Amp works!

[bluefoxicy]

My amp works!

I built a 12AX7/EL34L based guitar amp! A few minor bumps, bruises, and Lives-1, and I am victorious!

The amp is a Valve Junior clone kit with marshall gain characteristics, using an octal socket instead of a 9 pin for power tube. The diagram is for 9-pin, and the kit is designed for 6V6; an EL34 draws twice the power of a 6V6 across B+1. You can get the board and the components from WattsTubeAudio.com on the Valve Junior page.

I had to connect Pin 1 to Pin 8 (suppressor to cathode, thanks to Epiphone forums users for that tip) to rig this up for EL34; 6V6 doesn't care. I also lifted the filament grounds (filaments go through two resistors, then to ground) and instead ran them to the cathode (which goes to ground through a resistor/capacitor), and changed the input jack ground scheme.

Problem 1: I didn't center tap the 260-0-260. High voltage AC gave 520VAC, 766 rectified, and it shot sparks at me. I fixed this by using 260-0 and then taping off the other 260.

Problem 2: R10 (first resistor in power supply) handles 3W, which works for a 14W bias 6V6 (2W works here, actually). a 25W bias EL34 draws enough power to tint the resistor a light brown after 10 minutes of play, and make the amp smoke. B+2 and B+3 use the same 12AX7 so don't draw any more power. I got a 5W flameproof resistor to replace R10.

It works now. I need to put in an inline fuse and bias that EL34L hotter (I used a 2W pot instead of a 2W fixed resistor and biased conservatively), but aside from that it works quite well.