Cage 30A
P
re Amp Stages
ffect Loop
he Output Stage
From the guitar input we go straight into a single “Class A” gain stage to
drive both clean and overdrive channels. The clean channel then goes through
another stage of gain, followed by the passive Eq section, then to the channel
switch. The overdrive channel goes through a further four stages, the last stage
being a cathode follower to drive the passive Eq controls then to it’s master volume
and finally the channel switcher.
I went with the side chain design in order to retain the straight through
sound of the amp and mixing the effects in. There is a balance pot on the back of
the amp to blend straight through and effects together. The first valve stage in the
loop is of a cathode follower type to get a good low impedance drive to the send
output jack. The effect returns jack feeds straight into the balance pot and then
into a class A gain stage to drive the phase splitter.
I started off with the idea of an output stage with the power that would
break up at a playable level with a smooth but defined break up. There was only
one choice, double ended class A 30-Watt using EL84 output tubes self-biased
with some negative feedback. A presence control was used to achieve a sweet top
end. To complete the overall picture I used a 5U4-rectifier valve to give supply
sag to compliment, sustain and add harmonic content to the fundamental note.
After 11 prototype output transformer designs and about 6 months of testing and
redesigning, I achieved the results I was looking for.
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