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Ultimate 6V6 Plexi (15W)

For more information about the development of this amp, follow my Youtube video series.

Project Presentation

This project started with the idea of building the "ultimate" Plexi-style amp which would include a preamp that could be reconfigured from JTM45 specs all the way to a Master Volume series (aka JCM800 2204), a tube buffered FX loop and a low enough power output that it could be played at max volume on a small club stage.

Please note that although this is a low-power amp by Plexi standards, 15W is still very loud. While the aim of the project was to build an amp I could run at max volume for club gigs, it is not always practical and often requires turning the amp towards the back and relying on IEMs or monitors to hear myself. That's with lower sensitivity speaker (a Celestion Creamback 65 is a good match, or a pair of Greenbacks). Max volume also limits the usefulness of the FX Loop and requires applying effects after the power amp (I simply mic the amp and run the mic'd signal in my time based effects).

Schematic 

This is v4.0 of the schematic, presenting the current iteration of the amp. I am regularly using this amp on stage with my bands and making adjustments based on this. Tweaks are usually documented, explained and demo'd in the ongoing Youtube video series which can be found on my channel.

NB: You are free to do with this schematic as you please, but note that this is provided without any  guarantee or support. I do not recommend this as a first project.
NB2: This is the way I built the amp, not necessarily the best way it can be built. A lot of it comes from playing around with the amp until I get what I want. 

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Part selection/Circuit variations:


  • Transformers for Marshall 18W or Tweed Deluxe or Princeton Reverb will typically work well, provided you use the appropriate rectifier and such. My Output Transformer is for a Marshall 18W, while my power transformer is a "generic" unit that had the specs I wanted. I aimed for a B+ well under 400V, I got 365V.
  • This would work just as well with solid state rectification, but when I compared tube and diode rectification for a Youtube video (https://www.youtube.com/watch?v=P7uqoRH2Qdg), most viewers and I preferred the tube rectified tone. 
  • Speaking of the tube rectifier, a GZ34 is rated for 60uF maximum, I'm running 64uF. Take that as you may. I built it with 32uF initially, but I prefer 64uF.
  • My input scheme allows the Bright channel alone, or both in Parallel (both inputs work in Cascaded mode), but it doesn't allow Normal channel alone. There are other ways.
  • The Cold Bias switch (SW4) is pretty useless when not in Cascade mode. 
  • R15, R16 and the 1N4007 next to it are there to protect the coupled cathode follower stage as per this article: http://www.valvewizard.co.uk/dccf.html
  • The diode on V3 does the same thing for the FX Loop buffer stage (a fixed-bias cathode follower). Since I added the loop afterwards, this was soldered directly on the socket and that's why I didn't add the small resistor (à la R16). It is actually not necessary, but can prevent any extra noise from the diode.
  • I built the amp without a standby switch based on the information here (also, the tube rectifier provides a soft startup): http://www.valvewizard.co.uk/standby.html
  • The schematic doesn't include the filament wiring, you should already be familiar with this if you are going to attempt this build. I highly recommend DC elevating the filament (excluding the rectifier which is already at B+) by referencing the center tap (or a pair of 100 Ohms resistors) to the cathode bias resistor. It can be done directly on one of the power tube sockets.
  • R7 and R21 serve no purpose other than making sure the grids always have a path to ground while switching is happening.
  • R22 and R23 attenuate the Send signal by approximately 34dB. Through experimentation, this gave me the closest signal strength after recovery compared to the bypass signal, while being low enough to work well with pedals. R23 can be altered to change the Send signal strength. Another option is to flip R22 and R23 and replace R25 with a 100k potentiometer, this can increase output impedance though.
  • Due to many modifications, I had to build a new power supply node for the FX loop with parts from my parts drawer, hence the 16uF 475V capacitor. You can use anything between 8 and 33uF, as long as the voltage rating is high enough (in my amp at least, 400V would do).