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Powering up some very old high powered tube amplifiers

Discussion in 'Electronic Design' started by [email protected], Oct 8, 2011.

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  1. Guest

    I have three very old high powered tube amplifiers that were
    originally designed to be top of the line amplifiers for theaters.
    They are all identical, and were rack mounted, but I build cases fro
    them. Back in my younger days, in the late 1960's, I used three of
    these amps for my stereo. Left channel, right channel, and delayed
    center channel using a spring reverb. I built some huge speaker
    towers, and ended up with a stereo capable of blowing the windows out
    of a house, which I proved back in my wilder days. Either way, this
    system had better sound than any commercial stereo.

    Anyhow, I have had these speakers and the amps in storage since the
    late 70's. I decided it was time to bring this system back to life
    and brought it all home. Other than some dust and mildew, and a mouse
    nest in the cabinet of one of the amps, (which I removed), everything
    appears to be ok.

    I want to power up these amps, one at a time. However, I'm a little
    concerned about the filter capacitors. I did replace all of them in
    the early 70's. They were still available back then, and they were
    very unusual caps because they had an Octal plug in base. I'm almost
    sure these can no longer be bought.

    Fianlly, I want to power them up without a chance of a bad filter cap
    doing any damage. Is there any way to power them up without applying
    full voltage at first? Or should I just add a few fuses ahead of the
    filter caps? I should note that these amps have a main power line
    fuse and each of the four output tubes also has a fuse and a neon
    light to determine output tube faults.

    I know someone will ask what brand they are. I do not know.....
    There are no numbers of brand names on them. However, they have about
    40 lbs of transformers on each amp, and that is why they always had an
    awesome sound spectrum.
  2. Guest

    The rectifiers are 5U4 tubes. I do like the idea of using light bulbs
    in series with the line cord. At the same time. since these 'lytic
    caps are plugin types, I may just apply a voltage to them using
    another external power supply, and see if they hold up.

    Awhile back I got an old table top tube radio, plugged it in, and
    immediately heard 60hz hum from the speaker. I left the radio on and
    after awhile the hum almost vanished. I may be wrong, but it seems to
    me that they "heal" after some use, (assuming they are not shorted).
    Of course that radio is low powered in comparison (and should get new
    'lytic caps anyhow).

    As far as the paper coupling caps in the amps, they would be easy to
    replace, since they are available. The 'lytics wont be easy to get.
    Question on the paper caps though. What kind of caps are used as
    repalcements by those who rebuild these old amps or other electronic
    gear? Obviously they have to be the correct capacity and voltage, but
    there are so many choices. I know the ceramic types are stable, but
    maybe they wont work properly. So what is recommended? The "Orange
    drop" ones are similar, and are wound with some sort of plastic, not

    I never knew there was anything toxic in the old paper caps? They
    were just coated with wax, and I used to get that stuff all over my
    hands when I was younger and would solder them in. On the other hand,
    I recall some of the old stuff I worked on when I was in my teens
    contained 'lytics with oil inside, and several times I had them blow
    and shoot hot oil all over the place. Now I find out that was PCB
    oil, (dangerous shit). Of course these days we will all die from
    stress because of all the "deadly" things we are scared about due to
    health fanatics and medical people who want our money.......

    Considering all the old electronic gear I played with when I was
    young, I should have died 30 years ago...... and all that lead
    solder..... make that 40 years ago.....

  3. Tauno Voipio

    Tauno Voipio Guest

    You should run the rectifier filaments with the nominal voltage.
    Undervoltage with load on can lead to point emission and filament
    pitting with associated risk of burning it out.

    If you replace the rectifier tubes with silicon diodes, please
    remember to add some series resistance to limit the capacitor
    inrush current.

    I'd (temporarily) replace the 5U4's with silicon diodes and series
    resistors, pick out the other tubes and carefully feed the thing
    with a Variac, as the other posters have suggested.
    You were lucky - the caps could as well throw up the
    electrolyte and the other insides.

    I have once seen this happen. An old Telequipment scope quit
    working, as the HV rectifier tube slowly died. We replaced
    it with some 1N4007's, and got a beautiful sharp picture.
    There was also a pffffft sound, which quickly turned into
    a loud boom, and the room was filled with confetti.
    Unburned PCB oil is not very toxic. Its main problem is that it is
    chemically very robust and dissolves readily into oils and fats.
    If you succeed in burning it, then it is a different story, as
    it will lead to dioxins, which are toxic. For more information,
    Google for 'Polychlorinated biphenyl'.
    So do I, but I'm still alive after over 50 years of tinkering
    with electronics.
  4. Joerg

    Joerg Guest


    That is the way to go. But you need some resistance up front. Depending
    on the voltage maybe a few 120V incandescent light bulbs in series. And
    increase the voltage slooooowly.

    If one of them blows and you really want to keep this whole amp at least
    authentic from its looks you may be able to open the cans and then mount
    a new cap inside. New ones are more modern in technology and thus much
    smaller for a given capacitance/voltage. Did that once on a cap that had
    a nice logo, was from an Italian motorcycle brand. Of course the stuff
    from inside needs to be disposed of properly and safely.

    Sometimes they heal themselves, other times ... *PHOOMP*

    Hardcore audio fans use film caps, never ceramics.

    Surprisingly often they'll still be fine. But if you replace them just
    make sure to dispose of the old ones at the electronic waste collection.
    Ours is really easy, it's behind the next supermarket. The guys never
    minded when I came with my bag of parts that I had unsoldered over the
    last few months.

    Most of us will die from very unrelated causes. Too many cheeseburgers,
    for example.

    Nah. All the exercise from getting the old discarded TV sets back home
    over five miles on my bicycle was probably enough to offset that :)
  5. MrTallyman

    MrTallyman Guest

    Metallic form lead is NOT harmful, IDIOT!

    It is COMPOUNDS of lead that can cause damage.

    No solder joint or solder strand you EVER touched gleaned ANY lead into
    your body. You fucking FUDTard!
  6. Rich Grise

    Rich Grise Guest

    Society's do-gooders have panic attacks when there are trace levels
    of "contaminants" that only a few decades ago, were below the threshold
    of detection. (i.e., a ppm used to be safe - now it's ppb or ppt!)

    May the Great Roulette Wheel in the Sky save us from the hypersensitive!

    Hope This Helps!
  7. Guest

    Damn, that brings back memories. I used to do the same thing. Tie a
    coaster wagon to my bike and haul home old electronic gear. Not to
    mention used lumber and stuff to build shelves and a bench for my
    workshop, and of course speaker cabinets. Those old console tv set
    cabinets made great cabinets for part storage too.
  8. josephkk

    josephkk Guest

    Actually most of the PCBs were in transformer oils. Infrequent in
    capacitors under 1 kV, and not in electrolytics (wrong properties). Most
    of what you are likely to have come around would be in fluorescent
    ballasts (if you tore them apart).
  9. josephkk

    josephkk Guest

    Oil yes, pcb pretty much only in caps rated 1 kV and up, NOT a full
    guarantee. How many of those caps do you unwind? The paper was a physical
    separator and the oil was the dielectric, the fancy properties of pcb were
    not usually warranted cost wise below 1kV.
  10. ehsjr

    ehsjr Guest

    With old 'lytics, it's not a question of them holding up. Reforming
    doesn't make a working cap hold up longer, but it may change a
    cap that would otherwise fail when power is applied, to a cap that
    will work when normal power is applied to the equipment.

    Your plan to pull the caps out of the equipment to do the reforming
    is the right approach. With vacuum tube rectifiers, it is impractical
    to reduce the voltage to the equipment to do the reforming.

  11. Guest

    The caps that exploded and spewed oil were on very old tube systems.
    They were metal can electrolytics that had a tiny hole in the center
    of the top of them. (probably a vent). More than once those things
    blew on me, spewing hot oil out of the hole, and at least once blowing
    the whole capacitor apart. After having this happen a few times, I
    was always nervous when I powered up any 'lytics with the small holes
    on top. I learned to put some sort of cover over them just in case.

    I was working on this stuff in the late 60's early 70's and I bet
    these devices were 30 years old at that time......

    I have never learned what these caps were called (other than being
    electrolytics) but a specific oil filled type, whose name I still do
    not know.

    All I know is that they blew up much more often than any other type of
  12. Guest

    Hmmmmm, now I'm a bit confused. First you (amd others) say to apply
    the voltage slowly (reduce voltage at first). Now you are saying
    "With vacuum tube rectifiers, it is impractical to reduce the voltage
    to the equipment". Why is it different for tubes than it would be for
    solid state rectifiers? I have heard more than once about rectifier
    tubes burning out from bad caps.....

    While I'm here, I have been thinking. It's been years since I worked
    on these amplifiers, but I recall there was a pretty hefty voltage
    coming off the rectifiers (around 500v I believe). To reform the
    caps, do I need to go that high with the voltage to reform the caps,
    or would a 300v supply (for example) do the trick? I only ask this
    because I have several old (tube type) tv transformers sitting aroumd,
    to make up something, but they rarely went above 350v or so.
  13. Rich Grise

    Rich Grise Guest

    It's the filaments. Generally, in those days, they had BMF power
    transformers with a 6.3V winding for most of the tubes, and a 5V
    winding for the rectifier tubes. If you turn this down, the tube
    filaments won't reach operating temperature, which will affect their
    emission, and could be hard on the tube itself. If you can run the
    filaments at their rated voltages, then you can put the other voltages
    wherever you want them. (except over the max, of course.)

    Hope This Helps!
  14. Rich Grise

    Rich Grise Guest

    Please don't feel like I'm being a smartass, but they were called "oil-
    filled" caps. :)

  15. Guest

    Actually that's what I suspected they were called and I have found
    references to that term online. But using google (images), most
    appeared to be larger caps. Apparently these were the very early
    'lytics used on the very old radios and other stuff.

    They looked almost identical to the aluminum can 'lytics, except the
    tops had slight indented circles (like a target) and that tiny hole in
    the center.
  16. Guest

    Anyone have a schematic for a simple variable DC power supply I can
    make from salvaged tv parts, to be used for reforming old 'lytic
    capacitors from tube equipment? I have plenty of parts including
    transformers from old tube type tv sets. Solid state Rectifiers

    Thanks for all help.
  17. josephkk

    josephkk Guest

    Please remember that the electrolyte in electrolytic capacitors is rather
    conductive, not an oil at all.

  18. ehsjr

    ehsjr Guest

    The filaments, as Rich said. Tubes like your 5U4 have (and need)
    them, solid state rectifiers don't. The full, or close to full
    filament voltage must be applied to the tube for it to rectify.
    If you reduce the filament voltage a little, the tube will operate
    poorly. If you reduce voltage to the equipment far enough so
    that the reforming voltage would be low enough, the tube won't
    operate at all.
    Reforming to less than full cap voltage rating is a gamble. You
    might get away with it, you might not. A 350v secondary would be
    good for 450VDC caps. Rectified, it will provide peak DC of about
    500 volts, so you'll be able to vary from low up to the cap's rating.

    Have you figured out how you're going to control the voltage?

  19. josephkk

    josephkk Guest

    This is where it may get tricky. If you reformed only to 70% of rated
    they might still go bang if roughly subjected to rated voltage. I suggest
    reforming to 125% of rated, slowly. After all, they were formed to above
    rated so that they would be reliable at rated.

  20. Guest

    Your drawing came thru well. Thanks

    Where might I get a variac?
    I'm still trying to figure out what FWB means? I know that's the
    rectifier, just dont understand those initials.

    One other thing, to use this as a reformer, are filter caps put after
    the rectifier on this device, or do I just feed the TEST cap directly?

    Aside from the variac, I should have all the parts.
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