Same stage different stage

[nsubulysses]

I am a little confused by the description I am reading about equivalent parallel gain stage vs single triode gain stage. Also, is it actually not possible to convert my parallel gain stage to a single triode gainstage faithfully because I am using split cathode arrangement?

It seems based on Merlin's description I should convert the parallell stage to this (pictures atttached) to hopefully maintain the same frequency response and gain. Is this incorrect a bit?





Also please excuse my 1794 ohm typo which should be 1784 ohms

[Malcolm Irving]

The single triode version would be a bit different.

In the parallel version, the different cathode components mean that the bias is a bit different for the two triodes and also the bass roll-off for the two triodes is at different frequencies. Instead of the usual single shelf filter, you have a kind of double (or two-step) shelf.

However, notwithstanding the above, I think the way you have calculated the values is correct and you will have a single triode stage which behaves 'close' to the original parallel version.

What is the purpose? Saving a triode for some other function? Or just a theoretical exercise?

[nsubulysses]

Thanks for your response. I appreciate it

Since you ask, I am building a 2 input amp to capture (or try to) two tones which is the bright staccato clean sound that is jangly and more tight and responsive to pick attack, and a second tone that is more bassy, looser, slower sounding tone with floppy pick attack for the 2nd input for mellower, more bassy blown out playing.

I have the parallel gain stage for the bright staccato input (V1) and a single triode gain stage for the bassy loose input (V2A). Then a tone stack in V2B then V3 is the phase inverter.

Was thinking about de-paralleling my input 1 parallel gain stage because then it could be reconfigured as input 1 gain stage (V1A) and input 2 gain stage (V1B), summed into a 2nd gain stage (V2A), tone stack (V2B), then phase inverter (V3).

I want to mess with having a gain switch where in low gain mode it bypasses V2A and goes straight to tone stack for a really loud clean amp like I have already, and then with switch in high gain mode V2A gain stage can be used for more rock vibes. It could probably just be the same amp it is now but with a gain switch for more extension into rock/overdrive tones if I de-parallel the input 1 gain stage is what I'm pondering about.

I know with different cathodes in a parallel gain stage you are sort of "creating a new sound" with two differently biased gain stages being summed together for one big gain stage. But I am also wondering if it will just sound basically the same with one gain stage and then if it is it actually worth it?

[Chuck H]

Well... You haven't shown us much of what the circuit does WRT AC load following the parallel gain stage. That could have an affect on overall performance since you (may be) effectively doubling the output impedance of the first gain stage. I have to say that in this situation I think experimenting and tweaking, then falling back to the original circuit for comparison, are the way to go. You should be able to get acceptably close? That depends on your ears. So many things will change that a positive affirmation before ear testing seems impossible.

[nsubulysses]

here is what it is basically



Sorry the tone stack is drawn kinda sloppy. if it is too hard to read it is a derivative of some sort of combination of the carlsboro and carvin tone stacks posted on this old MEF thread -- http://music-electronics-forum.com/t25669/

I was trying to use my V3 as an FX loop or another gain stage after the tone stack but it didn't really work good and I abandoned that gain stage. That's why I'm now wondering about gain stage before the tone stack for more range of clean and rock tones, which in retrospect makes a lot more sense whoops. Check out the wax dipped 470pf 1% precision capacitor. Enzo gifted it to me so I threw it in my prototype for a magic vibe

[Chuck H]

With what looks to be about 400k-ish of AC load (that's just "ish" I didn't get nerdy on figuring it out). The parallel triodes really aren't all that different. I think you'll be ok changing to a single triode if you're not too OCD about it sounding "EXACTLY THE SAME DAMMIT!" Just crossover the values and use a 150k plate load, a 1.8k cathode and a 2.2u or 3.3u bypass cap...

"But wait.?. That's the same value bypass cap it has now?" Except that with the existing arrangement the 2.2u bypassed triode is also the hotter triode, so the effect of the shelving there will be a little more pronounced. And since you'd be bypassing a higher value cathode resistor the impedance of the circuit will shift the knee of the bypass cap down a little, giving a little more low mids in the fully bypassed triodes stead Maybe 3.3u would be better but that's for ear testing.

If you REALLY want to get jiggy with it you could shelf the cathode bypass curcuit. Maybe a pair of 820R resistors in series with a 2.2u parallel to one and a 10u parallel to the other. That won't be exact either, but it's really getting closer. You might even stagger resistor values to dial it right in. Keeping total resistance above 1.6k and below 1.9k. That bias range is also just approximated, but it should keep the clipping behavior similar to what you had with the parallel triodes. Of course, that stage never clips unless you plug a big, fat booster in up front, which can happen.

[nsubulysses]

still messing around with this circuit a bit.

Chuck thanks for the awesome tips!

I had a Sound City Concord in for repair. I noticed something, for the tone stack section it has the volume pot before the tone controls. Hmmmm, a lot of the other schematics I was checking out for ideas had tone controls then volume control last.

Does anyone know, without switching around my whole circuit just to see, if tone controls then volume, or volume and then tone controls would allow for less volume change when adjusting treble and bass controls?

I feel like how it is now with tone controls then volume, there can be a pretty decent change in volume when T and B are adjusted. I wonder if bumpin the volume pot earlier in the circuit would help this.

but also, would it change the frequency response in some way if volume was on 3 vs. 7?

Here is Sound city screenshot


Here is a carlsboro I am comparing it against, channel 1 specifically
http://schems.com/bmampscom/carlsboro/60tc.jpg

thanks for any insights

[Chuck H]

Well... Of course there is a change in volume when adjusting the tone controls BECAUSE it changes amplitude @ frequency. The only way to avoid this would be to design a tone control that adjusted the "other two" tone controls analogous with the adjustment of any one! This has NEVER been done in a guitar amp to my knowledge. You're looking for a refinement that has never been.