A few months ago, I ran across an old Eico 460 oscilloscope in my Grandpa’s garage, and was totally shocked to find that when I powered it up after thirty years of sitting in a dusty Texas shed, it worked like a treat! It was my first exposure to vacuum tubes, and it would seem I became instantly hooked.
Since I already had a more modern, powerful oscilloscope, I decided to salvage the components from inside the scope for a devious, fantastic, and altogether fun goal: a guitar amp. My design is shown in detail below. (Yes, it’s my own design. I did actually work out the circuit components from tube plate characteristics charts.)
There’s nothing really exciting about this: just a basic tube-rectified and RC-filtered power supply with AC filaments, raised to 265 Vdc from ground to try and reduce hum. The circuit produces up to 450 Vdc, but is designed for 325 Vdc under normal load conditions.
The interesting bits about this amp are probably in the preamplifier. The preamp starts with the 6AU8 pentode, lending a bit of edge to the sound of the amp while boosting gain. (Examples of these can be found in the Pipsqueak Pentode and the AX84 blues preamp.) The triode section of the 6AU8 is used in a cathode follower configuration whose output is fed to a 12AU7 with both triode sections wired in parallel for low output resistance and softer gain.
The output stage is really just a stock cathode-biased, (In fact, all the tubes in the amplifier are cathode-biased.) single-ended reactively loaded (transformer primary) pentode output stage. I had originally wanted to use a 6CB6 from the scope, but the maximum plate voltage of the tube was too low, so I redesigned around an EL34, which is popular and relatively cheap. The EL34 is the only component of the amp not pulled from the old Eico scope!
Here are some pictures of the slow by steady progress I’ve made since finding the scope in my Grandpa’s garage…