Amp 4: Two Channel Head
My third amplifier got sacrificed yet again — gutted to build amplifier number four. I chose a two channel preamp and a fairly standard power amp section. The preamp is a slightly modified version of the London Power preamp, which uses two 12AX7 tubes to achieve a clean channel and a high gain channel. This time, I decided not to add any of the more arcane controls: you can’t switch between fixed and cathode bias, you can’t switch between triode, pentode and ultralinear, and you can’t switch the value of the screen resistors. The amp also has no standby switch; the beast is either on or off.
Powering the amp on, as soon as it warmed up I heard a tone coming from the speaker. Oscillation! After checking the feedback loop to no effect I narrowed the problem down to a miswired switch in the preamp which connected the cathodes of two consecutive 12AX7 stages. I then biased the amp to 40 mA and started playing. I only had my bass with me so I didn’t get a real feel for the amp, but I noticed that the distortion preserved the clarity of the notes in each chord I played.
The amp emitted a high pitch squeal when I played through the distortion channel with the gain turned only a little, so I made a few changes to the circuit to compensate. I reduced the range of the gain channel volume control to 10% of its original range and I replaced the 220 kΩ anode resistors in the lead channel with 100 kΩ resistors. Now the amp starts squealing when the gain is about 60%, which I can live with for now. I couldn’t figure out what was causing the squeal; it could be interference as none of the wires in the amp are shielded. Oh well.
Sounds
A week after I built it I got around to plugging in a guitar. Wow! The amp is capable of some great crunchy rhythm sounds and some raw lead tones. Even as the distortion got extreme, notes remained distinct and I was unafraid to play minor chords without getting muddy. The amp’s sounds sound most like 1970s hard rock. There were some types of sound I couldn’t get out of the amp, such as focused modern high gain sounds.
- Audio Sample 1 [3:48, 137 kbps, 3.9 Mb]: I recorded this using a Radio Shack microphone right in front of an 12 inch Electro Harmonix 12VR8 speaker in an open back cabinet. The only post processing I did was to add reverb. I recorded it using Audacity on a Linux netbook, and there are audio glitches in the recording.
Lessons Learned
- List time I learned to spend more time on layout. Well, spending more time on layout really paid off.
- That said, I should be more careful with layout. I built the circuit cards backwards this time. I hadn’t yet drilled holes in the chassis, so I reversed the chassis to match. There also wasn’t enough clearance around the potentiometers. Next time I’ll space them differently or (more likely) buy 16 mm ones instead of 24 mm ones.
- The preamp card should be bigger next time. This preamp card was a little cramped, even though in the final layout there was enough room to make it about three inches wider.
- Next time I build an amp I’d like to mount some of the circuit cards over chassis mounted components such as tubes and transformers. It wasn’t possible this time because the spacers and the screws I had were too short, but I can get longer ones. I’ll have to make sure that the electrolytic capacitors on such a board aren’t too tall.
- High gain squeals. I’ll have to add shielding to my next high gain amp. This amp is otherwise fine, the wires are short and the grounding is solid.
- Chassis holes should be widened in increments of a sixteenth of an inch. The holes I drilled after I learned that are close to perfect, the earlier holes I drilled are messier. Also, one of the preamp tube sockets is mounted with only one screw because the hole for the other screw is too far away.
