300 WATT
MONO BLOC OUTPUT POWER.
Here are some graphs to try to
explain the relationships between my the amplifier output voltage
and output
power to loads likely to be connected.
The
above graph shows the output voltage at 1% THD at the onset of clipping
for the low impedance match
of the output transformer.
Two curves A and
B are for different values of Ea. With Ea = 480Vrms, a maximum output
voltage = 29Vrms
With Ea = 400V, output voltage is 25Vrms maximum.
Maximum power is at 1.5 ohms where 23Vrms output produces 352 watts.
This
graph shows the maximum power levels at clipping at the anodes for two values of
Ea, and the extent of the class A power using the 1.2k ohms to 2.5 ohm output
transformer ratio.
The power output is for 12 x 6550 with 20% cathode feedback connected, and
the tube operation is very similar an ultralinear amplifier with about 35% taps
for the screens, and power is only marginally less than pure beam tetrode, but
distortion is far lower.
The power shown is that which could be measured at the anodes, and does not
allow for output transformer winding resistance losses. But in fact losses are
less than 5% when the secondary load is 2.5 ohms, which is the nominal rated
load value for when the output transformer impedance ratio chosen is 480 : 1
which is the higher of two selectable ratios for the OPT.
For where Ea = 480V, Ia = 35 mA per tube, and for Ea = 400V, Ia = 75mA per
tube.
Curve A with Ea = +480V show that there is only 30 watts of class A power out
of a total of 290 watts of class AB
for 2.5 ohms at the output.
But with Ea = 400V and a higher Ia the class A increases to 125 watts out of
a total of 200 watts of class AB also
with a 2.5 ohm output load.
The maximum pure class A power available can only be with the amp that has
the higher Ia and lower Ea.
With Ea = 400V and Ia 75mA, 150 watts of pure
class A is available into 3.5 ohms with the load match as shown.
The THD is
very low in this condition.
The graphs for output voltage and power were
drawn to show a prospective buyer what maximum output voltage he could get with
the 2.5 ohm connection since he wanted to connect 3 pairs of Quad ESL57 as well
as a pair of Lipinski L707 in 4 different rooms of his huge house.
The output
voltage remains less than 25Vrms when Ea = 400V, so the Quads cannot be over
driven and arc out.
The output voltage when Ea = +480V is too high.
One pair of Quad ESL57 need an amp capable of only 25 watts into 16 ohms
which is 20vrms,
and 3 pairs in parallel will need 75 watts, so with the
Lipinskis, about 200 watts class AB into 2.5 ohms
with Ea = 400 volts would
work well.
It must always be remembered that pure class A power is
expensive because you need to dissipate a lot of
power in the tubes even
when there is no output power being produced, so that for
150 class A watts
at a maximum efficiency of 45%, means that there must be 333 watts
dissipated in the tubes
which would be 28 watts per output tube in this case
which is quite a light amount of dissipation because a 6550 or KT88
is
capable of dissipating 42 watts of power continuously.
One would never run
the tubes at maximum Pda lest the tubes wear out quickly.
It is possible
to use Ea = 600V and beam tetrode operation and thus get 600 watts of AB1 power
but it will nearly all be class
AB power with 5 times the THD as one has at
150 watts of class A power.
Since a pair of 6550 or KT88 can produce up
to about 140 watts in class AB2 tetrode, the absolute power
max would be 840
watts from a dozen tubes but the reliablity would then begin to suffer.
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