What’s your method for cooking? Do you look up recipes and follow them exactly? Do you substitute ingredients on the fly and hope for the best?
I’ve never been one to follow recipes. I might get a general idea from one, but I usually make it once – loosely based on the real recipe – and every time after it’s a tweak on my less-than-perfect memory of my initial experiment. This is why I mostly stick to the meals you can’t mess up: chili, pizza, pasta – throw in whatever ingredients you have on hand and it usually tastes somewhere between okay and great.
Unfortunately guitar amps take a bit more vigor and discipline than making chili. So instead of just building a classic design and tweaking a bit as I go, I decided to go all in, and question every component down to the individual resistor and capacitor values.
Topology is defined as, “the way in which constituent parts are interrelated or arranged.”
Many of the different topologies (amp recipes if you will) are well known; tweed deluxe (cathodyne phase inverter, cathode bias, lack of negative feedback), fender blackface (long-tailed pair phase inverter, fixed bias, reverb, lots of negative feedback), and many others. These help to give an idea of what these particular aspects will do to the resulting sound/tone of an amplifier, but the devil’s in the details.
Here are some considerations and questions I have asked myself when designing this guitar amp:
· What type of tone stack will I use? Baxandall (Ampeg and hi-fi), FMV (Fender/Marshall/Vox), Brownface, a simple one-knob tone control?
· Do I want to use a fixed-bias power tube arrangement, or cathode bias?
· Which type of phase inverter? A cathodyne, a long-tailed pair?
· How many gain stages?
· What type of rectifier?
· How much negative feedback should I use – if any at all?
· How much filtering in the power supply section? Should I use a choke (inductor)?
· What type of power tube? 6V6, 6L6? EL84? Something a bit more original like a KT66?
· How much power? 20 watts? 40 watts?
· Should there be a bunch of options to increase the variety of available tones, or should it make achieving one sound easy on the player?
The prototype that I finished last week actually has a number of different tone stacks, phase inverters, gain stages, and other factors that can be switched in and out of the circuit so I can “A/B” the differences and decide what sounds good to me.
I am not yet finished tweaking this guitar amp design. I haven’t committed to any real design considerations other than a cathode-biased power amp section, and the addition of spring reverb.
Aside from that, I am currently halfway through my bass amp design - details on that design to follow.
Stay tuned for updates as I write my own amp recipes.
Thanks for reading, Matt
P.S. Here's a song: https://www.youtube.com/watch?v=IuzNX4ZSbJc