From cign–(at)–elios.phy.OhioU.Edu Tue Jun 25 10:53:16 CDT 1996
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From: cign–(at)–elios.phy.OhioU.Edu (Dave Cigna)
Subject: Re: Lowering B+ Voltage
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Elvis, the King of Rock & Roll wrote:
>Howdy.
>
>I’d like to figure out some way to lower the B+ voltage.
>
>I have several ideas on how to do this:
>
>– Replace silicon rectifiers with gaseous-state

This is not an impossible mod if you have room for the rectifier tube and
an additional five volt filament transformer, but is impractical on amps
that came from the factory with bridge rectifiers (this includes many 70
and 135 watt Fenders, for example.) With a 100 watt amp, I’d be a little
worried about overtaxing the rectifier. Even a 5U4GB would be overworked
in an amp this size. I’d recommend leaving this mod for 50 watt and amps.

>– Big old power resistor floating several hundred volts above ground

Having them several hundred volts above ground doesn’t bother me, but
enough series resistance to drop 50 volts will make your power supply
hoplessly mushy. It ruins the sound. It is possible to add 50 to 100
ohms of series resistance to simulate a tube recitier, but this will
only drop about 10 to 15 volts. Not the solution that you are looking
for.

>– 50 or 100 volt transformer bucking supply transformer (it’s half wave
>rectified, so two would be required)

Sounds pretty cumbersome. I’d stick with easier & cheaper methods.

>– Some kind of switcher or phase-angle controlled deal

Hmm… I guess there are a few trick you could play. It seems like
throwing a lot of silicon at it is just not cool when we’re talking
about tube amps. 🙂

>The traditional way for musicians to do this is a Variac, but I’d rather
>not lower heater voltages. Perhaps I need a Variac and a separate
>regulated filter supply.

If you are using an amp made before about 1975, then it was probably
designed for 117 volt operation. Chances are the power coming from
the wall in your area is well over 120 volts. 125 volt or more is
not uncommon in the US. It’s not at all unreasonable to lower this
by 10% or so to about 110 volts. The heaters will still be at a
safe level and you’ll lose 45 or 50 volts of B+. You can accomplish
this with a 12.6 volt transformer bucking the 120 volt line.
I have one built into an extension cord. No need to modify the amp
at all. These are constructed by applying line voltage to the
primary of the 12.6 volt transformer and running the secondary in
*series* with the line so that the 12.6 volts is subtracted from
the 120 volts. If you wire it up the wrong way (which happens more
than half of the time) then you will add 12 volts. Just measure
the output and fix as necessary. Choose a 12 volt transformer with
a generous current rating. Antique Electronic Supply sells a 4 amp
Hammond transformer for about $20. This should be big enough for
one 100 watt amp.

Another mod that you might consider is installing a zener diode of
about 50 volts. This easiest and safest place to install this is
between the center tap of your power transformer and ground. I
recently did this to a Princeton Reverb with very good results.
You can buy a 50 watt zener with the correct polarity (cathode case)
>from Mouser for about $15. The NTE part number for a 56 volt part
is NTE5278AK

— Dave Cigna

 

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