marshall 50w model 1987

from some of my older posts, you may think that i don't have any nice looking amps. this just isn't true! if i find i love the sound of an amp it gets a new home. here is an example of the aesthetic improvement process on my 50w marshall build.

stage 1 - prototype

a while back I did an independent study on audio frequency vacuum tube amplifiers as an elective for my electrical engineering course work. i had visited a site on the net with a lot of tube amp samples and found that the one sample I dug the most was an old marshall 50W, so I started getting ready to build it.



above is the first incarnation of the amp. you'll first notice that it's pretty ugly. Then you'll notice that its got a few extra knobs. The amp had some experimental mods to do some testing for my final report. Some of these mods included:

- variable negative feedback (also switched)
- master volume
- pentode/triode Switch
- cathode/fixed bias Switch
- bias test points

some of the mods proved mostly useless. the variable negative feedback (250K pot) sounded best at the same values that happened to be very close to the stock marshall feedback resistor values. i couldn't tell a difference between the sound or 'feel' of cathode vs fixed bias so that mod proved pretty useless in the amp.

the amp had the sound that i was looking for. finally i could get that sweet marshall crunch. i liked the amp so much i sold my marshall jcm2000 dsl and used the money to expand my guitar collection.

stage 2 - new home

after building a few more amps and still having this hideous naked aluminum amp sitting around, i decided it was time to give it a good home. mojo music supply was having a killer sale on their jtm45 chassis and faceplates at the same time that weber was having a sale on their cabinets- perfect timing.



as you can see, it's looking a lot better with the new chassis, faceplates, pilot lamp and gold knobs. you'll also notice that the master volume was moved to the normal channel's low input. it took a little creativity to mount the vertical Power transformer on the chassis but a visit to the hardware store and $1 later, my problem was solved.



a look at the back shows that some mods were worth keeping. the pentode/triode switch worked wonders for playing in an apartment (along with the master volume) and the bias test points were great for testing my plate dissipation-especially when the wall voltage could vary up to 6 volts from day to day in the town i lived in at the time.

the bias points measure across a 1 ohm resistor between the plate and the output transformer on one side. they accept the leads of a multimeter and one of the added features is that you can pull out one lead, touch it to the chassis and you get plate voltage. i also added an external bias resistor so i would only have to remove the back panel to check or adjust the bias.



here is the amp in its home, a weber 6m45 small box head cabinet. You'll also notice the speaker cabinet below it - a 2x12 avatar cabinet with 1 vintage 30 and one green back. i had it wired up in stereo and then added a 'link' switch to the back panel that would run the speakers in parallel (which I did most of the time.)



here you can see the wiring inside. i had mistakenly assumed a 1/2" spacing for the turrets when i did the original layout. i had to cut about 1/2" off of the board for it to fit the length of the jtm45 chassis! you'll also notice that the wiring looks pretty messy. i was able to do a full volume test of the amp in the university's anechoic chamber (non-reflecting walls) and found that the amp had some parasitic oscillation above 9 on the Master Volume. i plugged in the bias meter during the test and found that the amp was putting out 68 watts at full bore! i later added shielded wiring to the master volume which took care of the oscillation.

stage 3 - final touches

in the meantime, i decided that i hated the white piping on the amp. i ordered some gold piping and after an hour or two of work i had it looking the way i had imaged it in the first place.



you may notice something different where the master volume is located. i did a search to find a knob that looked the most like an input jack. i think i found a pretty good one, don't you?

you'll also notice that i had traded the avatar 2x12 for a genuine Marshall slant 2x12. i swapped the G12T75s in the marshall with the greenback and vintage 30 from the avatar. this time they're permanently wired for 8ohm parallel operation.

after graduating college and settling down with a real job, i started looking for more amp projects to take on. i kept thinking about that nasty turret board in my 50W and decided it was time for it to go. this time i took the time to do it right and use some good components. here is a comparison of the two turrets boards.



after a few evenings of soldering, i finally got it done. here is what it looked like before the master volume was installed:





the wiring is much cleaner this time around. it would have been even more clean if the many incarnations of this amp had given me longer leads to worth with on the power transformer.

it took a little while ( 3 years ) but i've finally got the amp looking right on the inside and outside. maybe someday down the road I'll get the resources to drop in kt66's, sozo caps, and some good iron.

i also have to add that my 50 watter sounds much better now that i've got the guitar that was meant to be plugged into it.