6P1P PP guitar amp

01.04.2014

I needed a polyvalent guitar amplifier – good clean tones and full push-pull power tube distortion at a low sound level, for practicing at home, so I started building this project.

The Russian 6P1P beam tetrode was chosen for the power stage – it has ratings similar to the 6V6 and can be found at low prices.

To keep the output power low, being able to overdrive the power tubes in push-pull at the same time, I chose a low screen grid voltage of only 120v:

The curves of the 6P1P tetrode at 120v screen voltage

The 6P1P are fixed biased using stacks of LEDs (this helps with a very fast recovery from overload), and the current balance between the push-pull tubes is done by adjusting the screen grid supply voltages.

The preamplifier part has two channels, one clean and one distorted, inspired from the Dumble schematic. The preamplifier tubes used were some NOS Tung-Sol 12AX7. I have tried Russian 6N2P-EV, but they didn’t sound as good, especially when distorted.

The coupling caps are high quality – teflon for the clean stage and PIO for the following stages.

The phase inverter is made with a E180CC, a double triode that is not very linear – but that’s a plus for a guitar amp.

The power supply is a classic tube rectifier (EZ90 / 6X4) followed by a C-L-C-R-C-R-C filter and MOSFET stabilized adjustable supplies for the screen grids.

There is an FX loop connexion just before the power stage. The channel switching is done with relays, controlled with push-buttons or a foot pedal. An Arduino Nano takes care of the power sequencing and the channel/FX switching. There is a switchable T-pad attenuator on the output for an even lower volume.