything like that here
    (UK)


    NT
     
  14. Tim

    Tim Guest

    Well I'm in Canada, and it was very common until the 80's

    - Tim -
     
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