Build Your Own Clone Message Board

I'm probably about to display some ignorance so I apologize in advance.

I built my champlifier not too long ago and I really enjoy it. It has a 10" speaker.

I've been reading the Guitar Amp Handbook by Dave Hunter *reading can be dangerous). Talking about the preamp stage he states that the cathode bias resistor (C1 on the byoc layout?) controls the amount of gain in the preamp tube. And that by lowering the value of that resistor, you can get more gain.

Is it that simple? Can I swap out that resistor only and get more gain? Could you use a variable resistor and have something like a gain knob?

Are there other options that I could pursue? I did build the FET from BYOC and that certainly helped but it is on the bright side.

You do get more gain. But it might not sound like what you think it should. Upping the resistance for the plate resistor ups gain just like lowering the cathode resistor ups gain. It's like "voicing" the amp. A common value for tweed era Fender cathodes were 820R. Maybe put those two values on a switch?

The cathode bypass cap focuses the gain to a specific frequency depending on what value you use. There was another thread discussing this.

2 things to note: I have the book you note in the beginning. I have also made mods to my 5F1.
1. You reference the Cathode bypass resistor. This is really a fixed value. While changing it theoretically will alter the gain, it doesn't really work well in practice. I messed with the Resistor (not C1 like you stated but the resistor in parallel to C1) and really hated all of the change it made (got fizzy, farty just ugly).
2. C1 is the Cathode Bypass CAP. It can be changed for a nice range of effect. On the referenced link, I have a diagram of how to have a switchable bypass CAP that won't pop. There is an audible difference it the tone when using different caps. The 22uf is much fuller sounding while the .47 is much brighter sounding. The middle position on the switch takes C1 out of the circuit.

The reality is that the 5F1 is not a high gain circuit. You could add another gain stage to it, but then it is no longer a 5F1. What makes the 5F1 so desirable is that you can drive the power tubes into distortion which is a different sound that preamp distortion.
If you really want more gain from it, go with a good overdrive in front, that will really make it shine. The champ loves pedals and I get most all of my drive an distortion from pedals for this amp.
-P

Thanks for the link and clarification. I really like this amp and I'm not sure I want to really do anything to it. I am just trying to better understand what I built and came across that statement in the book. Plus, I think I use the terms 'gain' and 'distortion' interchangeably and I'm starting to figure out that isn't exactly correct.

Thanks again.

I made C1 switchable on/off, and put a push-pull pot on the negative feedback (changed the value of the neg feedback resistor as well). All kinds of wonderful tones from warm and mellow, to glassy, to wild and savage. And yes, it handles pedals wonderfully....so whatever your flavor of distortion/overdrive, go for it! Although some of the metal pedals might be a bit of a mismatch.....but that's just my opinion.