Two Manley Snapper on bench after many changes to schematic.
Notice the 4 smaller recessed 470uF / 450V electro caps between the
very good PT and very good OPT at rear of amps.
The original amps had much longer and taller 1,300uF electros which
stood higher to look impressive. A couple shorted out some years after
I first re-engineered these amps so I replaced the lot with easy to source
470uF caps which are very adequate for filtering B+.
Red and yellow LEDs sit beside each EL34 output tube to indicate
correct biasing when both glow equally bright. If an EL34 has Ia too
high, the red LED glows brighter and yellow turns off, and if
Ia is too low, yellow is bright with red turned off.
Anyone not liking the 5mm LEDs could use 3mm and recessed down
a little more to reduce the "christmas tree" effect.
The LEDS enable bias setting without a voltmeter, and indicate bias health
with one glance.
Active protection prevents one or more output tubes causing excessive
current from PSU damaging an OPT, and causing very expensive repair bills.
owner used these to power Quad ESL989.
and driver stages....
original PSU had B+ of +560Vdc applied to EL34 in 25% UL mode,
so both Ea and the Eg2 were higher than used by many other brands.
The high Eg2 meant the Eg1 needed to be around -57Vdc and I found
the bias adjustment too sensitive. The amps had been purchased from
someone in Hong-Kong who had found difficulty with biasing, and had
cooked several EL34 repeatedly, had gladly sold the amps.
So the original amps overheat all too easily. Australia's high mains voltages
of 255Vac on many days instead of 240Vac does not help matters.
There are two PT primary windings each meant for USA 110Vac
for USA, and when in series the Vac should be 220Vac.
I installed R7, 410r, made up from 8 x 820r x 10W wire wound
in series and parallel to be 80Watt total rated. These are siliconed to a
chassis divider plate within the amp and liberate 20 Watts of heat.
The B+ voltage drop is 80Vdc to +500V applied to OPT CT, and this is
better tolerated by EL34.
The original OPT has ZR = 484 : 1.
Load matches are k9 : 4r0, 3k9 : 8r0, and 7k8 : 16r0.
we consider use with 4r speakers with a typical minimum Z of 3r0,
the tube load is 1k5. If B+ = +550Vdc, the EL34 will very easily overheat
with loud music signals, and 4 ohm speakers should NOT be ever used.
correct EL34 operation with B+ = +550Vdc, UL operation with OPT
screen taps on primary should not be used. It is much better to have
Eg2 = +400Vdc for all Ea over 450Vdc.
However, with higher Ea there is need for higher primary load of OPT.
analysis will show that the 4 EL34 can produce 100Watts with the
Ea = +500Vdc and with UL operation. The OPT should have primary load
of 3k3 minimum and with OPT ZR = 484 : 1,
the minimum sec load = 6r8. Use of 8r0 speakers is then safe.
each EL34 biased for Ia = 40mAdc, Idle Pda = 20Watts.
With sec load of 8r0, Maximum class A Po is 12 Watts,
and the maximum class AB Po is about 85Watts, and "enough" for anyone.
Active protection and bias condition indication...