The Big Wattage Marshall Amps
There are times when only a big wattage Marshall will do. Need to drive a 4 X 12" half stack to performance level volumes? Want to hear the tone of those big bottle EL34 , 6550 or KT66 tubes cooking. Need to relive your fantasy of Cream playing "Crossroads" at the Fillmore theater and only a big Marshall head will do. Well, this is what these amps are for.
Based on the Fender Tweed Bassman circuit, Jim Marshall began building his amps in his kitchen back in the 60's. Fender equipment was very expensive to import into England and there were few British amp makers at the time. The English made Watkins Dominator amp was the model for Marshall's short lived 18 watt 1974 amp (See the 18 watt page) and the Fender Bassman circuit was used as the basis of the JTM line of amps- the most famous early amp was the JTM45 mounted in a combo cabinet and used by Eric Clapton in the famous "Beano" album when he played with John Mayall and the Bluesbreakers. The amp became known as the Bluesbreaker amp and is still sold by Marshall today. Do yourself a favor and get a copy of that album- it created the whole Gibson Les Paul through a Marshall foundation of modern Blues and Rock guitar to follow. I am sure Eric pissed off a few record engineers in the studio recording that album with his amp at full volume.
After the JTM series starting with KT66 power tubed JTM45 up to the first JTM100 plexi face-plated amps played by Hendrix, the next iteration was the JCM line which built on the JTM circuit with new tube types- the EL34 and 6550 tubes- and more distortion. The JCM line featured the first Master volume control that allowed players to cook the preamp section but keep the power section at less than ear splitting levels. These early period amps are what I have focused on building and what seems to be the most popular with folks. Later mass produced Marshall amps just do not seem to hold the same appeal to those who appreciate tone. Here are some I have built, many have special tweaks at the request of a customer.
Below is a stock looking JTM45 reissue from Marshall. It came with a PCB "robot" assembled component board. Stock power and output transformers. I build a handmade turret board with top shelf components, installed an Obsolete Electronics Transformer set and installed a PPIV (Post Phase Inverter) Master volume on the back panel using the second speaker jack opening as the mounting hole. This really opened up the sound of this amp. Great low end with the improved output transformer, nicer clean tone also. But with the Master volume it could get that great crunch at less than window shattering volumes. I converted the tubes to NOS KT66 tubes as well. None of was cheap and its not something everyone will be able to afford. Man, I loved this amp.
Above is a view of the Master volume modification. The shielded gray cables connecting to the back mounted double pot on the chassis can be seen. I also upgraded the two big white power resistors on the power tube sockets to 5 watt versions. These help save an expensive set of tubes if anything ever goes in the power supply. Frankly, at over $100 for a set of good KT66 tubes, this is small insurance to pay.
The above amp was based on a schematic I was given and asked to build for a customer. There was, or still may be, a company called Thunder Tweak amps that published it online. Its built in a JTM45 chassis but is really a JCM2204 master volume circuit with a few extra higher gain tweaks. It used 2 EL34 power tubes and this baby really screamed. I built it using Doug Hoffman's Power and Output transformer set.
As the gain goes up in these amps the shielding becomes very important for the signal carrying wires. You can see the gray and heavier black wires by the inputs that shield the signal from interference with the other parts of the amp circuit. I also used a heavy copper rod grounding scheme running along the pots to the input end of the amp- this keeps noise to a minimum.
Above is a JCM800 circuit with several changes. It has an active effect loop that is tube driven. It has external bias points and adjustment pot to make a bias job easier. But the big thing is that its built with dual 6V6 power tubes for 20 watts of power. Some folks don't need 50 watts.
There are so many options, mods and variations that can be done to these circuits I have only touched the surface. Check out my Frugal Amps Facebook page for many custom customer builds.
Based on the Fender Tweed Bassman circuit, Jim Marshall began building his amps in his kitchen back in the 60's. Fender equipment was very expensive to import into England and there were few British amp makers at the time. The English made Watkins Dominator amp was the model for Marshall's short lived 18 watt 1974 amp (See the 18 watt page) and the Fender Bassman circuit was used as the basis of the JTM line of amps- the most famous early amp was the JTM45 mounted in a combo cabinet and used by Eric Clapton in the famous "Beano" album when he played with John Mayall and the Bluesbreakers. The amp became known as the Bluesbreaker amp and is still sold by Marshall today. Do yourself a favor and get a copy of that album- it created the whole Gibson Les Paul through a Marshall foundation of modern Blues and Rock guitar to follow. I am sure Eric pissed off a few record engineers in the studio recording that album with his amp at full volume.
After the JTM series starting with KT66 power tubed JTM45 up to the first JTM100 plexi face-plated amps played by Hendrix, the next iteration was the JCM line which built on the JTM circuit with new tube types- the EL34 and 6550 tubes- and more distortion. The JCM line featured the first Master volume control that allowed players to cook the preamp section but keep the power section at less than ear splitting levels. These early period amps are what I have focused on building and what seems to be the most popular with folks. Later mass produced Marshall amps just do not seem to hold the same appeal to those who appreciate tone. Here are some I have built, many have special tweaks at the request of a customer.
Below is a stock looking JTM45 reissue from Marshall. It came with a PCB "robot" assembled component board. Stock power and output transformers. I build a handmade turret board with top shelf components, installed an Obsolete Electronics Transformer set and installed a PPIV (Post Phase Inverter) Master volume on the back panel using the second speaker jack opening as the mounting hole. This really opened up the sound of this amp. Great low end with the improved output transformer, nicer clean tone also. But with the Master volume it could get that great crunch at less than window shattering volumes. I converted the tubes to NOS KT66 tubes as well. None of was cheap and its not something everyone will be able to afford. Man, I loved this amp.
Above is a view of the Master volume modification. The shielded gray cables connecting to the back mounted double pot on the chassis can be seen. I also upgraded the two big white power resistors on the power tube sockets to 5 watt versions. These help save an expensive set of tubes if anything ever goes in the power supply. Frankly, at over $100 for a set of good KT66 tubes, this is small insurance to pay.
The above amp was based on a schematic I was given and asked to build for a customer. There was, or still may be, a company called Thunder Tweak amps that published it online. Its built in a JTM45 chassis but is really a JCM2204 master volume circuit with a few extra higher gain tweaks. It used 2 EL34 power tubes and this baby really screamed. I built it using Doug Hoffman's Power and Output transformer set.
As the gain goes up in these amps the shielding becomes very important for the signal carrying wires. You can see the gray and heavier black wires by the inputs that shield the signal from interference with the other parts of the amp circuit. I also used a heavy copper rod grounding scheme running along the pots to the input end of the amp- this keeps noise to a minimum.
Above is a JCM800 circuit with several changes. It has an active effect loop that is tube driven. It has external bias points and adjustment pot to make a bias job easier. But the big thing is that its built with dual 6V6 power tubes for 20 watts of power. Some folks don't need 50 watts.
There are so many options, mods and variations that can be done to these circuits I have only touched the surface. Check out my Frugal Amps Facebook page for many custom customer builds.