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Thread: Vox AC15 heater circuit

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    Vox AC15 heater circuit

    I am working on an early 60s AC15 and I measured the heater voltage at 7.3 vac (loaded). I was going to use a couple of 0.1ohm 5 watt resistors to lower the voltage but I noticed that one side of the heater circuit is grounded at the EF86 tube socket. This looks to be an original circuit connection. Why would one side be grounded? And should I put a dropping resistor on only the hot side or on both sides?

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    Supporting Member Jazz P Bass's Avatar
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    I would let the heater voltage be as it is.

    An awful lot of 'newer' amps run the heaters hot.

    Hey, it's worked for what, 50 some years?

    AC 15_1960.zip

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    Quote Originally Posted by Jazz P Bass View Post
    I would let the heater voltage be as it is.

    An awful lot of 'newer' amps run the heaters hot.

    Hey, it's worked for what, 50 some years?

    AC 15_1960.zip
    Thanks for the response. Just curious, Why would one side of the heater circuit be grounded? 7.3vac is well outside of the +\- 10%, will that not shorten tube life?

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    While weíre on the subject, this amp also has very high plate voltage, nearly 400v at the el84 plates. There is a lot of wax buildup that has dripped out of the power trans. I read where Ken Fischer said that was a bad sign. Maybe some shorted turns in the trans?

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    What is your mains AC voltage?

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    Quote Originally Posted by The Dude View Post
    But, I did learn something. There are protons, neutrons, electrons, ............ and morons.

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    Quote Originally Posted by words453 View Post
    ...There is a lot of wax buildup that has dripped out of the power trans....Maybe some shorted turns in the trans?
    Wax deposits from 60's Vox PTs and even OTs are very common. They cheaped out on the normal varnish potting but at least the rest of the spec (ie that actually affects performance) is good.

    A shorted turn would draw fault current from the mains and cause the fuse to blow (is the fuse value that's fitted appropriate?).

    Where in the world are you, what is your nominal mains voltage and frequency, and, as per @g1, what is your actual measured mains voltage?

    I seem to remember reading that 60s Vox PTs were incorrectly specified for North American 120V 60Hz use, they may not have properly accounted for the increased efficiency of operation at 60Hz or something, and they used 115V or 110V rather than a 120V nominal; consequently the secondary voltages are too high.
    Also read that even with all that compensated for, the PTs run hotter at the 115 setting than they do at 245, so it's recommended to set them to 245V and use a step up transformer.

    I've no idea on the truth of the above, but 7.3V on the heaters is crackers, if the step up transformer idea above isn't used then I suggest to use a mains bucker or commercial equivilant (assuming you're in a 120V region), set to provide a heater voltage of 6.0 to 6.3Vac.

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    Mains voltage is 122-123v @ 60hz. The voltage selector on the amp is set at 115v. There was no fuse or fuse holder in the amp. I think the fuse was originally attached to the voltage selector plug, but is missing now. I cannot read the schematic to determine fuse amperage but I installed a fuse block and a 2 amp slo-blo. I plugged it in to a current limiter which glowed dimly so I donít think Iím drawing a lot of current.
    Should I install the dropping resistors for the heater circuit and live with the high plate voltage or just suggest that the owner use a bucking transformer? I still would like to know the purpose of grounding one side of the heater circuit!
    Thanks for the replies

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    Grounding one side of the heater supply is not uncommon. It's what they did before they used center tapped heater windings or 'virtual CT' resistors. Have a look at early tweed fender schems. (Champ 5C1 for example)

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    Quote Originally Posted by The Dude View Post
    But, I did learn something. There are protons, neutrons, electrons, ............ and morons.

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    I would use an external variac to feed 115v,and eventually install 2x100 ohms resistors from filaments to ground.

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    Thanks for the replies. I think the general consensus from this forum and others is to use a bucking transformer. Thanks for the help!

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    To get the heaters down from 7.3 to 6.3 I think that mains will need reducing down from 123 to 106, ie ~17V bucker, so 2 a bucking transformer with 2 separate 8V or 9V windings; that should also bring the HT down ~50V.

    Quote Originally Posted by Jazz P Bass View Post
    ...An awful lot of 'newer' amps run the heaters hot...
    I think that a possible cause of that may be the use of 'worldwide' PTs, ie primary has 2 x 115V windings, wired in series for ~230V regions, parallel for 120V.
    Straight away when used in 120V nominal, the secondaries are running ~4% higher than intended, and that's before the typical high mains voltage comes into play.
    The export PTs that Fender use are good, as they can be configured for several mains voltages, eg https://el34world.com/charts/Schemat...erb_manual.pdf though unfortunately they arrive in the UK set up for 230V, so end up always running a bit hot on the 240V mains (within the 230V nominal tolerance) here.

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    Last edited by pdf64; 10-11-2018 at 08:03 PM.

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    I'd also note that Hammond has added an extra 'tap' on the primary of some of their series, allowing a choice of 115 or 125V. So at least they are acknowledging an issue with using standard transformers.

    http://www.hammondmfg.com/263.htm#263newpri

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    Quote Originally Posted by The Dude View Post
    But, I did learn something. There are protons, neutrons, electrons, ............ and morons.

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    I built a bucking transformer using a 12.6v 3amp transformer from my local Radio Shack. One of the taps brings the voltage down to 115v and the other tap brings it down to 108v. $12 at RS and it definitely does the trick. Thanks for the help!

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