This follows on from my previous post about a small bass amp. In the time since I made that post I’ve been considering my options with the help of the guys over at basschat.co.uk. I had it in my head that bass amps would require some sophisticated EQ circuitry to be usable but it turns out that the big issue – for a valve/tube amp – is making sure that your output transformer is large enough. People who like valve amplifiers seem to be happy with simple tone controls.
I have taken the time to draw out the schematic of my Northcourt Fifteen and it it turns out that it is different from the schematic I found on the web. The differences are primarily in the power supply and in the tone-stack which are two areas that will need to be redesigned to make sure the amp produces a good, solid bass tone. I used the Duncan Amps Tone Stack Calculator to model the tone controls on my Northcourt amp as it is today and I have uploaded the plot as a PDF file. There is good control of the bass but almost no control of the treble output. The values of most of the components in the tone control will have to change.
The power supply seems to have unnecessarily large reservoir caps for the two ECC83/12AX7 valves/tubes. The first ECC83 has two 50uF capacitors; one for each side! Normally, when the two halves of a preamp valve are working out of phase they are fed from the same reservoir cap because one valve is pulling more current while the other is pulling less so the action of one tends to compensate for the action of the other. That is why the phase-inverter (the second ECC83) can fed from a single reservoir cap but, even then, it appears to be larger than in needs to be.
I have sketched out my first thoughts on a revised schematic and you can see it here but I’m not going to build anything until I have had time to read more of Merlin Blencowe’s books. Feel free to give me any feedback in the meantime.