Humdinger!

Discussion in 'Shock Brother's DIY Amps' started by nathanh, May 6, 2017.

  1. nathanh

    nathanh Tele-Meister

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    I'm wrapping up my first build (5F6a) and am considering the humdinger mod to clean up some of that good ol' 60hz hum. Does anybody have any experience with this?

    I use this amp for touring and thats about 150 shows a year. When you adjust the pot for the best balance are you adjusting for the nature of the amp itself or for the power you're connected to? In other words, is this something to adjust on a day to day basis or just once?

    I'm trying to decide how accessible I want the pot to be! Will post pics and clips soon.
     
  2. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    An interesting question that had not occured to me before to consider the situation. I'd always seen it as a thing to set and leave after changing tubes (maybe reajusting after a burn in period). This would take in the slight differences in current draw of the two heaters, how much the filaments transmitted an AC electromagnetic feild, how much of this was picked up by the cathode, and other grids, how much of this AC was grounded out by the cathode bypass capacitor, the amplification factor of the tube, and the ballance of the two halves of the out-put transformer....all full of slight variences, but all fairly stable after the amp was up and running...

    But...as you point out, if you are touring, you are going to have to be putting up with some very differing grid supplys. Have a look at the wiki page - Electric_power_quality
    and one sees how all over the place the supply can be. (Here where I live, it is actually very clean by comparrison to most) The US, I believe, can have very messy oscillation of the supply voltage, getting rising and falling asymetrical and triangular tendencies with shifts in voltage and current.
    How is this going to effect your heater hum?......................Hmmmmmmmmmmmmm..........
    My guess is that it might be shifting the best point of hum cancellation ballance - by a small amount........but, that in one hours time, chances are, that the assymetry in the supply grid might well have swung 180°, making the new setting worse than it have been before.
    If some venues make your amp humm more than others, I think it is probably not the hummdinger that would best solve the problem.
    Get yourself a good line filter, possibly even with a isolation transformer, current limiting and a fuse - If you are every having to be powered by a small generator, this is a good safety procaution - for not only your amp, but for you. I built myself one into a small case, with a green and red signal diode before the on switch, showing which line carries the live current. This way I always have the phase the same way around, and due to my matching marked amp plug, I know the amps fuse is always on the live supply side of the input.

    I am interested to hear what others might have to say on this subject, but I would tend not to try resetting the humdinger at every new venue, thinking the available win would only be minimal, and possibly short lived.
     
    Last edited: May 6, 2017
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  3. robrob

    robrob Poster Extraordinaire Ad Free Member

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    The humdinger pot works to balance the voltage on the two 6.3v heater lines. If one is carrying more voltage than the other that extra voltage isn't being nullified buy the parallel twist of the wires. Once you set a humdinger for minimum hum you should be fine unless the pot resistance somehow drifts.

    A different current draw from new tubes shouldn't affect the voltage balance because the new tubes' load is across the two wires so they should be equally affected.

    I would do what everyone else does and just put the pot inside the chassis.
     
    Last edited: May 6, 2017
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  4. BobbyZ

    BobbyZ Doctor of Teleocity

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    Sounds like "humdinger" is a fancy name for the hum balance pot Fender put on the back side of the later silverfaces.
    I've disconnected those and replaced them with a normal artificial center tap, two 100 ohm resisters.
    I leave the pot in place so the uneducated can turn it all they want, thinking they're doing something, without screwing anything up in the process.
    My thinking is if someone can see a pot, they're going to turn it. So if you use one, hide it.
     
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  5. tubeswell

    tubeswell Friend of Leo's

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    A humdinger wired into the heater circuit is different to the hum-balance pot in a silverface amp. (Your typical humdinger replaces the heater circuit's artificial ground reference resistors - 100R pair in most amps - with the pot wiper being the ground reference and each pot end lug being connected to respective sides of the heater winding.)

    They do work to minimise hum (of course), but 'hum' or other noise from another source (such as RF noise), might still be present. (But don't let that put you off)
     
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  6. gatorfiend

    gatorfiend Tele-Meister

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    Ah, man I thought we were gonna talk about this:
     
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  7. Commodore 64

    Commodore 64 Friend of Leo's

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    My '79 Bassman 10 had both a hum balance for the power tubes and humdinger for the heaters, just FYI.
     
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  8. nathanh

    nathanh Tele-Meister

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    I think that about answers it.

    In theory then if I'm just balancing out the heaters then a different outlet or venue for that matter shouldn't have an effect. Inside it is.

    Does anybody have a good video of this working?
     
  9. nathanh

    nathanh Tele-Meister

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    Did it work well for you? How does the hum balance work/differ from the the hum dinger?

    EDIT: I'm looking at a Twin Reverb 100SF Schematic and it looks very similar to the humdinger except its balancing the inverted signals coming from the PI to the Plates of the power tubes. I'm a little unsure of the purpose of the resistors in line here but I get the idea I think. Could you basically just attach each signal to 1 and 3 and send 2 to ground?

    http://www.thevintagesound.com/ffg/schem/twin_reverb_sf_100_schem.jpg
     
    Last edited: May 6, 2017
  10. BobbyZ

    BobbyZ Doctor of Teleocity

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    I guess I'm getting my terms mixed up. By "hum balance" in reference to late silverfaces (and the early 80s black face plate silverfaces) I ment the pot in the heater filiment winding. It's on the back of the chassis, screw driver is need to turn it.

    Not the bias balance pot for the power tubes. That pot is pretty worthless because it heats up bias on one (or a pair in TR) tube while simultaneously cooling the bias in the other.
    Add a pot ahead of it to set overall bias voltage and it works pretty well. Otherwise a resister has to be swapped to set bias.
     
  11. Commodore 64

    Commodore 64 Friend of Leo's

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    I never turned it. I did; however, roast it last weekend while rebuilding the amp and shorting a couple pins on the power tubes when probing around. As a matter of fact, 3 CTS 100-ohm pots just got dropped off on my porch a few hours ago...$5.35 a piece as compared to maybe 15 cents for each resistor.

    IMHO, the humdinger in my amp, and some other silverfaces that I've seen the inside of...was probably cheaper than the labor required for good heater lead dress and twisting.
     
  12. robrob

    robrob Poster Extraordinaire Ad Free Member

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    That is the bias balance pot. It equalizes the bias current through each power tube and as a side effect can reduce noise and hum.

    The 100SF has an artificial center tap for the 6.3v heaters shown on your schematic. It's made up of two 100 ohm resistors.

    You need to add a bias trim pot to the back of the bias balance pot like this:

    [​IMG]
    The silverface Bias Balance pot only adjusts the balance between the power tubes, it does not adjust the bias level. The 15k resistor sets the bias level. The Balance pot is a special center tapped 10k linear pot with four terminals. The terminal on the left is the center tap. The 15k Bias Resistor is soldered to the back of the Bias Balance pot for the ground connection.



    [​IMG]
    Just add a 25k to 50k trim pot to the back of the original Bias Balance pot and replace the 15k resistor with a 10k 1/2 watt. You can bend the two trim pot leads 90 degrees to give more contact area and strengthen the solder joint. The trim pot's wiper and lower right terminal are soldered directly to the Bias Balance Pot where the original 15k resistor was connected. The trim pot's wiper connected to ground provides a failsafe in case the wiper fails. Reducing trim pot resistance moves the negative bias voltage closer to zero so bias current increases for a hotter bias. The Bias Balance pot wiper is also failsafe, if it fails the bias voltage will not be interrupted. You must bias the amp after this mod.
     
    Last edited: May 6, 2017
  13. nathanh

    nathanh Tele-Meister

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    Would you recommend this for a 5F6a build or is it overkill? I'm not having an issue with balance I don't think, I'm just curious. Also, would doing the humdinger style pot but with the signals from the PI have any positive effect?
     
  14. robrob

    robrob Poster Extraordinaire Ad Free Member

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    For the 5F6A I'm a fan of individually adjustable bias pots, one for each power tube so you can balance or intentionally imbalance (for extra ever order harmonic distortion) your power tubes.

    I don't recommend adding the silverface bias balance + trimmer because I know of no source for the center tapped pot they used.

    The 5F6A could benefit from a 6.3v Humdinger pot.

    https://robrobinette.com/5F6A_Modifications.htm#Adjustable_Balanced_Bias
     
  15. nathanh

    nathanh Tele-Meister

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    Do you have a simple explanation for the difference in even and odd order harmonics?
     
  16. robrob

    robrob Poster Extraordinaire Ad Free Member

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    No, I just know that imbalanced power tubes generate mostly pleasant sounding even order harmonics. Same goes for the phase inverter.
     
  17. nathanh

    nathanh Tele-Meister

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    Update. I added the humdinger to my LTD conversion a la Boothill and mounted the pot in one of the previously occupied RCA jack holes. Great mod. I probably won't touch it until I record again but the hole was there so why not. It really does a great job of shushing up that hum. This can now be a studio amp.

    I also bought a 12ay7 and a 5Y3. I'm trying to get that voltage down with the 5Y3. The rec does it's thing but the 12ay7 puts out zero noise. Must be a bad tube. Here's to hoping tube depot has good return policy.

    I also replaced the presence pot because my last one brought in an unbearable buzz. This one is still a little buzzy now but the amp is dead quiet when I disconnect the presence knob from the the NFB. Does the presence add any noise floor to your builds?
     
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  18. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Are you disconnecting the NFB or just disconnecting the presence pot from the NFB circuit?

    Normally an amp will hiss less when NFB is connected.
     
  19. nathanh

    nathanh Tele-Meister

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    The NFB is still in circuit, I just disconnected the presence pot from the top of the NFB resistor. Not sure why I did this but the resistor is a 56k. I must have been experimenting after a long day.

    EDIT: Come to think of it maybe thats why it's quieter than last time. Before I changed out the pot I had a 10k NFB Resistor. So naturally, 56k will be quieter, correct?

    Also this pot is kind of loud too. I can hear the wiper as I turn it through the speakers. Is that the nature of a 5k pot?
     
  20. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Many presence pots have a voltage across them so they can scratch.

    A smaller NFB resistor will allow more NFB signal which reduces distortion including hiss.
     
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