Tube amp power supply

Discussion in 'The Projects Forum' started by tube boob, Feb 24, 2015.

  1. tube boob

    Thread Starter New Member

    Feb 24, 2015
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    The plague of modern house voltage is it is higher than what most tube amps like. In lieu of using a variac
    or something expensive, has anyone had luck using a current limiting resistor on BOTH incoming
    AC line legs to reduce voltage? Will this have a deleterious affect on the amps performance?
     
  2. crutschow

    Expert

    Mar 14, 2008
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    3,237
    Why is you house wiring more than the tube amps like? :confused:
    The tube amp should be rated for standard main's voltage.
    Adding resistors would cause the voltage to vary with the amps output power which would likely increase distortion.
     
    planeguy67 likes this.
  3. tube boob

    Thread Starter New Member

    Feb 24, 2015
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    0
    I have vintage amps that like 110v or so. We have 123 to 125 VAC , wat too high, resulting in higher voltages in amp. This in turn
    reduces tube life. Also, when turning on, a CLR reduces the "surge", sort of like waking up slowly with a cup of coffee versus running a mile
    right away. I have used one CLR on an incoming leg to good result ( amps are choked until higher temp reduces resistance), saving tubes and fuses.
    I was wondering if two would lower overall voltage feed a few volts , which is desirable in these old tube puppies.
     
  4. planeguy67

    Member

    Jan 29, 2015
    59
    16
    Placing resistors in line with your input voltage isn't the answer. As crutschow said, any change in input current will change the voltage.

    Have you calculated the precise effect 125 Vac is having on your plate current vice 110 Vac??? If you look at the characteristics for an EL-84 pentode (for example), an increase in 30 volts in your B+ supply at a constant grid potential only changes plate current by a few milliamperes.

    If your amp is running right at the threshold of plate power dissipation, then you might have a cause for worry, otherwise, with a reasonable margin designed in, I don't see the trouble. Try turning the volume down half a tick.
     
  5. Reloadron

    Active Member

    Jan 15, 2015
    963
    232
    While I wouldn't worry about it, as you mentioned in your first post, if you are concerned the best solution is to run the amp(s) using a variac. Looking through my old tube manuals and books I see no reason to be concerned. Pretty much as planeguy67 points out.

    Ron
     
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  6. RichardO

    Well-Known Member

    May 4, 2013
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    Incandescent lamps have a much shorter life as the operating voltage is increased. Does the tube filament have this same problem? If so, a high line voltage would decrease the life of the tube. Note that this would be a complete failure and not a slow degradation in gain or the like.
     
  7. crutschow

    Expert

    Mar 14, 2008
    13,028
    3,237
    Here's an interesting solution. You could add a 12V filament transformer to reduce the line voltage.
    Connect the primary across the mains and the secondary in series with the mains to the amp.
    You have to experiment with the relative winding polarity. One polarity will add to the mains voltage and the other will subtract.

    Interestingly, when the transformer is reducing the voltage, its primary current will be in opposite phase to mains current. It will be subtracting from the mains current and reducing the mains power.

    The transformer secondary current rating should be equal or greater than the amp current.

    In general this technique can be used to increase or decrease the mains voltage. For example, a 120V-120V transfomer could boost 120V to 240V. That advantage of this is that the transformer power rating can be one-half of a 120V-240V step-up transformer for a given load current.
     
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  8. Reloadron

    Active Member

    Jan 15, 2015
    963
    232
    Yeah, that pretty much covers it. The typical tube filament is rated +/- 10% so in the case of a 6.3 volt filament between about 5.67 and 6.93 volts is the ideal range for normal performance of the tube. If the filament voltage is below the minimum the tubes emission will likely suffer and if the voltage is high the filament life will suffer. I began my life with vacuum tubes and can say I replaced more tubes for other problems than the filament failing. I am not saying a vacuum tube that constantly runs at a high filament voltage won't fail but I am saying a tube running hotter won't be an instant failure. If a tube has a filament rated at 10,000 hours and runs at a 7.3 volt filament voltage how soon will it fail? Would 9,000 hours be a good guess? Maybe more and maybe less? The only way to know how much filament voltage there is would be to measure it. Short of saying:when the line voltage is, my filament voltages are. Without knowing the actual voltages the filaments are seeing speculating seems a bit foolish. Again, while I never did a tube count during my tube years I saw very few tubes fail due to a filament failure.

    Ron
     
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  9. planeguy67

    Member

    Jan 29, 2015
    59
    16
    Thanks for the info Ron. I haven't played with tubes much, so I was curious as to their most common types of failure.
     
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