CHAMP ELECTRONICS -" THE VINTAGE VALVE
AMP HOSPITAL"
NOTTINGHAM, ENGLAND
SERVICING A MODIFIED SELMER
SELECTORTONE 25 watt
COMBO AMPLIFIER
I have included this second,
modified Selectortone as I thought all you guys out there would find it
interesting. It belongs to a customer called Jerry, for whom I have
done a fair amount of previous work. Jerry rang me to say he had a
Selectortone with just the pre-amp and a replaced (smaller) speaker. He
got it without the power section but, had managed to source a power
section for it. The problem was that this power section was from a 30
watt Twin Selectertone which has 2 x KT88's rather than the 25 watt
Selectortone's 2 x EL34's. He asked if it would be possible to combine
the two?.......Absolutely no problem!
Underside of the power section as received, which would have originally been based on a pair of KT88's.
As it happens, the output
transformer was duff anyway and as it needed to be rewound….this
was the perfect opportunity to wind it with the correct plate load for
EL34’s rather than the KT88’s (Jerry wanted this amp
converting to EL34’s).
The output transformer
finished and fitted….in-fact the whole amp is finished here. Two
things also worth noting: I retained the extra small tube (ECC82) that
the Twin Selectortone power amp section has. This is a push-pull driver
for the KT88’s. Personally I can’t really see why Selmer
ever included this tube. As the pair of KT88’s in the Twin
Selectortone were only ever supposed to give the rated output of 30
watts (absolutely no effort for them at this low power!) the idea of a
driver tube is pointless!! The preceding ECC83 splitter would quite
easily drive them up to 30 watts…..no problem! The other point
is….note the layout of this chassis. The output transformer is
on the right and the power transformer on the left. This is completely
opposite to all the other Selmer power amp chassis!
The main board with a few new components, plus a good few different
values and a slight circuit change…..to accommodate the use of
EL34’s. In the top left-hand corner there is an extra tag-strip
with a red fly-lead. This is the speaker impedance
selectivity……4, 8 & 16 ohms; whereas originally there
was no 4 ohm facility on these amps - just the usual 8 & 16 ohms.
I don’t think many of
you out there will need telling how many of these grey ERO caps needed
changing here…..in the pre-amp!!
And of course…..as always…..all replaced.
Here she is
- all finished bar the back panel. The original baffle (15”
driver) has had a second baffle fitted over it…..and the amp
loaded with the 12” Jenson as can be seen in this photo.
So for a small re-cap
here..........we have an original Selectortone cabinet and pre-amp, a
power amp from a Twin Selectortone but converted to EL34 output tubes
rather than the original KT88’s. And lastly a change to a
12” driver. Bit of an “odd-bod” I would say but guess
what……..? Jerry has long since contacted me and says it
sounds absolutely wicked!.....cool!
