Tooling up for a build, all my dumb questions in one thread

[Tahoebrian5]

I'm starting to feel like Ive been creating too many threads for all my silly questions so I thought I might just start one general thread. I'm putting together an experimental set up similar to some bread board setups I've seen. My tendency to over analyze everything is only going to get me so far so time to experiment. Ok, on to the first of many questions.

Can anyone comment on the pros or cons of using partial bypass caps to cut lows on a gain stage as opposed to using a smaller coupling cap? My first thought was the result could be about the same if values were carefully chosen. Thinking about it more though I wonder if that is true. If you allow lows to get amplified with the bypass cap it seems like you would have more harmonics because more freq's are present to interact with the distortion process. Anyway just curious if anyone has any thoughts on this.

[Chuck H]

The coupling cap is a blunt rolloff. The limited bypass cap will have a shelving effect relative to the unbypassed stage gain. Also, the limited bypass cap creates a LF local feedback circuit. This creates a different dynamic impedance which is subject to the input circuit for the following stage.

Now, this is just touching on it. Trying to calculate what even these couple of differences make tonally is immeasurable (for me anyway). It's much easier to try things both ways and listen to the differences. AND since the details change with different circuit designs it's probably easiest to continue to experiment.

I know some types REALLY want to be able to quantify this stuff. When you consider how things change with FB, input/output impedances, dynamic circuit impedances and gain, not to mention all the changes that happen when a tube begins to distort and idealizing for all points in between, it just makes more sense to keep experimenting. Even if you're frikin Einstein it's not possible to account for all the variables that will affect what you actually hear and end up having a preference for. Many have tried. It is possible to develop a feel for what effects different changes to different circuits will have. This is the "black art" part of tube design. Trying to lump it in with the purely functional tech end of the design aspect only works on a limited basis.

My advice is to learn what you need to for idealizing circuits and what you need to not burn them up. After that some experience will guide you toward how to alter and tune circuits for the sound you want.

How 'bout that!?! A post on design ideology and no math at all

[dwmorrin]

The coupling cap is a blunt rolloff. The limited bypass cap will have a shelving effect relative to the unbypassed stage gain.

That's a great point, Chuck, and something I hadn't learned yet.
Just to add some numbers to it, the coupling cap is -6dB/Oct and keeps going down to dc.
The bypass cap's "shelving eq" means that the gain drops -3dB/Oct and stops where the cap appears large compared to the cathode resistor.
That's very useful info!
Plots and math for those inclined found here: Designing Common-Cathode Triode Amplifiers

[Tahoebrian5]

I had not come across aikens page before. Some good stuff there.

[Chuck H]

I had not come across aikens page before. Some good stuff there.

There may have been updates since, but I printed the whole thing about ten years ago. It's about as thick as a small phone book! I use to browse it to pass time. Great combination of practical tech and how it applies to guitar amps.

[yankeerob]

There may have been updates since, but I printed the whole thing about ten years ago. It's about as thick as a small phone book! I use to browse it to pass time. Great combination of practical tech and how it applies to guitar amps.

I have built amps according to Randall's white papers for years... and swear by his 70% max dissipation rule... it really does work, sounds great and prolongs valve life... fantastic chap for sharing with us all... and I can confirm that he took them down for a while and appears to have gone through them all again with some amendments and put them back up... they're all in my reference folder!!!

Chuck has the right idea T-Brian... you really have to experiment to find what you are looking for... or have a mate that has tried it before... it's the fun end of amp building - provided you have the patience for it - good luck!!

[Tahoebrian5]

I am in full agreement. Got my peg board cut out today, have some pre wired tube sockets with terminal strips, and some other stuff. Trying to figure out the best way to get it set up. I'll post some pictures when I have some more progress.

[Tahoebrian5]

I'm fishing for ideas on how to implement something better than the traditional tone stacks. Maybe something along the lines of a pultec eq. Has anyone had any luck implementing something similar or know of any suitable eq circuits, hopefully without the opamps, that are recommended studying material?

Maybe somthing like this..


Is there any possibility of driving this off of a CF, and using a normal recovery stage and doing away with the transformers? I have seen the inductor sets are available although not cheap.

[Chuck H]

Inductors in preamps can pick up a lot of radiant noise. Mesa pulled it off by placing the EQ at the end of the preamp, careful layout and padding the signal down. Might be tougher for a novice to get good results. And I would be a novice in this area since I've never tried it.

Sure you could operate that EQ from any low impedance output. Finding the higher value inductors for small signal application would be the tricky part. When a company makes 10,000 pieces they can design and order custom parts from a component maker. IME finding these sorts of inductors for prototyping can be difficult. Mouser has some stuff.

Other considerations would be the difficulty of implementation. All those inductors, caps and rotary switches are going to be a real chore to wire and clunky in size with the parts available. Further, you may need to tweak values. That would be a real PITA. Whether this EQ will be within the gain producing stages or part of an "EQ after signal processing" like a modern uber gainer or modeling amp will also influence the affected frequencies. The Pultec is intended to be used as a FINAL EQ. This isn't often done with guitar preamps.

I just think it's overcomplicated and overkill for a guitar amp when there are many (MANY!) simple circuits that work just fine.

JM2C

[soundmasterg]

Ampeg used an inductor for the mids in the SVT and used it to good effect.

Greg

[Chuck H]

Ampeg used an inductor for the mids in the SVT and used it to good effect.

Greg

+1
Love that circuit. But it's ONE inductor. The actual EQ for that amp is more typical. The odds of finding an offending radiant field go up dramatically if there are twenty resonant circuits attached to long coils of wire on the preamp end of the chassis.

[Tahoebrian5]

The SVT is quite interesting. Controlling the mids via a filter in a nfb loop? A general question on nfb, isn't phase an issue when using nfb? By filterig the signal wouldn't the phase be altered and cause havoc when trying to cancel frequencies?

[The Dude]

I'm not sure if I entirely understand your question, but NFB works BECAUSE of phase cancellation, so that is the whole point. If we then phase cancel only at selected frequencies, we effectively create equalization.

[Chuck H]

I'm not sure if I entirely understand your question, but NFB works BECAUSE of phase cancellation, so that is the whole point. If we then phase cancel only at selected frequencies, we effectively create equalization.

And A LOT of things change when the FB stage starts clipping. This, I think, would be the relevance of Brian's observation. So, yes, things get hinky WRT phase when you use NFB for frequency shaping when that gain stage is clipping. But that can also be part of the effect/sound/tone/whatever provided the circuit doesn't cause instability. You can check in with the Stones re: whether the Ampeg circuit sounds good clipped

[Tahoebrian5]

The distortion issue in FB loops is also a good point but was not what I'm referring to. To simplify things just say there is a nice symmetrical sine wave for a signal. We take an inverted signal off of a CF for example and feed it back to the input. So now where the input signal is at the top of the wave form, I think that is at 90 degrees if I remember correctly, the inverted feedback signal will be at the bottom (270), so the high and low waves cancel each other perfectly, in an idealized model. Now say the feedback loop has a filter in it. This will change the phase of the wave in the FB loop so now the low 270 part is not lined up with the input signal top of wave at 90.