17.10.09

marshall 20w channel-switching amp


this amp came about after building the marshall 20w lead and bass (model 2061) and not being completely satisfied with the results. while i loved the clean sound from the lead channel on that amp, i was hoping for more of a marshall 50w sound out of the distortion channel. this is just not possible because the 20w lacks the extra gain stage of the 50w. i decided to create an amp that combined the sounds of the 20w's lead channel clean and the 50w's distortion channel.


i had already purchased an 18watt chassis with 20w style faceplate from mojotone. with this chassis i can use the 18watt transformer set, also. the 18w transformer has just the right voltages for the 20w. the 18W chassis is nice because it already has the extra preamp tube slot that I needed and will accommodate the 18W transformer set and 18W cabinets without modification.

the first circuit that i built was a stock 2061 circuit. for this implementation, i used the same circuit board that would be used for the channel switching amp, but only wired up the components that made it a stock 2061. playing it with proper transformers didn't produce much of a different sound that my initial 20w build that i documented here - i definitely wanted that extra gain stage in there.

my next attempt was to implement a stock 20 w amp with a switchable gain stage in series in the circuit. this didn't work well because of the cascaded volume controls. it was impossible to get a good clean sound and a get distortion sound that was at the same volume. also, any change in the first gain stage was sent downstream to the second gain stage. looks like i'll need some more modifications....

at this point i determined to redesign the amp entirely to get just what i wanted- a 20w lead channel for clean and switchable 50w channel for distortion, with the switching handled by a relay. since i always run my 50w controls at 10, i wouldn't need the tonestack and would use separate volume controls for both the normal and high treble channel to change to sound of the distortion channel. this way, the amp would essentially be switchable between a 50w circuit and 20w circuit, with the exception of the 20w bass channel and the 50w tone controls.

the power supply was based on the 20w amp to get the voltages right, while the phase inverter was derived from the 50w phase inverter. the amp would also be fixed biased instead of cathode biased because today's tubes can be pretty hard to tame in cathode biased amps to avoid red-plating. i used the bias circuit found at 18watt.com for the fixed bias. there would also be a separate power supply for the relay circuit fed from the 18w transformer tap for the tube rectifier.

the implementation of the channel switching circuit wasn't without its problems. there was certainly an evolution that occurred. attenuating the signal in the extra gain stages to get the distortion to sound right was one trick. i had also used the same gain stage for 50w normal and 20w lead channels, since the volume controls are after these channels in the circuit and their first gain stages are identical. this cause some signal bleeding between the 20w lead and 50w normal channels. i used the relay to ground out the signal from the 50w normal pot when the 20w clean channel was engaged.

here's a closer look at the relay power supply:



on the 50w channel, i implemented a pre-phase-inverter master volume. this helped to even out the volume with the clean channel. i noticed that when i had this volume low, i wasn't getting the distortion i wanted out of the amp. i soon realized that even the extra gain of the phase inverter had an astounding effect on the amp, so i used another tube stage to simulate the gain of the phase inverter before this volume control. the results were wonderful! i had learned the difference between a "master volume" plexi and a "non-master volume" plexi.

i also had a problem with the 50w normal and high treble channels acting like they were wired in parallel. it turned out that i had mistakenly installed 470 ohm mixer resistors instead on 470k mixing resistors. installing the proper resistors fixed this issue.

once all the channels were isolated the amp was sounding great - good enough to have my mother-in-law comment on how nice the clean channel sounded and have my friends and other guitar freaks to comment on the sweetness of the distortion. my buddy who had just bought a new amp liked it better than his own.

here is a peek at the inside. its a bit of a mess thanks to all the modifications. you can see how some modifications were done right on the tube sockets and were potted in hot glue to eliminate vibration or shorting of signals. the relay power supply is to the right of the chassis. i tried to do a nice job with the power transformer wiring on this amp. mojo seemed to have flipped the main power and standby switches on the 20w faceplate, so i had to rearrange my nice wiring into a mess.



i had also implemented a post-phase-inverter master volume (the rich mod) to keep the volume under control when practicing at home. It was quite a stretch to implement all these features into the minimal 20w front panel, but it was certainly done!

here is an explanation of how the controls are set up:



in the end, the amp was mounted in an 18w style 1x12 cabinet loaded with a celestion well broken-in g12h30 speaker (i pealed the sticker off of an avatar hellatone 30, really).



i came to realize that what i now had was very close to my first tube amp - the marshall jcm2000 dsl401. A fixed-biased el84 two channel switching amp, except this one was much better.

now i finally have an amp that gets the classic marshall sound - well, actually, two of them with the ability to switch between them easily - the stock 20w clean channel and a custom 50w 1987 normal and high treble (bridged) channel.

here are some schematics of the build:

1 comment:

Ray said...

I just realized that your background is engineering paper. Totally geeky.