Twin HUM - dinger

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by TheAmpNerd, Jan 6, 2008.

  1. TheAmpNerd

    TheAmpNerd Member

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    Lucky me, I've got one of these.
    It is a late 60s, thin wire, silver face twin reverb.

    It is a real HUM dinger. argh!

    Been through caps, all electro's all around,
    that helped, some.

    Isolated to the power supply, with only the
    PI tube, and the PI feeder cap removed.

    Hum;

    Heater voltage is high, about 6.6VAC (123 VAC from the wall)

    However with the heater supply wire disconnected
    from the first power tube socket, it is still about 6.6VAC
    across the two wires.

    Interestingly then there is 193VAC and 187VAC to ground
    from each heater wire (still disconnected from the tube sockets).

    Now with a NEW/spliced in power tranny, hum (120 Hz) is reduced...
    and there is about 10.3VAC to ground from each heater wire.

    Measuring the 100 ohm resistors (heater side, lamp to ground [both with and w/o the bulb])
    ...reduces the hum somewhat more.

    With PI cap installed and pre amp tubes , same hum @ 120 Hz,
    however it permiates the amp...and slighly varies with volume
    and tone controls too.

    So, where is it coming from?

    We know still in the power supply is the source.
    Replaces all the PI compnents...prior to New power tranny
    spliced in.

    HOWEVER

    Before I was going to rebuild the output tube socket components
    etc, and all related original wiring (no tube socket component is the same,
    three prong intallation is barely passible, not so hot, etc) I was hoping to trouble shoot the hum before continuing.

    Thinking now, I guess not, perhaps the problem is sloping component
    intall in the power tubes or the 100 ohm resistors at the lamp.

    I spliced in another choke too, that made no difference.

    All diodes have been replaced and bias upgraded as well.

    Fornication, I hate thinking I have to redo the output socket
    compopnents then still have the same problem with hum.

    I know I'm going to do that any way, I'd prefer to find the probem
    before continuing.

    Have I missed anything?

    Are there anythings you can think of that I've overlooked?
     
  2. rog951

    rog951 Member

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    Hi. Did you check the polarity of the heater wiring to make sure that all the output tubes are wired the same (pin 2s are connected to pin 2s and pin 7s are connected to pin 7s)? IIRC, Fender was a bit indiscriminate about their heater wiring and you lose the humbucking effect if the power tubes are wired such that one's pin 2 is wired to another's pin 7.
     
  3. PRNDL

    PRNDL Member

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    IMHO Fenders (and tube amps in general) are susceptible to ground issues.
    There are several ways to diagnose this:
    - use a DVM to measure resistance
    - use a chopstick or screwdriver to see if ground blobs are securely soldered to the chassis
    - put a wire on a single spot (the AC ground wire) and touch the other end to different grounds and chassis points to see if the hum decreases
    - changes based on past experience. Some old techs replace all ground lugs with solder blobs (many don't).
     
  4. John Phillips

    John Phillips Member

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    The high floating voltage on the filament winding is worrying. Is there any measurable resistance to ground?

    Does it do it with the PI tube itself removed? Could it be a problem with cathode to filament leakage in the PI tube?
     
  5. phsyconoodler

    phsyconoodler Member

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    Just a thought,do you have the original bias balance system?If so,turn the bias pot and the hum will go away.
    With a ground reference through the 100 ohm resistors you can't get a proper unloaded reading.
    Check over the basics before you start looking for more complicated answers.
     
  6. plexi67

    plexi67 Member

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    bias supply cap can also cause hum in the power amp section
     
  7. TheAmpNerd

    TheAmpNerd Member

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    A lot of good things here to consider and try.

    I forgot about that, the single AC ground wire....start touching around the amp.
    Then wondering do you fix the ground there at that spot. Or if that doesn't work,
    run a legnth of wire back to the AC ground and solder it in. (almost sound like
    bus bar would be the ticket here, then route all the ground here (in another project though.


    No, actually I didn't. But I will and when I redo the output socket components, I'll make sure at least here they
    keep the same polarity.


    The balance pot has little affect on the
    hum. It kind of only changes the timbre of it, but it is still there. I've addad a bias voltage pot and it works fine--as the bias
    voltages drops the tubes turn on and the hum is increased.
    That is what I've been trying to do, cover all the basics.

    Replaced both of these, they did have holes in them.

    Not with the PI tube removed. I was also thinking some issue with tubes as well, andwas going to try some
    other tubes in the amp...as I've used varias known good sets in there with the same resultant hum.


    none from the old tranny (still in place, no longer wired in.) Then checked the new heater wriing to ground.
    Nothing. Wait, NOTHING? There are 100 ohm resistors. WTF? It should be 100 ohms to ground. Trying at the lamp end
    and still nothing. Trying at the grounded end of the resistor, yeah like .1ohm. Measured the 100 ohm resistors, and nothing.
    These are 100 ohm 1/2 watt metal oxide power resistors. Damn and they are both open, no resistance.

    I"ll replace these when I get back on the amp today...it just me be it, or the other part of it (hum).
    I was going to have a closer look today in this area of the amp becasue there was some difference
    in hum when I was measureing the voltage at the heater lamp joint....

    Thinking these 100 ohm resistors must have been the by product of a tube melt down in the amp. 4 power tube sockets,
    All of them configured differently. So the hum resulted and never fixed. The last few people in the amp couldn't figure it out.
    My guess is at least three other poeple trying wroking on it and gave up.

    Embedded problem, 100 ohm resistors, amp check, wiht the new power tranny wired in.
    then lift these wires out and rewirire the installed older tranny and see what it does.
    I imagine it is still hummy to some degree, I'd be surprised if I could still use the original
    power tranny, but we'll see.

    Thanks guys, you've helped me come up with a play for this bugger.

    I'll have some more "fun" with this amp and I'll keep you posted.


     
  8. John Phillips

    John Phillips Member

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    ... and if they're metal oxides, they aren't the originals. So something fried the originals, then these were put in, then something fried these too... hmmm.

    The obvious cause would be a shorted (to the filament) power tube, but it would be worth checking for a damaged tube socket too. Carbonization between pin 3 and pin 2 can do this, as I'm sure you've seen before. With the history of the power tube sockets, I'd probably just replace all of them and their components to be safe. It's actually quicker to shotgun this sort of thing than troubleshoot it usually...

    There may be nothing wrong with the PT.
     
  9. phsyconoodler

    phsyconoodler Member

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    Using metal film 100 ohm resistors is good insurance if you have a filament short.They just go ftttt and that's it.They often even look ok until you measure them like you did.Carbon comps on the other hand get all burnt and charred,and you can see that they are shot.The tube sockets often get charred too when a tube shorts.Then carbon tracks get formed on the socket and it's shot.Especially the black sockets they put in Fender's and marshall amps.
     
  10. TheAmpNerd

    TheAmpNerd Member

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    I dont' know yet, as i still have the spare tranny spliced in.
    After the 100 ohm resistor replacement, it works and sounds
    fine...not more hum and 3.2 VAC from the heaters to ground.

    Yea!

    Now if I can only figure out why the neon trem bulb keeps
    coming on and staying on.... : )

    Thanks for your help ya'll.

    I should post a pic of it all rigged up....
     
  11. TheAmpNerd

    TheAmpNerd Member

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    Post script:

    Original tranny is fine, resoldered it in for testing.

    Don't see any more problems on the heaters--a tad low at 3.1vac
    per side to ground, should help tube life.

    Trem bulb light was the result of disconnecting the neg feed
    to the trem circuit, during testing.

    New components on the power tube sockets along with
    new wiring where original is shot and this Twin will be good to go.

    PPS - Yes, I'm replacing most all the brown turd caps for better one's.




     
  12. phsyconoodler

    phsyconoodler Member

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    3.1v is not low and will not extend tube life.It could be your meter not calibrated.Some read 3.4 and some read 3.1?
    The brown turds get a bad rep for no reason and unless they're shot,leave them alone.They sound fine.Does the amp sound good?Yes,leave 'em.Sound bad?Change 'em.
    It you don't like brown turd because it looks suggestive,change them.They likely will sound the same as orange drops.Always change the two .1uf caps on the drive lines anyway.
     
  13. TheAmpNerd

    TheAmpNerd Member

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    Three of the sockets look to be replacements, where the heater wire was changed as well.
    Once I removed the heater wires, it revealed some no so good looking soldering on screen
    wireing...My guess is that happened (short, fried tubes, sockets, heater wire, etc) sockets and components replaced, etc, still hum...with those new looking 100 ohm resisors in the heaters.

    That whole area is still gummy and sooty (the cleaning and clean up process has begun).

    Thanks. The other tranny read at 3.28 VAC to ground.
    I always thought a tad low on the heaters extened tube
    life. No biggie, I'll take your word for it. Either it isn't a large
    enough difference to matter or within specs w/in variation.
    not to disagree.

    The amp sounds way to fuzzy and not enough detail. However, I'm going
    to finish up the output sockets and heater wiring first, because this will
    make a change....then once I got everything working right, cleaned up
    etc, I'll finish voicing the amp.

    I might keep one to four brown turds in the amp depending on the tonal needs. Feedback, two trem caps and one in the signal path.

    I'll upgrade to something better in various parts of the amp.

    I don't like the .1 browns in the final or pre amp section,
    neither do I like the blue hot dog caps--too fuzzy.

    Thanks again.
     

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