Power Transformer center tap vs. filament resistors?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by EightySix, Feb 17, 2012.

  1. EightySix

    EightySix Member

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    Is there an advantage to either? Can using 2 100-ohm resistors @ the power lamp REALLY save transformers in the case of a power tube completely shorting?

    I'm looking at an early SF Showman (still ab763) that had the power trans replaced somewhere along the line. Apparently the original Silverface PT didn't have a center tap, because there are still 2 100-ohm resistors on the pilot light- connected. So right now BOTH the center tap & the 2 resistors are hooked up, and the 2 resistors are fried.

    I'm debating whether to use the center tap of the replacement PT and take the old resistors out, or disconnect the center tap and replace the 2 resistors.
     
  2. eddy999

    eddy999 Member

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    If your resistors are closely matched then using an artificial centre tap can give you a better balance than the pt centre tap. The balance of the pt centre tap depends on how well wound it its, whereas with the artificial ct you control the balance by using matched resistors. I use 1% tolerance or just match other resistors using my dmm.
     
  3. EightySix

    EightySix Member

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    Thanks eddy, is either one better for the longevity of the amp- specifically the Output Trans?
     
  4. trobbins

    trobbins Member

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    eddy999 - why do you want to match the resistor values in a humdinger? Normally the humdinger gives lowest hum for some custom resistor setting, which is why a humdinger pot is a better aim.

    If a 6V6 or 12AT7 cathode shorts to heater then current passes through the humdinger arms (and heater winding), but with 100R more series resistance than if you just used a CT, so there is some benefit.
     
  5. eddy999

    eddy999 Member

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    I don't use a humginger i.e. a pot - I just use a pair of matched resistors to give the best balance. The way I see it a humdinger just lets you manually balance the two sides. If your resistors are carefully matched then there's no need to have a pot. I guess the only need for a pot would be if you've got a non symmetrical hum for some reason, however i've never experienced this personally.

    As for the protecting the amp, trobbins is right that it would provide some protection for the power transformer. As for the output transformer, I don't think it would make a difference - you are better off protecting the OT using a fuse on the B+.
     
    Last edited: Feb 17, 2012
  6. trobbins

    trobbins Member

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    Yes, some input stage valves are fine - but I've noticed a few common ones often need quite an unbalanced pot position, and sometimes the wiring couples more hum to grid from one 'side' of the heater winding. I have a batch of Bourns 10T 0.75W 200 ohm that come in handy - the 10 turn can often give a noticable improvement in nulling.
     
  7. VacuumVoodoo

    VacuumVoodoo Member

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    That's the way to do it
    multiturn trimpot
    sets you hum free
     
  8. guitarcapo

    guitarcapo Senior Member

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    I think that two resistors is better than a center tap for hum but it does tax your filament supply a little more and uses electricity. A humdinger is nice if you have space, but you do have to run more filament wire inside the chassis to set one up and I sometimes wonder if the advantages of being able to "balance" resistors that you've already measured on a meter is worth the extra filament wires running around inside your chassis possibly causing hum.

    There's also the idea of elevating the filament supply, but in the few cases I tried that I got more hum myself. probably from lead dress issues.
     
  9. VacuumVoodoo

    VacuumVoodoo Member

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    A 200 Ohm resistive humdinger draws less current (31mA) than the difference in heater current between two EL84s can be. You can win about 10db in hum cancellation with a humdinger vs fixed CT or unnecessarily matched resistors. Of course your level of hum (in)tolerance dictates the course of action.
     

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