gibson les paul junior

while waiting for my amp fund to replenish after completing several builds in a relatively short amount of time, i decided to build an amp from spare parts.

i had a 5y3, 6v6, 6sj7, a champ power transformer, a spare 6.3v transformer, lots of octal sockets and nearly all the spare parts to complete another build. i decided on the original gibson les paul junior circuit (similar to fender 5c1.)

most of the parts from this build came from a 1$ surplus store purchase. i recently removed the power transformer from this purchase from my marshall 2061 prototype after decided to redesign it. in the meantime i found out that this particular transformer was more suited for a champ type build and was actually the exact dimensions of a champ power transformer! (perhaps this could have accounted for some of the reasons i didn't like the stock 2061 as much as i'd hoped.)

after purchasing a tweed champ cabinet from mojotone for a previous build, i started looking for a different chassis, since the 9x5x2 didn't fit the control panel cutout without some cosmetic filler. my previous builds had been with hammond chassis, but as i looked at the bud website, i noticed that they had a 9.5x5x2.5 chassis that was about the same cost, but would cover the width of the tweed chassis panel cutout. (part no. ac-401)

one of the first challenges was to cut a square hole to mount the power transformer. i measured out the mounting holes, then measured the space that i would need cutout to accept the transformer. i had previously mounted this transformer the way that it was removed from it original home- on standoffs above the chassis.

to cut the hole for the transformer, i simply drilled a hole large enough to accept my nibbling tool and nibbled away at the edges. i had a nice hole in no time. as you can see the transformer fit great.

since i didn't have any turret board material handy, i decided to use some leftover terminal strips from my hotbox built and make this amp a true point-to-point amp. it took me a little time to work up a layout for the amp, but soon i had something that would work just fine and take up little space.

after the metal work on the chassis was finished, i started mounting the components. unfotunately, while doing the chassis metal work i had drilled the input jack holes at the wrong centers. to make things worse, the holes i drilled in the proper place had the wrong centers in the other axis. if only i had taken a little more time. i wasn't too worried with this build however as it was mostly completed with parts that were sitting in box anyway.

one of the most interesting aspects of this amp build is that it doesn't have an output transformer! well, it does. technically, i have two power transformers. one to provides that amp with power and the other to act as an output transformer. now this might seem like a rediculous idea, but i wanted to prove the point that a transformer is simply a device with a power rating and turns ratio. i plugged some commercially available transformers into my tube amp calc page and found that at 120 volts a 6.3v transformer is about perfect for a champ type circuit at 5k primary impedance with an 8 ohm speaker. I happened to have one on hand, so it got installed in this amp!

after building my second amp with a single pentode gain stage, i can say that i am suprised at how clean these amps are when cranked. the les paul junior is barely starting to distort as the volume pot approaches its maximum value. these just don't have the push themselves to get things going for me. an ef86 has a theorectical gain of 5000, but a two triode stage would have something like 100x100=10,000 for a theorectical gain. I much prefer the sound of the double triode over the pentode.

in all, the build was a success. i used up some spare parts, found a new standard diy champ chassis and used a power transformer as an output transformer. the amp doesn't sound half bad either. the lower notes are a little muddy sounding, but maybe that is just the extended low frequency response of a transformer designed to run a 50 or 60 Hz coupling with an 8" speaker.