Designing and Building
Audio Amplifiers.

50 Watt Per Channel EL34 Ultra Linear Amplifier.

The Power Supply and Circuit Description.

The power supply for this amplifier is no trivial matter. I have decided
to use a single power supply for both channels. The schematic is shown
below.

For a verbal description click
here.

You are probably wondering about those
4 zener diodes from B+2 to ground so
I'll get them out of the way first.

The original plan was to set the total zener voltage at about
440 volts to protect the capacitors from over voltage before the tubes
warmed up. What I didn't count on was the large positive temperature
coefficient of the zener, more correctly called avalanche, diodes.

Note: The physical mechanism within a so-called zener diode
for low voltages, below 6.2 volts, is zener breakdown. Above 6.2 volts
the mechanism is avalanche breakdown. Zener breakdown has a negative
temperature coefficient while avalanche breakdown has a positive one.
The change-over from one to the other is not the flip of a switch but a
gradual merging from one into the other. At the critical point, just a
little less than 6.2 volts, the temperature coefficient is zero. Diodes
that fall on this critical voltage are sold for 10 times the price as
reference diodes.
When the diodes were doing their job the
heat generated caused the total voltage to rise to about 500 volts. The
56 and 82 volt diodes have been replaced by a single 100 volt diode. This
should protect the capacitors but still have little effect on the power
supply in normal operation.
The
string of zener diodes constitutes a voltage limiter. You will note that
the voltage on the other side of the 4.7 k ohm resistor can be as high as
620 volts when the power is first turned on and before any of the tubes warm
up and start to draw current. That could do serious damage to the 450 volt
capacitors after the resistor. The zener diodes will prevent the voltage
from going above 400 plus a little,
volts. After the tubes start to draw current they will hold the voltage of
B+2 under 400 volts where it
belongs. Under normal operation the zener diodes should not be drawing any
current. They are there to protect the capacitors against over voltage. If
someone, such as me, fails to think and unplugs the 12AX7s and 6CG7s the
zener diodes will still protect the capacitors. The vacuum diode shown in
the earlier version would not provide any protection under this scenario.

I estimated that the current draw from B+2 and B+3 would be 14 mA per
channel for a total of 28 mA. The actual measured value is 15 mA or 30 for
two channels. The output tubes have an idling current of 200 mA for a total
of 400 for both. The chart shows a voltage of 535 volts for a current drain
of 400 mA. Linear interpolation yields a voltage of 531 volts at a current
of 430 mA. From this I arrived at the resistance of 4.7 k ohms for the
resistor. A 5 watt resistor is required.

The transformer used is rated at a
current of 465 mA. This doesn't seem to leave much leeway for increased
plate current when the amplifier is working hard. However the heater
winding is not being used which gives us an additional 50 watts to play
with. I am aware that not all of those 50 watts can be transferred to the
high voltage output but it does give some additional head room. I
breadboarded up the circuit and tested it using my Load
Box. Taking it all the way to 600 mA might seem to be a stretch and I
think it is. I ran it at that current for half an hour on two separate
occasions with no ill effects. Remembering that it takes a while for heat
to soak out of a big transformer I checked it several times after
terminating the test. It still got only warm to the touch. I'm sure no one
would call it hot. When the amplifier is being tested at full power with a
continuous sine wave the plate current of one channel is 300 mA. If I were
you I wouldn't run a prolonged full power sine wave test with both channels
driven as required by the old FTC (Federal Trade Commission) power amplifier
rules. But that's a discussion for another page.

Some may find having the filter choke in the negative lead confusing. It
doesn't make any difference since the choke is in series. Series is series
and the order doesn't make any difference. This is a standard precaution to
make sure there will never be a short from the choke winding to the core.
Although the Hammond chokes are rated at 800 volts, why take chances if we
don't have to?

There is nothing remarkable about the bias supply. It uses a zener diode
so the bias won't be effected by changes in power line voltage.

Well, that was my other big mistake. Once
again the large positive temperature coefficient came into play. The bias
would start out low and go higher as the diode warmed up. That would
cause the tube plate current to start out high and gradually fall. As I
had adjusted the bias after a prolonged warm-up, the tubes were probably
exceeding their maximum plate dissipation for the first several minutes of
operation. Another problem was those line voltage changes I wrote of
above. The plate voltage was, and is, not regulated but the bias voltage
was. That meant the plate current went up and down with the line voltage.
The line voltage changes more than you think. Measure the voltage in the
early evening and again at 2 or 3 A. M. The solution to this problem is
to let the bias voltage change along with the plate voltage.

Note: In the ultra linear connection the tubes are
operating as triodes as far as DC is concerned. The screens are always
at the same DC potential as the plates. In my pentode designs where the
screen voltage is regulated the bias should also be regulated. The
plate current is quite immune to changes in plate voltage due to the
nature of the pentode.
There is still a zener diode but it
serves a similar function to the 4 diodes on B+2. It will limit the
voltage when the power supply is operated without the amplifier being
connected. The unloaded voltage at the anodes of the bridge rectifier is
-195 volts.
There
is probably more filtering than necessary but it can't hurt. My goal in the
overall design of this supply is not to hear even the slightest suggestion
of hum with my ear right up to the speaker.

The 100 k ohm resistor from C+ to chassis is so the supply can be
operated unplugged from the amplifier without the bias supply floating away
into hyper-space. The 10 Meg ohm resistor does the same thing for the
heater supply. A lower value would have upset the DC level placed on the
heaters by a circuit in the main amplifier chassis.

Introduction.

Power Supply Schematic and Circuit Description. (You are here.)

Amplifier
Schematic and Circuit Description.

Power
supply chassis layout.

Amplifier
chassis layout.

Initial
tests and adjustments.

A foolish man dreams of wealth,
a wealthy man dreams of
wisdom,
and a wise man dreams of tubes aglow all in a
row.

Amplifier
Smorgasbord.

Home

Or use your "Back" button to return to where you were.

This page last updated April 16, 2006.