2204 2nd stage gain boost

ledvedder

ledvedder

Well-known member
I've been messing around with my 2204 build, trying some different things I've been finding on some schematics. One of the changes is adding a footswitchable gain boost. The schematic I'm referencing switches a 47k/1uF in parallel with the cathode resistor on the 3rd stage. My 3rd stage is already 820r with a 3 way switch for either 0.68uF or 22uF bypass caps. So I figured I'd try the concept on the 2nd 10k stage. I'm using the footswitch to bring in a 10k/1uF in parallel. It functions as expected, and does gain it up, but it's a bit too much bottom end. I'm wondering if a lower bypass cap would work better? Something like 0.68uF or 0.047uf? I realize it's the sum of all parts, but I'm wondering if anyone has any suggestions?
 
What’s the coupling cap size of the previous stage? Have another cap switch in series with the coupling cap after it which will tighten the low end up at the same time the gain boost is added. You could change the 1uF so it isn’t boosting low end by going smaller in value, but you’ll also notice you’ll start getting a low mid emphasis in the tone. That’s why the series cap being switched in at the same time is best. It’s bypassed when no gain boost is on, present when the gain boost is on.
 
A partial bypass on a 10k cathode resistor doesn't work well. Think of it this way: a bypass cap will boost things to the max level determined by the anode resistor (more or less). So if you add a small bypass cap to a cold clipper that has low gain, there is a big difference between the low end and the boosted high end. 0.68uF would be okay in this regard, because the 'cutoff' frequency is still pretty low. There would be about 8dB between your low E string and the boosted high end. With 0.22uF, there would be about 14dB difference. You can get some ice-picky highs with that kind of partial boost.
 
V2a said:
A partial bypass on a 10k cathode resistor doesn't work well. Think of it this way: a bypass cap will boost things to the max level determined by the anode resistor (more or less). So if you add a small bypass cap to a cold clipper that has low gain, there is a big difference between the low end and the boosted high end. 0.68uF would be okay in this regard, because the 'cutoff' frequency is still pretty low. There would be about 8dB between your low E string and the boosted high end. With 0.22uF, there would be about 14dB difference. You can get some ice-picky highs with that kind of partial boost.
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Yeah I agree with all of this and it’s a better explanation why it’s not the best route to start messing with that 1uF too aggressively. 47k in parallel with 10k is still around 8k, so still biased relatively cold. The lowest value I’d go is 0.47uF instead of 1uF and that will have a 3dB point at around 40Hz which is 2x lower than the lowest E in E standard at 80Hz.
 
glpg80 said:
What’s the coupling cap size of the previous stage? Have another cap switch in series with the coupling cap after it which will tighten the low end up at the same time the gain boost is added. You could change the 1uF so it isn’t boosting low end by going smaller in value, but you’ll also notice you’ll start getting a low mid emphasis in the tone. That’s why the series cap being switched in at the same time is best. It’s bypassed when no gain boost is on, present when the gain boost is on.
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Previous cap is 0.0022uF, next stage is 0.022uF.
 
ledvedder said:
Previous cap is 0.0022uF, next stage is 0.022uF.
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Change out the next stage CC that’s 0.022uF with a 0.0047 and see what you think. If you like it, make it so that a 0.0047uF is switched in series with the stock 0.022uF when the gain boost is on. When it’s off, the 0.0047 is bypassed and it’s the stock 0.022uF. Don’t switch between the caps because there’s 300VDC and you’ll get sparks and also ruin speakers. Plus it’s just dangerous.
 
SpiderWars said:
Maybe try the Cameron mod for Bogner XTCs. Basically a 1uF cap in series with a 10k resistor...put that whole thing in parallel with the 10k cathode resistor.

https://www.rig-talk.com/forum/threads/did-the-cameron-mod-on-my-bogner-ecstasy-101b.42278/
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Maybe that's actually what the jack is doing? I have a 10k on the jack, instead of the 47k. And a 10k on the board, instead of the 5.6k.
Screenshot_20220917_133900_Gallery.jpg
 
I cant see where those other two wires on the jack go but it looks like the 47k is to prevent popping. It's big enough compared to 10K that it won't make much of a audible difference when in series with the cap. You should be able to hear a difference between the straight 10k and adding 10k+1uF series combo in parallel with it. But it will be way more subtle than just putting the 1uF in parallel with the 10k cathode resistor. This Cameron mod doesn't add much but it helps with the issue V2 was talking about above. There is also another more gainy mod that is similar to the Cameron mod but I can't remember it.
 
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