Adding gain

[gui_tarzan]

My custom amp only has two inputs and that's all I want, so in order to use the extra half of V1 that would just sit there doing nothing, I want to use it to add some sizzle to the signal. I know I'm missing something so any input given is greatly appreciated.

The schematic is untouched after V2 which goes directly into the tone stack so I didn't include the rest. Will this work or is there a better way to do this?

[Stevem]

I would just simply swap in a 12ax7 for now if you have a 12ay7 in there now and see how much added voltage gain that gives ya towards driving the next stage harder.
I would also even now swap out that 250uf bypass cap for a 22 to 25uf, as just this change will let you open up the tone section more and pass more signal.

[gui_tarzan]

The plan is to use a 12AX7 for V1 anyway, but leaving that one triode unused is like leaving a good steak in the freezer because you have hot dogs already thawed out in the fridge!

I was hoping to be able to use it with a foot switch for lead boost.

[Smokebreak]

I'm not sure that would work the way you have it drawn. You have parallel triodes up front, then the bottom triode gets routed through the pot then back through the first triode. The amp will beef up w/ paralleled inputs but you'd need o put the mixers back in (270k).
If you want to truly add a stage, you can do it via hi/lo inputs like the JCM800, or you can simply switch it in upfront. You'll need to attenuate the signal to taste of the added stage. 800 does 470K to 1M pot, or you could come up with your own divider, with or without a pot. 68k/68K for 80's sound, 1M/100K for modest gain bump, or anywhere in between.

[Smokebreak]

Here's one scribbly way I like to do it. Circled area is divider network that can be tweaked to taste. Put in a relay for the DPDT and boom you've got a boost on the floor. Good luck!

[Reeltarded]

Splt cathode and drop the bypass like ^^^^. The small divider pair is a good way to go. 68k pair is enough to prove it works.

[gui_tarzan]

Like this?

[Smokebreak]

Well, your switching looks correct, as far as introducing the extra stage. When you split a once shared cathode R, you need to double the Rk value, o keep the same response. 1K5 is close enough. 3 820r stages is way hot, and those massive cathode caps will make for a farty mess. Hell, two 1K5 and an 820r stage is still quite hot. Nothing wrong with building it and seeing though, these are simple changes.

Also, you may wnt to think about a divider after second stage. Generally you could add back in a 270K to ground after the 270k you have in there.

You don't have to bypass the cathodes, either. It really depends what sort of sounds you are going for, and of course this is all subject to personal taste

[tubeswell]

I've used this switchable parallel/single V1 setup on 3 or 4 amps now. (Idea is in Merlin Blencowe's 1st edition preamps book)

[img:75:75]https://farm6.staticflickr.com/5338/8843549954_c80eed2f63_s.jpg[/img]Switchable parallel single V1 by tubeswell, on Flickr

[gui_tarzan]

I haven't changed the values of the parts on the schematic yet, I should just take them off the schematic for now. I just want to make sure the layout is correct first.

I suppose I could have used a two-input Marshall circuit to start with but I wanted to try the Fender tone stack first and I really don't want to have a MV.