How To: Schematics and Layout for a Hot Rod Marshall

It's been awhile since I've done any "How To" blogs, so I thought I'd do one where I provide a schematic and layout for a prototype of a two channel, foot switchable hot rod Marshall style amp. When I first set out to build my ultimate amp, I built this as a turret board prototype I called the PlexRod. It is a great sounding amp, and there are some clips available of it if you Google search a bit. Many of the ideas in this amp turned into the Cherry Bomb, although the switching and some other things are different. I'm sure not many will attempt to build it because it is crazy complicated, but it might provide some ideas for your own custom builds. I put a tremendous amount of work into this design and received help from a bunch of folks, including my friend Olaf Krampe and Bruce Egnater from Egnater Amps. I actually took Bruce's amp building class about 6 years ago and he was a great help in getting me started in amp design. I highly recommend Bruce's amp building class and his great products as well! I hope you enjoy checking this out!

I'd like to say a few things before we get into the details. First of all, the layout diagram and schematic may still have a few errors as I did not update them when I finished the prototype. As one example, in the schematic, R5 is shown as 100K, but the same resistor in the layout diagram is 330K. The 330K is actually correct for the design, but the preamp schematic was never updated. Anyone who uses any of the design will need to go through the schematics and layout CAREFULLY to find the issues and address them. If anyone does, I'll be happy to post changes to the schematic and layout here. When I have time, I'll be posting detailed pictures of the assembly of the PlexRod that are basically step by step descriptions on how to build it. An earlier blog I posted shows how to cut a custom chassis from scratch and that is the PlexRod you see in the pictures.

Also, I have to give my standard disclaimer here:

Tube amplifiers contain potentially lethal, high voltages even after they are unplugged, that may cause personal injury or death. Do not attempt to repair, modify, or work on any amplifier unless you are absolutely certain you know what you are doing. Henning Amplification accepts no liability for damage or injury incurred while using the information provided on this site.

That said, Let's get started. There are 4 schematics for the amp (please reference the pictures above):

  • The preamp
  • The power amp
  • The effects loop
  • The footswitch

Let's start with the preamp. What we have in the schematic is a preamp section with a footswitchable/manual switchable gain stage. It has High and Low Sensitivity inputs and a pre tone stack "Jose Master". You can see that there are zener diodes over the master. You can actually put them on a switch so you can move them in and out of the circuit, but in the schematic they are always on. I love the way the Jose master sounds with the zeners engaged. You get a nice, saturated sound. The 250 ohm pot you see at the bottom of the screen is for the DC heater setup that really eliminates the hum in a higher gain design like this.

BTW, these schematics were created with the FREE EAGLE light version from CadSoft. If you are a person just tinkering and not producing commerical product (like I was at the time), it is a great program for creating your own scematics and as I said, it is FREE! You can get it here:

http://www.cadsoftusa.com/download-eagle/freeware/

I had to create my own library of tube amp parts since they aren't available in the EAGLE library (like 12AX7 tubes, etc.), but the documentation clearly explains how to do it. Anyway, here is the preamp schematic:

Now let's discuss the power amp section schematic. It is a pretty standard design for a 100 watt power section, but take note of a few things. First, you can see the rest of the DC heater circuit in the top left of the schematic. You can also see that the amp has a Post Phase Inverter Master Volume (PPIMV) implemented. I find these personally to be the most transparent master volumes you can install in an amp. In the middle of the schematic you can see there is a separate transformer and circuit for powering the relays. Since there are a ton of relays to do the switching in both the amp AND the footswitch, I felt a separate power supply was in order. In the top right you can see there is also an adjustable line out for powering effects for stereo and W/D/W effects and power amp guitar rigs.

The next picture shows the footswitchable/manual switchable effects loop that was used in the amp. Pretty straightforward:

The next image shows the schematic for the footswitch, which is also pretty straightforward. Although I am providing the design, I must tell you that I never actually built the footswitching for the prototype. Since I had decided to re-design the switching for the production Cherry Bomb amp, I just moved on to that and the prototype footswitching was never implemented.

Ok, that's it for the schematics, so now let's move on to the layout diagram. The layout took FOREVER to get right! At first I started trying to just draw it freehand, but it became clear that that would never work and it needed to be to close to actual scale anyway. A friend of mine from one of the amp forums had created these beautiful layout diagrams for classic Marshall amps, so I asked him how he made them. He recommended a program called TurboCAD, which actually had a pretty steep learning curve, but he stepped me through the basics and I got pretty good at it over time. If you want to attempt your own custom designs of any complexity, you need to get familiar with some type of CAD program. My friend was nice enough to provide me with his library of amp parts to do the layout and this greatly sped my process of creating this layout. Once again, there may be a few errors here, but this is very close to my working prototype:

So, there you have it! This is the basic design of the working PlexRod prototype amp I built, which ended up becoming the Cherry Bomb. I hope it gives you some cool ideas for your own custom builds! In the future, I'm planning blogs that will show you how I constructued the prototype PlexRod.