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Old Lafayette valve radio problem

Discussion in 'Electronic Basics' started by [email protected], Mar 3, 2007.

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  1. Guest

    I have a very old valve radio from a company called Lafayette.
    It has 6 valves which are
    6v6 gt
    6x5 gt
    6sq7 gt
    6sk7 gt
    6sa7 gt

    This radio was working fine but recently has developed the following
    problem, when the volume is increased the sound is distorted
    completely,
    it can only be heared ok when the volume is very low, and putting the
    ear onto the speaker. Any ideas?
     
  2. Valves fade away with use, as the emitting surfaces of the cathodes wear
    out. Old paper capacitors leak. A leaky coupling capacitor will shift
    the bias on the valve it feeds, which can let a more-than-minimal signal
    drive the valve to saturation. Another suspect is the power supply, where
    a dried-out electrolytic capacitor can cause all sorts of symptoms.
    Carbon resistors can increase in value over time.

    Can you find the schematic? It may be glued to the inside of the cabinet.
    These sets are usually easy to troubleshoot with a DMM if you know what the
    voltages at the cathode, grid and plate of each valve are supposed to be.

    The real problem might be finding parts. I don't know whether those
    once-common valves are still manufactured anywhere. When the
    new-old-stock supplies salvaged from bankrupt distributors' warehouses are
    finally used up, these radios will become pure curiosities. The
    antique-machinery buffs have it easier. A modern machine shop can make
    any part of a steam engine. Making a 6SQ7 requires a specialized factory,
    the last of which probably went to rust and ruin with the collapse of the
    Soviet Union.
     
  3. Bob

    Bob Guest

    Couldn't this modern machine shop build said specialized factory?

    Bob
     
  4. Could be a lot of things. The usual way to repair these is with a signal
    tracer or a scope. You need to find the source of the distortion.
     
  5. In theory. In practice, it would be such a long, capital-intensive
    bootstrap process that nobody is likely to manufacture the old tubes
    again. I can imagine an obsessed (and wealthy) hobbyist building a shop
    that could produce those receiving tubes on a one-off basis, but but it
    would probably take him years to build the first working samples. Can you
    still get mica for the element spacers, or would you have to learn to make
    ceramic wafers?

    Oops. I forgot about the one or two companies that supply the
    more-money-than-sense segment of the audiophile market, but they aren't
    interested in things like pentagrid converters and remote-cutoff pentodes.

    It's far more likely that people will design plugin solid-state substitutes
    for those tubes, as was done to convert the last generation of vacuum-tube
    voltmeters to FETVMs.
     
  6. Rick

    Rick Guest


    Ask at rec.antiques.radio-phono, too
     
  7. Ken Fowler

    Ken Fowler Guest

    Audio Power Pentode. Powers the speaker. Weak tube could cause distortion.
    Power Supply Rectifier. Provides High Voltage for tubes. Weak tube could cause low Voltage which
    causes distortion.
    First Audio Amplifier and Detector. Weak tube would likely cause low volume.
    Pentode RF/IF Amplifier. Weak tube usually reduces sensitivity.
    Oscillator and Mixer. Weak tube usually reduces senstitvity.
    If you can find the Model No. on the cabinet or chassis, try to find the schematic and service/parts
    information in Sam's Fotofacts or Ryder Service Manual. Sometimes, you can find these in the local
    Public Library. Or there are sources on the WWW.

    There are a few sources of old valves on the internet. Also you can find vendors at flea markets
    and Ham Swap Meets. Probably the easiest way to determine if a tube is worn out is to substitute a
    known good tube. In the old days, there were tube testers in drug stores and your local radio
    repair shop. If you have a schematic and know vacuum tube fundamentals, you can make determinations
    of tube or circuit (Resistors, Capacitors, Etc.) by making voltage and resistance measurements.

    The circuits for radios with your tube lineup were all very similar and were referred to as the "All
    American Five". After repairing a few, most radio service people could visualize the schematic in
    their head just by looking at the under chassis wiring. They could sometimes diagnose failures by
    touching grid circuits with a screwdriver or test lead and listening to the audio change.

    Be careful in trouble shooting. The power supplies for tube anodes could be from 150 to 300 volts.
    And capacitors can retain a dangerous charge even with the power switched off.

    Good Luck,
    Ken
     
  8. Elephant

    Elephant Guest


    If you need tubes, here's a good place to start.

    Antique Electronic Supply in Tempe, Arizona
    http://www.tubesandmore.com


    They carry a huge assortment at very reasonable prices. For example,
    the 6V6GT sells for about $9.00 The 6SK7 runs about $5.00


    Living in the Phoenix area, I've done business with them and was more
    than pleased with their professionalism. They know tubes!!!!
     
  9. Rich Grise

    Rich Grise Guest

    I've never heard of such a thing, and I'm as old as some tubes. ;-)
    The things that give out are the old caps. Shotgun[1] the caps, and it'll
    probably work like new.
    [1] replace every cap in the unit with a new one.

    ....
    Well, maybe you could buy one from these folks:
    http://www.thetubestore.com/nos-6sq7.html
    (second of "about 10,000 hits" at http://www.google.com/search?q=6SQ7 )
    :)

    Cheers!
    Rich
     
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