Cathode bypass gain boost questions

[phydauex]

Just had a question on how to dial in a very slight bump in gain/volume on a triode stage....Going to switch in a small cap across the existing 4.7k resistor, like .47uf or 1uf. that sounds great in that it doesn't totally change the overall tone of the un-bypassed state, but it's a bit too much gain. Can I dial it back so when the resistor is bypassed, there's only a small bump in overall gain. It's for more of a jazzer that wants just slight increase, not a big jump.

Can I put a small resistor in series with the cap or something like that to soften the gain increase? So in affect I'd have a 4.7k cathode resistor with a switchable 1uf+1k>5k resistor in parallel with it.

Also when doing that, is the additional resistor actually in parallel or does the cap isolate it from the other resistor? Not sure if I'm making sense there, just thinking about the actual mechanics of the arrangement.

thanks for any insights.

[Enzo]

Smaller caps enhance the gain in the upper frequencies.

You can put a resistor in series with the cap to pad it, The series resistor is only in parallel for AC, ie signal. It won't affect DC, hence bias.

Get out your clip wires, use a pot clipped in series with the cap, then clip all that across your cathode resistor. Now you can dial the pot to find a resistance you think moderates the effect sufficiently. Then unclip and measure the resistance of the pot you have it on. Then select a standard resistor value close to that.


Are you looking to modify the amp for this gain, or are you looking for a switchable boost?

You have not provided a model or schematic for the system in question. Many amps have signal level reductions between stages here and there. It might make sense to instead of boosting the gain there, back of on the reduction elsewhere.

[phydauex]

thanks Enzo, I thought that might be the case, I just wasn't sure . I was looking to modify an amp for a small gain boost that could be switched in/out for soloing.

The circuit is a Jim Kelley Reverb amp that I built up and saw the Suhr built reissues have a foot switchable boost, "late in the circuit..." but none of the schematics I found show that feature. I'm guessing that it was a bypass cap added on the cathode of V3a, before the driver/PI stage. so that's where I put the small cap and it works great and switches quiet, I just want to dial back the amount of boost slightly.