marshall 20w model 2061

when marshall released their 20w handwired lead and bass amp (model 2061x) i really wanted to try one out. unfortunately, none of the guitar shops in my area had one in stock. i had heard them described as a mini-plexi and had found a few sound clips on the internet that sounded pretty good, but i had to build one myself to give it a fair try.

i was hoping to make a portable 1x12 combo amp that i could lug around for practices without bringing my head and 2x12 cabinet and still get a similar sound.

i began doing research to discover the details of the 20w circuit and layout, collecting pictures and schematics. i started this thread over at the plexi palace forum for anyone who was looking for the same info or had something to add. (If you're interested in building a 20w amp check it out, i posted some layouts for classic 20w circuits.)

this amp would start out much like other experimental amps: in a cakepan chassis. i had a spare power transformer i picked up from some old tube-driven hewlitt-packard scientific instrument for $1 at a surplus sale and a 15w hammond transformer left over from my 18w build. the remainder of the parts cost me about $60.

i followed the classic 2ow layout as closely as possible. i used grade xx garolite (paper phenolic) in this build. it was much easier to machine than the g-10/fr-4 boards i had used in the past.

i made one modification to the circuit, adding a matchless dc-30 style master volume.
when i powered on the amp, i found that i really had to turn up the master volume to get a good sound out of it. i later discovered that this type of master volume is best implemented on an amp with matching plate load resistors in the phase inverter, like the vox ac30 and ac15.

you'll notice some rather crude looking construction methods in the above picture. again, the experiment here was to build the classic circuit for as little as possible. the circuit is electrically fine, but it does leave something to be desired in the area of looks.

i really loved the clean tones that i was getting from the lead channel. the bass channel didn't seem capable of much more than clean tones and did not have as much volume as the clean channel. it was nice to be able to blend in the bass channel to change the overall tone with the channels linked.

when the channels were linked and the volumes were cranked the amp started to break up. the distortion seemed rather brash than smooth and i'd always thought there must have been something wrong with amp. i eventually found the schematic for the scientific instrument which seemed to indicate that the power transformer may have been underpowered. measuring the transformer showed that it was the exact size of a vintage fender champ transformer.

i decided the give the amp another shot with a proper set of transformers, this time with a nice 18w set from heyboer. i'd see if an improvement was made and if not i had other plans for the circuit- afterall, i did love the clean channel. i got a proper chassis and faceplate set from mojotone also.

unfortunately, the new transformer set didn't yield much improvement. i had hoped for a sound more similar to my marshall 50w. comparing the two circuits, i determined that i liked the sound of the extra gain stage in the 50w for distortion. the marshall 18w and 20 w circuits only have one gain stage before being sent to the phase inverter.

i decided to give the circuit another try, making the extra gain stage switchable. the amp this circuit would become is outlined in my marshall 20 watt channel-switching amp post

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