Firson asks:
>How much (watt) will be approximate required to drive this amplifier ???..
About 95 - at LF. The input capacity will be about 50pf for all those tubes, so
the SWR on 28MHz will be a bit higher than on the LF bands. (50 ohms in parallel
with j112).
If you have a bit more drive, you can use unbypassed resistors in the filament
return. i.e. connect the filaments in parallel and put something like a 10 ohm 2
watt low inductance resistor in series with a mica or RF curent rated ceramic
.01 microfarad capacitor from each side of the filament string to ground. This
improves linearity.
Some brands of 4-125 equivalent draw 7 amps filament current, some 6.5 amps, so
you need a pretty massive filament transformer and very heavy wiring. It's worth
while putting a suitable large variable resistor in series with the transformer
primary, and making sure the filament volts are correct - 5 volts within 5% or
better. (Rich will doubtless confirm the importance of this - it has a big
effect on tube life). Dropping enough volts in the resistor also minimises the
surge at switch on.
One technique used in a military tx (British Army D11 - used in many other
countries, too) with these tubes was to series the filaments, and use a 5 - 0 -
5 volt transformer. This reduced the current, but did mean that the plate and
screen currents were added to one side of the filament current. In practice,
this did not appear to affect matters, although ideally, the plate and screen
return is to the centre tap of the transformer.
With these tubes, it's advisable to strap both the screen grid pins together
with copper strap, and use a series non inductive resistor of about 15 ohms
right up at the grid pin. Make sure the pins can still 'float' to allow for
thermal expansion, and keep the pins cool. The 4-125 has a metal base shell and
is designed for forced air to be blown over the pins, up through the base shell
and then around the tube in a chimney; the QB3/300-QY4/125-RS1007-CV2130 (all
the same thing!) don't have the base shell, and in theory, don't need the pins
blowing at HF, or chimneys. My experience is that the theory may be OK if you
don't minding changing tubes or resoldering pins and so on. Far better is a
small Muffin fan playing on the underside of each socket. Usually, a fan running
on reduced voltage is enough, and it keeps the noise down. This is additional to
a fan blowing a fairly large amount of air over the tube itself. You need to
make sure that you don't get 'hot spots' where there is no air circulation;
blowing down from above usually works OK.
2500 volts is a little low; 3000 or 3250 - even 3500, works a bit better; just
watch the dissipation. They glow red under zero signal conditions. Expect about
600 to 700 watts of linear RF out - maybe 850 if you run 3500 on the plates.
As you may have guessed, I have experience with these tubes and their 'big
brother' the 4-250A and its European variants. They're nice little tubes, even
though the design is over 50 years old - a more efficient, higher frequency 813.
Good luck
Peter G3RZP
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