Re: Convert Vintage Valve Radio to Guitar Amp
I would be tempted to stick with 0.022uF for the coupling cap, that is a fairly typical value for guitar amps. If you decide that it needs more or less bass response you could tweak it later to suit (higher values = more bass). As stated 22uF would be a good value for the cathode bypass cap (anything from 22uF to 100uF should work fine, the value is not critical).
There is absolutely no need for a buffer on the input, the input impedance is ideal for guitar as it is. If you find that it does not have enough gain the simplest solution would be to use an overdrive / distortion pedal in front of the amp.
The Celestion speaker you linked to is a good choice, I have one of those in a practice amp that I built myself and it sounds good. I don't know what size speaker is in the radio you will be using but I would definitely try that first before buying another one, you might be surprised at the results. Many of the low power valve amps back in the 50's and 60's used standard radio speakers and many of those are considered very desirable amps now!
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