December 2020

EL84 Guitar Amplifier

Recently I’ve designed and built a simple yet very nice playing guitar amplifier.

I’ve designed it keeping in mind that I don’t wanna distortion: it’s a specialized amplifier for playing contry music, blues and clean.

To reach this goal I have to minimize the overloading of the first input stage and keep low the overall gain.

I’ve choosed for this role an high-current low noise single triode tube tought by constructor to be used as preamplifier in audio stages with resistor-capacitor coupling.

The second stage it’s based uppon a dual triode – medium gain tube for UHF radio band receiver use with internal shield. It’s second triode is used as phase splitter that drives a couple of EL84 power tubes in class B.

Here the details, schematic and notes as PDF file. Click here for PDF >>> Building NOTES

I’ve used the following russian tubes:

  • 6C3П-ЕВ (6S3P-EV) equivalent of EC86 as first input stage
  • 6Н3П-ЕВ (6N3P-EV) equivaent of 2C51 as voltage gain stage and phase splitter
  • 2x 6П14П (6P14P) equivalent of EL84 as push pull output stage

The transformers are home made, home caculated, self winded. I’ve used oriented grain EI core to limit power loss. The low range cutoff frequency of output transformer is calculated for 100Hz. Primary fractioning it’s a secret.

For the ultimate schematic please click to download the PDF file >>> Building NOTES

This is the original schematic. I’ve modifiedit during assembly because of some small errors. The phase splitter built around V2/2 was not working properly. I’ve solved by putting the 1M resy from grid to ground not between the 1.5k / 56k series at the chatode bias. I’ve let the 56k+1.5k still there for bias.

It have some little instability at full input gain… it needs to be further modified and inproved.

Here the calculations for the two transformers.

 

Output transformer:

  • Column 29x36mm EI oriented grain
  • 1429 + 1429 turns dia. 0.14mm CuEnameled 4k+4k primary push pull
  • 44 + 44 turns dia. 0.8mm CuEnameled 4R+4R speaker output to get 0-4R-8R inpedance selection.

Power supply transformer:

  • Column 29x36mm EI oriented grain
  • 1123 turns dia. 0.25mm CuEnameled 220-230V mains input
  • 1116 turns dia. 0.18mm CuEnameled 200V for amplifier main power
  • 35 turns dia. 0.8mm CuEnameled 6.3V for filaments