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Following
on from last time, we are getting down to the amplifier’s design and
construction.
A few changes have been made since the prelude last time.
I
incorrectly stated that the Fender amp and also this amp had pre gain
volume
controls.
This is not strictly true as there IS a gain stage before this, based
on ˝ a 12AX7.
It basically makes up for the losses involved in the tone control
network, with a little extra to drive the post gain stage.
I've decided to name
this volume control a “drive” control.
I’ve also added a midrange control to
the bass channel as well.
My original intention to build the beast on 2 U
shaped chassis is still valid but I discovered an alternative at Dick
Smith’s.
Their “Economy Rack Mount Case” is just big enough to fit everything in
and,
slightly cheaper.
The
rack mount case is best described as a 3 ˝ Unit rack mountable case.
It is a
smart way to house the amplifier and looks good too.
It has a black anodized
front and rear panel and an internal aluminium sub chassis that all of
the
major parts are fitted to.
It has a well ventilated top and bottom made out of
powder coated aluminium. If it is desired to see the valves merrily
glowing
away then the open chassis is the only option there. I would extend the
sides
upward and place some sort of screen over the top of the chassis to
protect
those precious bottles from things falling on them!
The
preamps:
The
circuits of the preamps are fairly similar to the original Fender
design.
I
have decided to make everything of modular construction.
Printed Circuit boards
house the majority of the circuitry.
The Bass and Guitar channels are so
similar that the one PCB design caters for both.
It is only the tone networks
that have minor differences in the components fitted to them.
This flexibility
allows the constructor the choice of having 2 Guitar channels or Bass
and
Guitar, or even 2 Bass channels as desired.
The 12AX7 valve used in each channel
has all of the components fitted on circuit boards.
The valve socket is mounted
in the chassis in the usual way and the circuit board mounts underneath
the
chassis, supported by the valve socket mounting screws.
This also makes an
earth connection to the chassis.
As
stated in the prelude, footswitch operation is incorporated into the
preamp
channels.
On the Guitar channel, it is a "Brightness" switch and becomes a "Deep"
or" Bass Boost" switch for the Bass channel.
I tried lots of different circuits
for the tone controls.
I was hoping to use active tone controls, but found the
need for cathode followers around this network.
It could be made to work
reasonably well but the addition of another valve per channel was a
little
excessive !
I dug around for lots of different amps of this era and examined the
tone networks used.
They all used a similar design to this one … Minor changes in
component values were the only differences !
So the simple passive tone control
network was used !
Actually, it is quite acceptable and seems to do the job.
When
I was fiddling around with the cathode followers, the thought occurred
to me
that the unit could be built up to this stage and could
be used to drive an external power amplifier.
This is another option open to the potential builder.
A 2 channel valve
preamp
or go the extra expense of a valve power amp …. Any feedback is welcome
!
As a
self contained preamp, the power supply requirement is quite modest.
A
buffer
stage to drive any sort of power amp would use ˝ a 12AU7A as a cathode
follower, the other section of the 12AU7A could be used to drive a pair
of
headphones.
Why
a 12AU7A ?
Well, there are 2 good reasons for picking this valve.
It is designed
to operate at a higher plate current than the 12AX7 which makes it
ideal for
cathode follower duty.
And it is designed to withstand a
higher cathode to heater voltage.
This is also very important for cathode
followers.
Incidentally, the phase splitter stage can also benefit from this.
I
intend using a 12AU7A in this stage instead of the 12AT7 originally
mentioned.
In
keeping with the modular concept, I am hoping to produce designs for
valve
tremelo and reverb units as add ons.
The only problem I see is the cost of the
reverb tank.
They are around the $70 mark.
These spring based units give quite
a pleasant reverb to the overall sound and require probably 1 extra
valve to
drive them.
I will use an old unit salvaged from an organ in my prototype.
The
tremelo unit is easy.
I will use the same design as I used on the amplifier
many years ago …. It worked well then, it should do the same now !
Till then......Bye
© Rick's Workbench 2009
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