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Question to ponder on :
Why do we not have floating "space" platforms (using hydrogen or helium) at the top of our atmosphere from where we can launch rockets into space.
Much take-off pollution would be avoided as the rockets would have a much shorter "burn" to get into their orbits.
The rockets, payload and personnel could also be taken up to the platforms by airships.



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Articles from "Rick's Workbench"

Tale of interest No.14
The valve guitar amplifier : part 1

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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