Safe to put V1a coupling capacitor on a switch for variety?

[Chuck H]

The way your NFB circuit is set up now is like having a resonance control on ten all the time. You might benefit by adding a variable control to that effect to get the tighter bass you mention. Different caps would change the knee frequency, but everything below that doesn't get feed back at all so the actual bass or low bass will never tighten.

[Mr. Bill]

The way your NFB circuit is set up now is like having a resonance control on ten all the time. You might benefit by adding a variable control to that effect to get the tighter bass you mention. Different caps would change the knee frequency, but everything below that doesn't get feed back at all so the actual bass or low bass will never tighten.

Thanks Chuck. That's kinda my plan at this point. 2 different values to change the knee strapped a across a push/pull 1M pot with a switch before it to take it out completely if I want.

[Leo_Gnardo]

Just thinking about amps that use switched coupling caps and another classic is the Orange OR120/OR80 that works nicely.

Agreed Mick, I was just about to mention Orange's "FACS" control.

[Mr. Bill]

You guys have been extremely helpful as always. I feel like I owe you money at this point. If I can ask one more question for now, does anyone see any problem with switching the cathode like this? I may even make it more complicated by adding a second option, but here's my starting point. Thanks.

[Chuck H]

Ordinarily you would, again, need to include an equalizing resistor for this circuit. This would be my more economical approach.

[Mr. Bill]

That's brilliant. I'm starting to see how you think and I like it! Again thanks for all your help.

Is the basic idea behind an equalizing resistor that as R2 approaches infinity, that V2 approaches V1? So that for significant values of R2, it has no effect on the circuit? Also, in your economical version, does the .68uf not affect the tone at all with the switch open?

[Chuck H]

That's brilliant. I'm starting to see how you think and I like it! Again thanks for all your help.

Is the basic idea behind an equalizing resistor that as R2 approaches infinity, that V2 approaches V1? So that for significant values of R2, it has no effect on the circuit? Also, in your economical version, does the .68uf not affect the tone at all with the switch open?

I never look at in that way. I just look at voltage potentials. You don't want a cap holding a charge or needing a charge the moment it's switched into the circuit. It needs to be compatible with the environment or the instant differential will impress itself on the audio (POP!).

And yes, the .68u cap WILL affect the tone. But it is only bypassing less than 10% of the resistance in that circuit with the switch open. The other 90% of the resistance is still elevating the circuit and imposing local negative feedback. I doubt you'd even hear the difference (without wondering if you were ACTUALLY hearing it) if you put the .68u on a switch and A/B tested it.

[Mr. Bill]

Thanks again Chuck. Would you mind reposting your not as economical version you had put up first so I can compare?

And as far as A/B testing. I do a before and after shot of every mod and change I make to amps with the same stereo guitar part reamped pre and post incremental change. I try not to fool myself, but if I'm doubting I heard a significant difference in something, I just assume I'm not. Too bad you can't do a full null test as a tube circuit will never give you the exact same thing twice.

[Chuck H]

Trying to dig it out of my already uploaded images. It may give some trouble because it has the same file name... Nope. It replaced it. I'll redraw.

[Chuck H]

Ok... This is the typical way of doing this. And it occurs to me that it's six in one and a half dozen in the other, beacause...

Bypassing 1/10th of the load should be roughly the same effect as paralleling it with a bypass at 10X. The math would prove out the details, but it's mice nuts. This just occurred to me now looking at the circuits. And, incidentally, this is half the reason I post here. It keeps my observations in the game. Using a larger value resistance for the switch bridge would probably skew the differential enough to cause some noise? I'm not sure on this, but I know that 100k is a typical value for this sort of circuit. So in that light, I'd be VERY surprised if anyone could hear any difference between my economy circuit and this one.

[Mr. Bill]

And if I wanted to add a switchable cap only on the third stage. I should do it like this? Am I getting it?

[Chuck H]

Nope. The point (to some circumstances, like this one) is to keep capacitors referenced to the circuit voltage potentials. See below.

Another consideration is the cap value. The .68uf value will NOT pass the same frequencies across the the 820R resistor as it would across a 1k or a 1.5k. And certainly not the same as a 2.7k which is the "classic" sound. So if that is your goal you need to adjust the cap value to knee at the same frequency for the 820R as the .68u value does for a 2.7k if you want it to do anything similar. The knee frequency of the capacitor is relative to circuit impedance. Which in this case is the same as the circuit resistance (though it isn't always). I like to use on line calculators and spice to support my lack of electronics training and math skills. But a good rule in cases where the resistance is the relative impedance is that the cap value doubles when the resistance value halves.

Another thing to be aware of is that the cathode resistor value dictates the level of affect by the bypass cap. With the 2.7k value you get a -1db (more on why I chose this measure later) at about 550Hz with a total boost over unbypassed of about 6.5dB. But with an 820R resistor and (as I guestimate) a 1.5uf bypass cap you get a -1db at about 500Hz and a total boost of a tad less than 3dB. I chose the -1dB level rather than the typical -3db level because this cathode bypass circuit is only capable of about +6dB. So what get's percieved as the boost effect (IMHE) is anything above the the -1dB knee. Other's can disagree as they wish. Take your time getting your head wrapped around this and DO download Duncan TSC and ltspice as soon as you're up to it. Until then... Here is the schematic representing the values specified. Know that there is NO crime or wrong is simply experimenting with almost any values in the preamp and going with whatever sounds good to you

EDIT: P.S. This is your welcome entrance into the "rabbit hole". Enter at your own risk. Sometimes I think I might have been better off to just plug into stuff and use what I thought worked and omit what didn't without trying to "know" more about the actual mechanisms. But I'm not wired like that and my life/playing and other things are permanently affected by the path I've taken. Choose and prosper or lament

[Mr. Bill]

Thanks again for the correction and useful information. Some of it I already understand a little, more is always helpful. Regarding caps, I guess they always need to be in circuit, but the switch determines whether or not they're actually affecting it. That makes sense. As for the rabbit hole.....the rabbit hole is the only part I care about. My best kept secret is that I don't even play guitar! I could fake it in a punk band, but my instrument is the drums. From a recording standpoint though, I'm obsessed with guitar tones. I've never been truly happy with any amp so I started building my own. I'm working on a bit of a arsenal here of great sounds for the studio where I work. The journey is the best part!

[Mr. Bill]

Wait. Serious question. How is my drawing in post 26 different from Dave's drawing in post 5?

[Dave H]

Wait. Serious question. How is my drawing in post 26 different from Dave's drawing in post 5?

I had an extra cap and 10M resistor to ground before the switched capacitors. You need to put the 100k across the switch.