Starting from the right- hand side as you
look at the rear panel....
V1 on Schematic - Input valve (7025 = ECC83 = 12AX7)This is used as two
separate stages of amplification in one glass bottle. One stage provides the
first amplification after your guitar signal arrives at the input jack. NB
the amp's volume control comes after the first 'half' of this valve, not
before, so it's not just a duplicate of your guitar's volume knob. After the
bass, mid,treble and volume controls, the other 'half' of this valve
provides another stage of amplification. Obviously you want all the valves
working well, but the signal passes through this valve twice, so if this
one's a bit noisy and tired, the others won't be able to help no matter how
great they are. If you're minus a valve cover, don't let this valve go
without, as in theory it's the most sensitive to interference.
Note - the 7025 valve(tube) was originally devised as a low-noise 12AX7, but
in reality any reasonable-quality ECC83 or 12AX7 will work OK.
V2 - Reverb Driver and 'lead' distortion (12AT7) This one has 2 jobs. (1) It
amplifies the signal current considerably in order to move the springs
in the reverb unit. It does this all the time, even when you haven't
selected reverb - the 'reverb' knob comes after V3a (below). BUT (2)
irrespective of reverb setting, when you select the 'lead' effect,
some of the (massive) signal from V2 is mixed into the sound via V3b,
resulting in preamp distortion and offering you a kind of valve overdrive
sound. Therefore (depending on V2 manufacturer or batch) the reverb depth
drops when 'lead' is selected. Different 12AT7s can affect this, sometimes
improving it. Maybe a lower-gain valve gives more useful adjustment. But
that will also affect the 'lead' sound. The amp will work without this valve
- but minus reverb and lead (overdrive). You'll never hear this valve's
output direct and clean -only through the reverb springs and/or during 'lead'
effect operation - so an expensive NOS valve isn't needed here.
Note that the lead level knob doesn not change the amount of distortion (like
a gain control). It's a volume control for the leadsound. There is no
control over how much gain, depth, distortion you get. But this will be
affected by your choice of V2; if you really want less aggressive distortion,
ask your valve dealer for a low gain 12AT7 or a 12AU7. For more available
distortion, get a higher gain valve. This will also change your reverb depth.
V3 - Reverb Pickup, final preamp, phase inverter
(6C10) This valve/tube is three triode stages in one glass bottle. The 6C10
is like 1.5 12AX7's . The first triode (V3a) picks up the signal from
the reverb unit and passes it on via the reverb knob. The second triode
(V3b) provides another stage of gain for the main signal path through
the amp. This is the pasrt which is purposely pushed into distortion when
you select 'lead', so swopping this valve will change the way that
effect behaves. (Whether you like this kind of distortion (ie in the pre-amp),
or prefer power amp distortion is up to you, but they are different.)
After V3b comes the master volume knob, the lead level
knob, and the presence knob. NB Line/recording output comes after the output
valves, not from here or anywhere in the preamp chain. V3c is the phase
inverter; it splits the signal so that both the signal and its mirror image
are sent to the pair of power valves, so that one power valve can 'push'
while other is 'pulling' and vice versa.
V1-V3 do not need to be matched to each other in any way, and when you
change them, no internal adjustments are needed. They are three different
types of tube and cannot be changed with each other.
V5 and V6 - Output or Power Stage (6V6GTA) The schematic doesn't have a V4
for some reason - the 4th and 5th valves/tubs are labelled V5 and V6.
These are the power stage, so the are bigger and fatter than the other
valves. They don't have covers, as they're not sensitive to interference,
and they need all the ventilaton they can get. They work as a team to push
and pull current through the output transformer, which in turn moves the
loudspeaker. (At the risk of stating the obvious... the amp needs both of
these valves to work; if one fails, you don't lose half the power - you
lose all of it.) These 2 valves should only be replaced as a matched pair;
even then the amp needs checking, and maybe adjusting (rebiasing).
Finally...the output transformer. The loudspeaker and
the line out/recording socket both come off the output side of the
outputtransformer.
Andrew
|
|