CHAMP ELECTRONICS -" THE VALVE AMP
HOSPITAL"
NOTTINGHAM, ENGLAND
SERVICING OF A DECCA DECOLA Hi-Fi POWER
AMPLIFIER
The Decca Decola Pre-Amp Unit.
The
Power Amp Unit.
This
one was a mammoth task/challenge but I really enjoyed doing
it!
Dating
back to somewhere in the sixties, this Decca Decola power amp and
pre-amp was sent to me by a real nice gentleman, Chris, from Northern
Ireland. This is how it looked when first received, not working at all
and I’m not surprised! There were many, many duff and kaput
components, so she required a massive overhaul. About the only
component still surviving was the 8uf @ 500 volts paper reservoir
capacitor. However as this amp didn’t have a choke input
filter,
I saw no point in retaining this. I did-away with the cap and replaced
it with 100uf….the HT is “flat lined”
now and much
better for fast transients!
The
chassis is made of a very soft metal and, as the three transformers are
mounted in a “tri-angle shape”, the centre of the
chassis
has bowed-down over time. I removed the power transformer and smoothing
caps and then made a trip down to my metalwork man, Steve, who got it
back to around 95% perfect.
One of the driver boards showing
the original wiring.
Top-side of same board before any
work.
Same
board again after work completed. All the duff/tired components have
been replaced, including the ones that were likely to fail in time.
Under-side
of same board again. This time with all new colour-coded ribbon
cables……much easier to follow now let-alone
tidier!
The new gold-plated RCA (phono)
input sockets along with a complete new buss rail.
Finished!
Ahh….now that’s better, much neater/nicer though I
say it
myself! And……a perfect 12 watts RMS per channel!
Both
drive boards, now mounted horizontally as apposed to the original
vertical mounting (check out the second photo down from the top of the
page). I think this looks so much better and tidier!
Under-side of the pre-amp before
commencement of any work.
Top
view of the pre-amp. The two valves on the left are ECC83’s
and
the two on the right are 8D8’s. The 8D8 is pretty much like
an
EF86 but with a 150 m/a heater, whereas an EF86 has a 200 m/a heater.
The 8D8 has been chosen for this 150 m/a heater issue and for what at
the time was a very novel idea…..explanation as follows. It
is
always a good policy to run pre-amp heaters on DC, especially where
high gain is required, as-in moving coil turntable pick-up cartridges.
So, what the designer of this amp has done in order to run all the
pre-amp heaters on DC is this: He lifted the power transformers main
ground return (centre tap) and now grounded it through a 280 ohm, 20
watt resistor. Then with a 500uf electrolytic capacitor @ 64 volts
across this resistor now gives us 37.5 volts DC above the earth ground
rail, for the four pre-amp tubes’ heaters…wired in
series!
(12.6 + 12.6 (both ECC83’s) + 6.3 + 6.3 (both
8D8’s) = 37.8
volts). When putting tube heaters in series they all need to be the
same current…..hence the two 8D8’s, being 150 m/a,
same as
the ECC83’s. The GZ34 rectifier is working very hard,
supplying
everything! As the amp warms up and the four output tubes start to draw
current this voltage then appears across the said resistor and
cap…..settling at the required 37.5 volts (or thereabouts).
The
power transformer does run pretty hot, not only with the four
EL34’s pulling full currant nearly all the time (they are
triode
connected, in cathode (self) bias) but of course there is this added
150 m/a, constantly from the pre-amp heaters! Nowadays I would doubt
one would design it this way, a better way would have simply been to
have an extra winding on the power transformer for the pre-amp heaters
and rectify/smooth-it separately. Still, I must admit, that once
finished the amp works superbly and sounds great!
All
the RCA (phono) inputs were very iffy and tarnished with age. I
painstakingly changed each one, in blocks of four, for nice
nickel-plated ones. Quite a tedious and tricky job really, having to
drill-out the old, very fragile bakelite panels! There were 16 sockets
to do altogether!
One panel
done…….three to go…….phew!
Most
of the caps have now been changed and just a few resistors. On any
stereo valve pre-amp there are so many factors involved in both the
channels being identical when the balance control is in the middle. You
quite often hear one side at a slightly different volume to the other,
even though the balance control is telling you it is in the middle. To
correct this phenomenon I installed a pre-set pot (the white thing at
the top-right). This pot is wired into just one channels final feedback
circuit and then, with a mono signal injected into both channels, the
test gear monitoring, the balance control set to “12
O’clock” one channel can now be tweaked for the
identical
gain of the other!
Showing
all the new nickel-plated inputs plus a grounding terminal for the
turntable, which this amplifier didn’t have originally.
All
labelled up and ready for the return trip to Chris, in Ireland. Note
the large M6 nut/bolt with the large nickel-plated washers (just in
front of the power transformer). I added this as a support leg for the
chassis centre…..it won’t bend again now!
All the junk
removed……..phew!!
Cheers, John.
