"Entertainment
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February/March 2009 (10,009PH)
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Articles from
"Rick's Workbench"
Tale
of interest No.13
Prelude to a valve guitar amplifier :
| Having
nothing to fix lately, I thought I
would build something
for a change, at least then I could only blame myself for the dry
joints
and poor design! No really.. It's not THAT bad.... I do like constructing electronic devices. The thrill of creating something is always very satisfying. There are challenges involved in parts procurement, designing the project, and of course, the actual assembly and testing. Anyone who has a practical hobby will know the same thrill! Why a valve amplifier? Well... Why not! I have always had a soft spot for valves... Many audiophiles and musicians prefer them. They sound “different” to solid state amplifiers. Technically speaking, the distortion products are higher and of a different nature to transistor amplifiers. For guitar amplifiers in particular, distortion is often deliberately introduced to help produce the desired sound. Famous brand name valve guitar amplifiers are still available, though their price tags are usually a horrible surprise! I've had a hankering to build a valve amp for years.... The last one was in (I think) the early 1990's or late 1980's. This new one will be more elaborate and based on models from famous brands of the early 1960's. As a pleasant surprise, I found Altronics and Jaycar stock the valves needed for such an amplifier. They're not cheap, but at least they can be had! (A look on the internet will reveal stocks of valves at slightly cheaper prices.) I want to use new and “off the shelf”components for the complete design, based on what I could source from the aforementioned suppliers and good ol' Dick Smith Electronics. (There is a sad saga of Dick Smith retail stores NOT STOCKING electronic bits and pieces for much longer!) I was figuring on a design capable of 50Watts RMS as a starting point and, as standard parts were to be used, the possibility of ultimately producing the complete amplifier as a kit... Assuming anyone else wanted such a kit! (feedback please) The design is an all valve beast, based on 12AX7 valves for the pre-amp a 12AT7 for the phase splitter, and a pair of either 6L6 beam power tetrodes or 6CA7 power pentodes in a push-pull output stage. It looked promising when I found all the valves I needed in my collection of valves. I settled on 6CA7's for the output. These are a potent bottle... A pair of these with an 800V Anode supply can deliver 100W of audio power! I would be aiming for half that, using about a 400V supply. As I like to play Bass Guitar as well as Lead / Rhythm Guitar, I've based the design on a Fender Bassman amp of the CBS era. This had a Bass channel offering pre-gain volume, bass and treble controls, and a bass boost button for which I have decided to add a relay to operate, allowing a foot switch option. The Guitar channel offers a pre-gain volume, bass, middle and treble controls. It also has a treble boost button. Once again a relay is incorporated for foot switch operation. The negative feedback for the power amp is also able to be switched in or out to give a 'clean' or 'dirty' sound. There is also a master volume control. The power supply is on a separate chassis and has power on / off and standby switches along with a dual colour LED indicator. - When red, the power is on but in standby (No H.T. present). - When green, this indicates the amp is ready to be used. The other indicators are also LED's. The only other semi-conductors used in the entire amplifier are some rectifier diodes. (Yes purists, even Fender and Marshall used them!). The back contains an IEC power socket for the mains, a fuse for the H.T. Supply and an Octal socket that connects the power to the amplifier chassis. The amplifier chassis' rear contains 3 jack sockets for foot switches, another for the speaker output, and a measuring socket for bias voltage and cathode current for the output valves. The bias pre set pots are also near this socket. With minor changes to the bias and wiring to the output valve sockets, 6L6 valves could quite easily be used instead. There seems to be more of these available on the internet. Bought new, the price is about the same for 6L6's as it is for 6CA7 valves. There may be a difference in the sound produced from one type to the other. For Fender fans, 6L6's were the popular choice, whilst Marshall amps preferred 6CA7 bottles (also known as EL34). Next issue I'll get down to the nitty-gritty of the amplifier. How problems were overcome, and hopefully, the final results. Till then......Bye © Rick's Workbench 2009 |
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