PI cap question

[12AX7]

Assuming for this question that cap types do make a difference, (they do to me) would the .1uf at the "unused" side of the PI on a 2204 make any difference as far as cap type? For example, if i find cap A to sound superior to cap B in other positions, will i hear it there?

[sluckey]

I don't think so. No signal is passing thru that cap. The purpose of that cap is to hold the grid at AC ground, very similar to a filter cap on the B+ rail.

[HotBluePlates]

Try and decide is the only answer that could apply to a subjective result, such as "will it sound better?"

[12AX7]

I'm not asking if it will sound better, only if the difference in 2 caps that i can hear in any coupling position would audible at all there. By the way, it has NFB.

[HotBluePlates]

I'm not asking if it will sound better, only if the difference in 2 caps that i can hear in any coupling position would audible at all there.

What I mean is you're asking if you can hear a difference. I'm saying try it and you tell me. After all, how could I tell if you will hear a difference?

I know it sounds like a jerk answer, and I apologize for that. If it were my amp, I wouldn't bother with getting each cap just-so. But it's your amp, and these things might matter to you. Since it's subjective, the right answer for me may not be the same as for you.

I'm trying to avoid pushing my opinion, but you seem to be asking, so I'll give it. I do hear differences between some caps, primarily when we are talking two caps with different dielectric materials (e.g., polyester vs. polypropylene vs. ceramic vs. mica). When we're talking two caps of the same dielectric material, the differences are much smaller (e.g., Mallory 150 vs. P225 Orange Drop - they're both polyester dielectric).

While I might pick one type or another for a complete amp, it is my firm belief that a change in the value of the cap (0.022uF vs. 0.1uF) will have a very much greater impact on the amp tone than a dielectric change (polyester vs. polypro.), and greater still than switching between two caps of same dielectric/different brand.

So the cost of trying tweak parts isn't worth it to me (and I have tried some that cost a *lot* more than Sozo's). However, folks get very passionate about this stuff, and personal experience is the only thing that will settle the debate as far as I can tell.

Bottom line, I never argue with results. I might think it's not worth it, but if you build a great-sounding amp, then it is worth it. Since tone is in the ear of the beholder, what might not be worth it to me might be the perfect choice for you. So I advocate allowing everyone to choose what is best for their application.

[PRR]

> No signal is passing thru that cap.

I see it differently. There IS signal at the common cathodes. We have to DC bias the grids to this vicinity, but do NOT want the AC voltage there. So all the audio injected via the 1Meg has to be drained-off by the 0.1uFd. It has to conduct good.

BTW, while you would use 0.01uFd to "pass" signal TO 1Meg, here we want to "suck" signal and we need to go toward 0.1uFd (~~10X bigger) for that to happen well.

Added complication in 5F6a scheme: the 0.1uFd returns not to ground but to a tap on the long-tail and the NFB. And the 0.1u carries NFB to the offside grid.

I can't hear any but the worst caps.

If you can, I think you want to try this position.

But why are you asking? You can tack-solder caps in and out faster than you can type.

[sluckey]

> No signal is passing thru that cap.

I see it differently. There IS signal at the common cathodes. We have to DC bias the grids to this vicinity, but do NOT want the AC voltage there. So all the audio injected via the 1Meg has to be drained-off by the 0.1uFd. It has to conduct good.

I figured someone would call me on that statement. I see it exactly the same as you do. The cap's job is to keep signal off that grid. It does so by passing that signal to ground. My thoughts were about passing signal thru the signal chain. My statement is technically incorrect.
 

Added complication in 5F6a scheme: the 0.1uFd returns not to ground but to a tap on the long-tail and the NFB. And the 0.1u carries NFB to the offside grid.

Yes, I forgot about that. Same applies to the AB763 amps.

I take it all back. The type cap may have an effect on the sound. Whether or not you will hear it is dependant on your ears.

[12AX7]

Thanks for the replies so far. Let me clarify why i asked. I have some sozo vintage caps that i'm pulling from an old board to use to make a new board for it. One of the .1uf's was damaged in a horrific soldering iron incident  and therefore i have to use a mallory in either that spot or the presence cap spot. I figured the PI was the more likely spot to show little or no benefit from sozo over mallory. I'm almost done and i have put the mallory at the PI. I can swap them tho. I just hate unsoldering from a fresh new board, as thats why i'm replacing it in the first place. (too much slop from all the tweaking)

By the way, i would just buy another but at something like $7 and probably $5 for shipping i figured one of those 2 positions has got to be unimportant.

[Geezer]

Unless you have dog ears, I seriously doubt you will be able to hear the difference between a sozo & mallory in either of those positions.

$0.02

G

[Ed_Chambley]

> No signal is passing thru that cap.

I see it differently. There IS signal at the common cathodes. We have to DC bias the grids to this vicinity, but do NOT want the AC voltage there. So all the audio injected via the 1Meg has to be drained-off by the 0.1uFd. It has to conduct good.

I figured someone would call me on that statement. I see it exactly the same as you do. The cap's job is to keep signal off that grid. It does so by passing that signal to ground. My thoughts were about passing signal thru the signal chain. My statement is technically incorrect.
 

Added complication in 5F6a scheme: the 0.1uFd returns not to ground but to a tap on the long-tail and the NFB. And the 0.1u carries NFB to the offside grid.

Yes, I forgot about that. Same applies to the AB763 amps.

I take it all back. The type cap may have an effect on the sound. Whether or not you will hear it is dependant on your ears.

Your replies have gotten me confused.  As a rule of thumb, I have never used an expensive cap in the PI as I have been told audible signal change goes to ground and the rest of the signal passes.  I have never even tested any different caps there, I have always just used Mallory's as they are inexpensive.  Even though it is only a $10 decision, all those $10 dollar decisions add up quickly.  It is easy to waste $100 in parts just blindly installing "Special Caps" everywhere.

I know this is the ongoing debate in amp building and I can understand wanting to use special caps in certain places.  So my question is have I been repairing amps incorrectly if I save a guy a couple of bucks using F&T's over Atoms for filters and using Mallorys in the PI even though the rest of the amp may have SoZo?

[HotBluePlates]

So my question is have I been repairing amps incorrectly if I save a guy a couple of bucks using F&T's over Atoms for filters and using Mallorys in the PI even though the rest of the amp may have SoZo?

No.

Repair, by definition, should be about whether the amp works or does not work. Anything else should, I think, fall under "customization." Kinda like, does your car run, or does it run and you swap the stack steel wheels for 18" aluminum billet wheels?

... audible signal change goes to ground and the rest of the signal passes.

It is worth noting the root word of "circuit" is "circle." To keep a schematic from becoming confusing, there is a ground symbol for all points within the circuit which are connected together and treated and a common reference point (perhaps to measure voltage from, or to compare against when determining information about a signal).

Ground is not a black hole down which things disappear. Everybody studying electricity and electronics goes through a phase where they don't know or forget that ground is a return path, and that currents can/do flow through it. Take a look at the simple picture here, and imagine the portion of the wire path from the motor to the battery (-) terminal  are replaced by two ground symbols, one at the motor and the other at the battery (-) terminal. That's how ground symbols replace real return paths along the "circle". If you don't provide those physical return paths, your circuit won't work.

There is debate about what constitutes "signal path." A very good case can be made that everything is in the signal path, due to return current through ground paths.