curious about xener diodes to lower B+

Discussion in 'Amp Tech Center' started by texwest, Sep 24, 2020.

  1. texwest

    texwest TDPRI Member

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    In another thread I'm having trouble with a champ biasing because of high B+ and higher wall voltages.

    Would putting a xener diode on the centertap bring down the B+ a little?

    I saw one at Antique Electronics that is 33 volt 5 wt.

    Would even just one of those on the centertap bring the B+ down some? How much voltage drop would I see?

    how would I do that safely?
     
  2. Jon Snell

    Jon Snell Tele-Holic

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    Check the mains transformer primary tapping for the correct voltage setting. If set to 100volts, the HT will be too high.
    Zener diodes are OK to reduce Screen grid and low current voltages but not for high power.
     
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  3. King Fan

    King Fan Poster Extraordinaire

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    Zeners are a common useful way to reduce B+, and in cathode-biased amps like yours, the PT CT is appropriate. Rob, as so often, has an excellent general discussion here in the section on "Other Ways to Reduce Your Amp's B+". For more discussion and 5F1 details, Google "zener 5F1 B+ site:TDPRI.com".

    EDIT: I don't know details from your other thread. How high is your B+? Your wall voltage? What bias do you calculate (and how)? What PT are you running, and if relevant, on which primaries and secondaries? Quite a few 'high 5F1 voltage' issues are the kind Mark Twain had in mind when he said, "I've faced many problems in my life, but most of them never actually happened." :)
     
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  4. D'tar

    D'tar Friend of Leo's

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    I'd string together some 10v 5w rather than a 30v. I didn't see mention of whiat rectifier tube you have either?
     
  5. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    +1 on this question. Zeners and power resistors can work to bring down excessive B+, but they're kind of a band-aid fix. Getting the desired voltage at the source is ideal.
     
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  6. JuneauMike

    JuneauMike Poster Extraordinaire

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    Since the Champ is cathode biased and I used an appropriate PT for it, I don't recall sweating the bias. What PT are you using? That would be the most ideal solution.

    Maybe run yours through a variac for a bit just to see if it really is that big of a deal to lower the voltage. Those are pretty inexpensive these days and you can use them for many other things going forward.
     
  7. JamesAM

    JamesAM Tele-Meister

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    If you haven’t already (sorry, I haven’t seen your other thread), providing the wall voltage, voltage from the PT to rectifier, type of PT, type of rectifier, and cathode resistor R would be helplful, as well as your bias calculations using the cathode resistor method.

    I had this issue with a 5e3 rebuild and a higher value cathode resistor and nos rectifier set me straight enough- my PT was a bit hotter than its documented spec. I’m putting out just about 100% dissipation now (depending on tubes) which is fine for a cathode biased amp.

    again, my apologies if you’ve provided all that info in the other thread, but I can’t seem to find it.

    EDIT: sorry! found it. Yes, the type of PT and whether it’s original would be helpful, as well as rectifier, wall voltage and which primaries you’re using. 48mA is very high. I retract my initial advice on the cathode R until we figure out what’s going on with the PT.
     
    Last edited: Sep 24, 2020
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  8. JamesAM

    JamesAM Tele-Meister

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    I also see in your other thread that the cathode R is new. Have you measured the true resistance with a meter to make sure it’s correct?

    Also, what method were you using to measure bias: the output transformer resistance method or the Cathode resistor method? If you’ve got voltages off of the rectifier as well as the pt, that would be helpful too.
     
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  9. texwest

    texwest TDPRI Member

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    Wow thanks for all the replies. I was away from my computer all day.


    Here's the data - 1975 champ original PT

    new 5 wt cathode resistor measured at 493 when I installed it. New filter caps installed 30-22-22. New cathode caps original 5y3. I tried another 5y3 to check and I got nearly identical voltages.

    wall voltage 122.8
    htr 6.7
    PT secondary red wires 368
    first node 415
    plate 400
    screen 409
    cathode 25.7
    48ma on meter that measures ma current on pin 3

    The voltages are nearly exact to an old Bronco layout I have.

    Trying some GE and sylvania NOS tubes. Very slight redplate on some of them, more of a big line on others .

    Fixing for a friend. I doubt he wants to mod much or spend much money. I have had two silverface amps like this and they both biased up around 45-50ma. I would just find a tube that didn't go higher than 45 or so and didn't redplate. I was ok with this when it was my amp, but with this one want to make sure it's reliable for him.

    Amp actually sounds great but shows that very slight redplate.

    is there a new production tube that can handle this?
     
    Last edited: Sep 24, 2020
  10. texwest

    texwest TDPRI Member

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    PT L010020 csa827

    An interesting thing about it. The primary wires are different. Both a yellow/black and red/black wire connect to the fuse and the black hot. A green/black and black wire connect to the switch and white neutral.

    Wondering if this might be wired wrong by the factory?? A couple weeks ago I had a problem with a vintage 65 PR and found a wrong 5000pf cap on the reverb feed that's been in there all this time.

    Here's a pic. (Please don't judge the sloppy look on the terminal strip for the new caps. I reused a terminal strip from different project and had to use wire and solder to connect all the terminals to ground, so this doesn't represent my best work.)
     

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    Last edited: Sep 24, 2020
  11. JamesAM

    JamesAM Tele-Meister

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    The JJ 6v6 is basically a 6l6 and should be able to handle it. I was red plating at 114% dissipation in my 5e3 with Tung Sols and old Marconi coke bottles from the 40s, but was holding steady at about 94% with JJs. The gain is a bit different but it still sounds great.
     
  12. JamesAM

    JamesAM Tele-Meister

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    also, wall voltage 409! I’m sure it’s high but I’m assuming that’s coming off the HV primaries at the rectifier, not at the wall outlet, right? That should read about 115-130vac depending on the time of year.
     
  13. elpico

    elpico Tele-Afflicted

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  14. texwest

    texwest TDPRI Member

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    Oops that should have been 122.8v it's changed now!
     
  15. Jon Snell

    Jon Snell Tele-Holic

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    We have a lot of issues with too much HT on the Blues Junior made in Mexico. They come to the UK set at 230volts. We are 240volts!
    A simple correction fixes that but I doubt an older amplifier will be wired wrong. Is the amplifier primary wired for 100v, 110v or 120volt. Worth a quick check the label perhaps.
     
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  16. King Fan

    King Fan Poster Extraordinaire

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    Ah, *1975* Champ. Just right. While many 'too hot 5F1s' aren't really too hot, many/most BF/SF Champs and VibroChamps run notoriously high B+.

    A bucking transformer/brown box/vintage voltage adapter as suggested by @elpico is a perfect and elegant fix, but I see you're fixing this up for a buddy -- he might dig an onboard solution. A new 'cooler' PT is popular with repair experts, but less popular for 'restore' guys.

    So zeners on the CT are a fine fix. Even simpler, and equivalent in effect, is a dropping resistor *before the filter caps.* Same voltage effect as zeners, generates the same amount of heat (no big deal if you do either one right); the theoretic downside is it adds a little sag, but in my VibroChamp that turns out to sound *great.* And those '70s PTs by themselves didn't sag too much IIRC.

    Super simple. Just solder a 5 or 10W wirewound resistor (mine worked great with a 470R) between the rectifier and the first filter cap...

    77385ED6-A498-47F3-9ADC-0E03DC112491.jpeg
     
    Last edited: Sep 25, 2020
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  17. texwest

    texwest TDPRI Member

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    Thanks that's very helpful. Really the simplest solution and best solution is the brown box. I was thinking about mentioning it to him. I might try that 470r though. That's easy too.
     
  18. texwest

    texwest TDPRI Member

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    OK I wired in a 500R 5wt wire wound in between the rectifier tube and the first cap. Brought the voltages down to 355v on the plate with 42ma on my meter. This seems more manageable.

    Less volume but sounds great with more sparkle with a softer feel too! I'm liking this a lot now.

    Want to make sure is 5 wt ok or should I order a 10 wt just to be safe???

    thanks for the help
     
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  19. King Fan

    King Fan Poster Extraordinaire

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    If I'm thinking straight, 5W would handle up to 100mA and I'll guess a Champ runs about 40mA through there. But I'm the exact opposite of an expert.... :)
     
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  20. Euphonica

    Euphonica Tele-Meister

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    Please forgive the off topic! (It’s not worth a whole thread.)

    this is great but my house voltage is constantly changing by about 3 volts. It’s swings back and forth every few minutes, I can see it on the Amp Maniac. (It’s for a 1962 amp.)

    Has anyone made a voltage correction box that can maintain a steady output even though the input voltage changes? I am just curious, it seems like you need some sort of computer chip or some sensor.

    Apologies for the off-topic again, but seeing this link struck a chord.

    Dan
     
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