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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