The 45/100



      The 45/100 circuit takes a special place in the history of Marshall amplifiers as the first real "100 watt" amp, made for power hungry players in the mid 1960's who wanted the already ear-blisteringly loud Marshall JTM-45 to be even louder. Though only produced for a very short time, the 45/100 paved the way for the Marshall circuit to evolve into the massively powerful Super Lead amplifiers. I've always loved the non-master-volume Marshall sounds, and because the 45/100 was a product of an era when Marshall didn't produce any bad sounding amps, combined with rave reviews that this circuit receives, I decided that I would have a go of it. The chance of finding one to try in person was nil. Throwing so much time and effort into building one without knowing what the results might be was a bit of a guess, but so many classic sounds had been recorded by famous players using this 45/100 circuit. Could a bit of the magic rub off on my own shoddy playing? Hmm.

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     One of the first big things with the 45/100 is an aluminum chassis. Although steel chassis are more popular these days, aluminum is in fact a better conductor of electricity, and imparts a different sound to the finished product, because it does not have the same impact on the electromagnetic fields of transformers (and the honkers on this one are huge) as does a steel chassis. The original 45/100 amplifiers also used aluminum chassis, and so I decided I'd get an aluminum chassis made up.

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     After some looking, I found a suitable 45/100 chassis. Along with the chassis came some circuit board material (which was later discarded in favour of my finding of the unobtanium paper-phenolic laminate known as "paxolin"), mounting hardware for the circuit board, as well as a very nice set of panels for the front control panel and rear power and output panel. The front panel was copied directly from an actual 1966 45/100 amplifier, and reproduced using the equivalent materials. The back panel was not quite as accurate, so I'd need to fabricate some decals, which you can see towards the end of this page.

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     Another important element to consider in the 45/100 is the transformers. The quality of the iron in an amp is a huge factor in the final sonic outcome. The power transformers on the original amps were huge, heavy, and capable of delivering very high plate voltages. The actual plate voltages varied from amp to amp, but from the mixed reports available it seems that actual voltages in one of these amps could be anywhere from a low of 525 all the way up to 625, with an average somewhere in the middle around 560. Considering that most modern amps only operate into the 450 volt range, one of the key concerns in choosing a transformer became finding one that wouldn't fry modern power tubes.

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     Such high plate voltages in the 500 and 600 volt range were seemingly tolerated by the tubes of the 1960s. But at nearly $1000 for a set of the famous Genalex/GEC KT-66's, modern tube production KT-66's became the standard by which my selection of a power transformer would be judged. Subjecting them to the same high plate voltages that the old European production tubes could sustain would be dangerous for the longevity of the tubes, so I selected a power transformer that would deliver around 490V, which seemed to be within the acceptable operating range for the modern KT-66, but still high enough to provide the characteristics of the classic 45-100 sound.

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     Some of the original JTM 45/100s also had dual output transformers, each rated at 50 watts. In this configuration, Each transformer would drive one output jack. When Marshall switched to a single 100 watt output transformer, you could simply plug in one cabinet, and it would see the full 100 watts of power. If you plugged in a second cabinet in the newer, single-OT configuration, then the 100 watts would be divided between the cabs. Although the dual output transformer models certainly are unique and easily recognizable, they did not strike me as much more than a peculiar novelty from the standpoint of having a good working amp. Moreover, many more 45/100's were made with single output transformers than dual, and so I resolved to abandon all consideration of the dual-OT setup in favour of a single OT (side note: It is entirely possible that the people who designed these early amps were drunk out of their gourds). To top it all off, I decided to grab a "1966-era" head cabinet for this project, with the "Vox-style" brass vents on top.

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      A lot of other parts were required. A major hunt ensued for high quality switches, indicators, jacks, dial knobs, potentiometers, resistors, capacitors, diodes, tube sockets, mounting hardware, bolts, washers, nuts, and wire, and so forth. When would my bundle of joy arrive? I waited with great suspense! Finally the package crossed onto the Canadian side of the border, and was picked up by Canada Post. The home stretch!

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     The parts arrived! The heavier and larger components got bolted together. I had decided to use an IEC socket for this amp, because I don't much like strain reliefs or having people trip over power cords. When a cord goes right into a device using a strain relief, a tug or a trip can bring the entire thing down to the floor, typically with disastrous results. The IEC socket also all but ensures that a grounded power source is being used, unless you lend it to that friend that always cuts off the little plastic tab on the two prong extension cords so he can jam a three prong in it and leave the ground disconnected. These 45/100's are HEAVY -- maybe even 45 lbs -- so the plug will pull out of the socket before the amp goes overboard

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     To install the IEC socket I needed to cut a bigger hole in the aluminum and rear panel. Because Aluminum is so soft, this was easily accomplished with a drill and a few small files. A few test fits, a few minor adjustments with the files, and it fit in perfectly. The original JTM 45/100 amps used Bulgin connectors, but because Bulgin cords and connectors are difficult to find and expensive when available these days, the IEC seemed to be the best choice.

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     I had to bolt the transformers on, and see how this thing looked. I took it out into the sunshine for a photo-op. The aluminum chassis and panels don't weigh much, but after you get that power transformer on there with it's massive 3" stack of laminations, it really wants to stay put! Looking great.

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     Inspired by Joe Popp, another amp builder, I decided to braid the PT and OT wires. Imitation is the most sincere form of flattery, some say. Well, I'm not trying to flatter Joe, as his work speaks for itself -- just impeccable. But braided wires and amps just go together really well, and since I saw his job of it on his JCM800 "Rachelle" build, I simply had to do it myself.

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Check out his website, at http://www.joepopp.net/rachelleamps.htm



     Anybody can twist two wires together in a drill. But braiding three or four wires in a tight, consistent pattern is a bit more complicated. It might be simple for a 4th grade Schoolgirl, but for a guy in his late 20's, braiding is difficult enough to seem like the apex of human achievement. I started practicing with three wire braids. Success didn't come at first, but after I had tried it a few times, I put my finished 3 braided wire length in my pocket, and walked around with it all day, just beaming. I had to take it out at various points in the day, just to reassure myself of my awesomeness. That night I decided to tackle the four braid wire. Again, success didn't come at first, but with a bit of work, and a few practice lengths with different colors of 22 gauge PVC coated amp wire, I once again became "master of the braid." Martha Washington would be proud.

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     The "Red Swirl" perf boards that came with the chassis and panels were very well made, with excellent, even, on-center hole spacing, but I decided I would try to shoot for some boards that looked like the old paxolin boards in some of the vintage Marshalls. This brown board material proved to be the single most difficult part in the amp to find, and delayed my construction of the amp for over three months. Although there are a world of good options available for the construction of circuit boards, anything resembling the original paxolin material is simply seldom or never seen here in North America. You'd think that finding some thin, crappy plastic in a somewhat unappealing brown colour would be really easy, but that is actually not at all the case. Eventually I was able to secure some boards from a kind and gracious individual in the United Kingdom.

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Above photo: an example of an original Marshall, with the paxolin board and mustard capacitors



     In the three months that passed, I hunted down some optional components for my 45/100. I sourced and bought some original NOS (new-old-stock, which is to say, unused, as if fresh "off the shelf") "mustard" caps. I also tracked down some NOS mica capacitors, which are dead ringers for the ones seen in photos of the original 45/100 amps. These capacitors, which are very hard to find, were graciously sold to me by two very kind amp builders, one from the United States, and the other from England. England is, to be sure, the hotbed of vintage Marshall supplies. England can also proudly say that they are the home to the BBC.

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     After the boards arrived (in fashion, on my birthday), everything was in place. The building was to begin. I had already installed the tube sockets, rear impedance and voltage selector switches, IEC socket, fuse holders, and transformers. The next step was to wire the heaters, and to start dealing with all of the connections to the power tubes and rear panel switches.

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     I decided to use 20 gauge PVC wire for the heaters, instead of the 22 gauge I used through most of the rest of the amp, just to be sure that the wire could handle enough current draw from the big KT-66s. This probably wasn't necessary, but it's my amp, my DIY time, and I'd do it my way (not only that, but I suspect that the people that Marshall paid to rip off this circuit from the Fender circuit were total rookies, and hadn't the faintest idea of what was going on -- of course, I can't prove that). I also like tightly twisted heater filament wires, so I used clear shrink tubing and a drill to keep everything twisted neatly as I went along.

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Next would be connecting the output transformer to the power tubes, adding 1 ohm resistors to ground from pins 1 and 8 of each power tube for biasing purposes, adding yellow wires to each unused pin 6 of the power tube sockets for the screen grid resistors,installing 470 ohm screen grid resistors, and adding 1.5k resistors to pin 5 of each power tube as control grid resistors.

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After these power tube essentials were taken care of, the pre-braided OT wires were connected to the impedance selector switch, and the pre-braided PT wires were connected to the voltage selector switch. With these wires in place, I connected the power/standby switches and the power transformer to the IEC socket. Wires were sent to the indicator lamp, as well as twisted and heatshrinked wires sent off from the power section, ready to be connected to the soon-to-be finished boards.

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     I proudly use PEC potentiometers, as they are the only components for my 45/100 I could find made in Canada. I put Big Sugar's rendition of "Oh Canada" on repeat, and hummed along as I installed these beautiful potentiometers.

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     I ordered large double pole switches, as they have a bigger, more solid feel than the smaller siwtches. However, this meant I had to make a small incision, and remove four of the perfed holes from the cap board.

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     The modern production KT-66 tubes arrived. The shipment cost exactly $6.66. Could this amp possibly be made of pure 'rock'? Although the amplifier is used mostly for Bach transcriptions and Jazz, it does a convincing AC/DC, Free, Cream, and Hendrix.

Tubes Arrive - KT-66s


     I am quite pleased with the "Mustard" capacitors, the Lemco and Radio Spares mica capacitors, and the carbon composition resistors. They make a very attractive looking board. The amp sounds fantastic, too.

Board preparation:



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Amp, far right:



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Amp, middle right:



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Amp, middle:



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Amp, middle left:



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Amp, far left:



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Board, far right:



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Board, middle right:



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Board, middle:



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Board, middle left:



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Board, far left:



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Cap board, right:



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Cap board, left:



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Input Jacks:



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Mains Socket and Fuses:



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Impedance Switch:



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Impedance Switch 2:



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Output Jacks:



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Voltage Switch:



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Maple Leaf Punch:



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Rear View:



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Rear Decals:



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Head Cab on Half-Stack:



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     I've got the amp fired up and playing. It's a beauty. I hope to add some sound clips in the near future.




Feel free to E-mail me.

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