My friend Greg was a year older than me, and he was going to Princeton that year. He was also an avid bass guitar player. We got the idea of building a bass amp together, he had the idea of replicating a Fender Princeton – for obvious reasons. While he wasn’t the most technically minded of people, he did have the cash for the parts, so I dove straight in.

Remarks on Circuit

The Fender Princeton is a classic guitar amp with straightforward construction. The two triodes of the V1 7025 are pre amp stages before and after tone and volume control, V2 is the oscillator for the reverb, V3 is half reverb and half gain stage, and V4 is half vibrato and half phase inverter. The phase inverter is a combination gain stage and cathode follower, for both in phase and out of phase outputs of roughly identical gain. This feeds the push pull 6V6 output. Output is about 10W.

I don’t have many pictures or documentation of this build, since we were on a tight schedule when building it, but I had thought of building a tube amp for several years, and have the unrealized parts in my parent’s basement to prove it. I didn’t make many changes to the Fender design, other than replacing the audio output transformer with a heavy duty Hammond output transformer rated at 25W and 10Hz-17kHz response. The audio output transformer is one the main limitations of any amplifier, which is why solid state was such an improvement compared to tube amplifiers. Due to its low impedance, solid state outputs can directly drive loads and eliminate the heavy, expensive, and limiting output transformer.

I also wanted to replace the 12AX7s with 6SN7s – the only reason being that I’ve always preferred the look of octal tubes to 9-pin miniatures. Greg shot that down, since we couldn’t find a chassis that had those punch outs.

One of my early tube amp designs – believe me I got way deeper into tube design than this.

Not much else to say about this. I wired up the whole chassis, which I painted earlier with gold hammered paint, installed the components, and was ready to go. I picked a good speaker too.

Not bad, huh.