The design of this amplifier, as is the case with any amplifier,
centers around the output stage. EL84/6BQ5 tubes were
selected for the output stage for several reasons.
They will produce adequate power at low distortion with only
250 volts on the plates and screens. Plate current requirements
The most important consideration in the use of 6BQ5’s was
their low drive requirements.
Only 22.5 volts grid-to-grid are required to drive the tubes to
Compared with the better known 6V6, the 6BQ5 will produce
more output with lower distortion, less plate-current
consumption, at about one-third less grid drive.
This means that if we use the 6BQ5, our power supply may
be reduced in size and the voltage amplifier stages preceding the
power amplifier stage may be simplified.
We have achieved a large part of our goal simply
by choosing the correct output tubes.
A little unusual circuitry now comes into play to enable us
to reach our goal of the “ultimate" in a simple, yet good,amplifier.
It will be noticed that there is no phase inverter in the amplifier
and, at the same time, the hook-up of the output tubes is
unconventional. The output tubes act as their own phase inverter.
This type of circuit is not new and can be found in some of the
audio hand-books. The circuit is not used much despite its
advantages of good inherent balance, simplicity, and low
drive requirements. There are those words "low drive” again!
Only one tube is driven in this circuit as opposed to two in the
normal type of output stage. This means that the required driving
voltage is halved which, in this case, now amounts to 11 volts.
Almost any tube will supply the required 11 volts when driven
from a ceramic phono cartridge, FM tuner, or tape deck so we
eliminate the multi-tube input stages found in most amplifiers and
end up with a single 6C4.
Inasmuch as we do not have a great deal of voltage amplification
we do not need a lot of filtering in the power supply, hence
almost any filter choke and capacitors will do the job. Since
we are not striving for maximum gain in the input stage, the
cathode resistor has not been bypassed, thus giving us desirable
inverse feedback and saving the cost of the bypass capacitor.
Some further information about the output stage is in order.
The plate current of V’ varies in accordance with the signal.
The voltage across the cathode resistor, R will also vary as a result.
Thus there is a component of signal voltage on the cathode of V.
Since the cathode of V: is in parallel with that of V .
it will also have signal impressed on it and since the grid of V.
is grounded the cathode will swing around the grid which is
equivalent to having the grid swing around the cathode as
we are accustomed to having it. The cathode resistor must
not by bypassed (by a capacitor).
None of the parts in the amplifier are critical,however,
you should make every effort to get the output transformer
of the impedance and power specified in the parts list.
This amplifier has plenty of high frequency response so tone
controls have been kept to a minimum and only a simple treble
attenuator is employed. The output power is just over 6 watts
before the scope shows any noticeable distortion.
(Melvin Leibowitz, Electronics World, June 1961)