marshall 18W model 1974

the reviews i had read of the marshall 1974x had me dying to try one. then, of course, there is always the the net's 18W hype. unfortunately, there are no guitar shops near me that stock this sort of thing regularly. the only thing left to do was to build my own!!

the entire point of this build was to keep costs as low as possible. it is not an exact replica, but it should be at least 90% accurate.

chassis

budget cut number one. why pay $50-$100 for a metal box? but what else is there? I'm not exactly sure where the idea came from but the thought occurred to me- a cakepan! hence this amp was originally called the "cakepan amp." a used cakepan can cost you nothing or maybe $1 at a yardsale or thrift shop, a new one from $3-$7, not to shabby. so is everything gonna fit? let's have a look....



it will fit! one of the nice things about the cakepan is its easy to drill through, sometimes too easy.. the drill bits can grab and deform the metal, so be careful. now that i know everything is going to fit i laid it all out on graph paper, actually engineering paper. i used my paper template to mark the holes and drilled the cakepan. to my surprise the knobs were knocking into each other! lesson learned: engineering paper is not spaced by 1/4" per grid, so don't use it to do layouts. time for a new cakepan....

i should mention that this chassis is set up 'combo style' with the tubes mounted on the side ( or back) of the chassis, opposite the controls.

power supply

budget cut number two. the power transformer can be one of the most expensive parts of an amp. why do they have to be so expensive? what else could i use? i decided i would use two separate transformers to stand in place of one. one transformer will supply the 6.3 volts for the heater and the other will supply the high voltage supply for the plates of the tubes. finding a high voltage transformer is a bit difficult. I could use a step-up transformer to supply 240 volts to the rectifier, but they were a little pricey. so how do i get a sufficient high voltage supply for the plates?

looking at my tube power supply diagrams, i found one labeled 'voltage doubler.' now i can use a simple 120V isolation transformer ( less expensive ) to provide the high voltage supply. of course, now the tube rectifier is out (a voltage double is possible with a tube recitifier.) after rectification i should see about 320 volts, surprisingly close to the voltages coming off of the tube rectifier on the stock circuit.
here's the voltage doubler circuit that i used:



later, i realized that i could have accomplished the same thing with a bridge rectifier. it also provides approximately a rectified dc voltage of 2.8 times the ac voltage applied to it - and you don't need capacitors.

turret board

the turret board layout had to be modified a bit from the classic 1974 layout, or it wouldn't have fit in the cakepan! i had to mount the voltage doubler, filter caps and power resistors on a separate board. here's what it looked like:




initial tests

when i first fired up the amp, i heard a wonderful buzzing noise. this was followed by smoke coming from the isolation transformer. I quickly turned the amp off and rechecked the wiring to the power supply. I had the center tap to the isolation transformers secondary grounded. I disconnected it and it powered up fine.

the amp was also plagued by parasitic oscillation. many attempts were made to reduce this and finally using shielded wire on the inputs fixed it permanently!

i mounted the cakepan in an old stereo speaker cabinet, using a 12" speaker i bought for $1 at a surplus sale. here is a pic of the speaker:




and now its ugly cabinet:



eventually, i decided to convert the chassis to a head style, so that it could be mounted more securely in a cabinet. I also rearranged the layout of the front panel and added a master volume to make it more family friendly. this incarnation of the 1974 became known as the cakepan mkii.


cakepan mkii panel layout

in order to make room for the master volume, the trem and normal channel's single jacks were stacked on top of each other. a nicer looking indicator was also used. by the way, the knobs are all from an old scientific instrument i purchased for $1 at a surplus sale.





cakepan mkii wiring layout

this time around i added a little more space between the power supply board and the turret board. i also gave myself a little more room around the tube sockets and moved the turret board closer to the pots. notice the shielded wiring from the inputs. a pentode/triode switch is also mounted on the rear panel.




mkii chassis from above

the transformers are indeed mounted in an unusual arrangement! since I could mount all the laminations perpendicular to each other, i mounted them as close to perpendicular as possible.




the tone

the amp always sounded a little harsh and thin. in diagnosing this problem i read the B+ off the voltage doubler and found that it was dropping by a very large amount at full volume. i later replaced the isolation power transformer which fixed this problem.

the normal channel didn't sound that great to me, especially at maximum volume. i eventually made an attempt to put the normal channel to marshall 20 watt specs, which improved the sound in my opinion.

the trem channel sounded beautiful. it had a little more bite than the normal channel and a little more brightness to it. the tremelo circuit itself also worked quite well.

overall, the amp was a little too brash for my ears. i had wondered if it was the solidstate rectifier or something else from my build. one day i finally saw an official marshall 1974X at guitar center. it sounded very familiar as i played it. i came to the same conclusion that the 1974 was not the holy grail.... at least not to me. this amp has since been disassembled and its parts are in use in many other amps.

sound clips

the following sound clips were recorded with the damaged power transformer at low master volume settings. i did not record any clips after this problem was fixed. the setup for these files was a Gibson Les Paul Faded DC (P-90's) into the cakepan amp and then into a weber alnico blue dog.

normal channel max bridge pickup
normal channel max neck pickup
normal channel rolled off
trem channel
normal channel maxed (through oaktron speaker)

conclusion

i must first say that using surplus parts had an ill effect on this amp. the values of the pots drifted as the amp was operating. the master volume would work well, until the amp warmed up, then the amp's output would diminish into almost nothing. also the normal channel volume pot dropped from 500K to 250K somehow and needed to be replaced.

( i later discovered that the type of master volume i used was part of this amps problem. the 'crossphase' master volume, as I like to call it, mixes more of the out of phase signals from the phase inverter together as you turn the volume down. This type was used by matchless in the dc30. it did not work well in this amplifier because the phase inverter was not balanced! the phase inverter tube sections have different plate load resistors values and therefore different outputs. as the amp warmed up, the signals would increase and become more unbalanced reducing the volume.)

the use of the cakepan as a chassis was a good way to prototype the amp, but it did have its ill effects also. the thin metal did not offer much protection from mechanical vibration. it also was very easy to bend the metal when drilling when the bits would catch. the chassis also got very hot -especially the combo (mki) style chassis.

overall, the build was a success. i learned a great deal and was able to build a classic guitar amp for less than $140, much better than the marshall reissues price tag of over $2,000!!