In my limited experience (I politely decline the title of guru, I'm too young) the main difference between a hi-fi tube amp and a guitar tube amp is the output transformer, that's what really determines the frequency response of what hits your speaker. Hi-fi transformers are excellent if you're building a bass amp, but for guitar you want something that's going to chop off those lows. There are some component changes that will affect frequency response as well, again the main target is going to be chopping off some lows.
That preamp already has negative feedback, but personally I would take it out, it's a tonal preference and I don't care for it.
Things I would change if this was me taking on the project, in no particular order:
- increase R1 to 68k
- omit C1, C2, and C3
- omit R5 and R6
- Increase R2 to 1M
- omit C4
- increase C5 to 100nF
- decrease C6 to 22nF or less (I wouldn't go lower than 2n2, but tailor to taste)
- Replace the voltage divider at R2/R8 with an A1M pot as a volume control
There are a couple optional changes in the power supply if you want to save a couple bucks. Hi-fi amps tend to filter the absolute hell out of the power, which is fine and doesn't hurt anything, but it's overkill for guitar amps in my opinion. So what I would probably do for simplicity and frugality:
- Omit C13, C15, and C17
- Drop C12, C14, and C16 to 22uF
- you can leave the choke in if you want, or you can take it out and jumper that bit if you want, you would likely be fine either way
- they're running the EL34 heaters straight off the 6.3VAC tap but rectifying it so the EF86 gets 6.3VDC. You should be just fine running the EF86 on AC, which both Vox and Matchless do in their amplifiers.
- omit B40, C18, C19, IC1, C21, R13, R14, and C22
- connect the EF86 heaters in parallel with the EL34 heaters
Again this is just what I what do if it were me building it, so there's my $0.02
And I will of course second Fig's advice to
BE SUPER CAREFUL around high voltage. It can kill you and it will hurt the whole time you're dying.
