coupling caps in tonestack... inaudible??

[diddymix]

Hi everyone.. I'm quite new to tube amp circuits and am still learning.. but its been a fun ride so far. I'm just writing in what people thought of this.. I was thinking of buying some PIO caps to try in a tonestack of my amp... then as I was looking at the schematic earlier I thought.. hang on doesnt the cap only pass the signal that gets attenuated/sent to ground? If this is the case, am I right in saying one would hear almost no difference at all between all different brands and dialectrics of the same value cap? Pointless to go there??...
Love to hear your thoughts and what youve all found with your ears too.

Thanks

[pompeiisneaks]

Basically, the supposition that signal does not go through the caps isn't correct, (depending on the circuit of course) Here's a shot of the typical fender tone stack, I've taken some colors for each treble/mid/bass impact on the signal,

but basically the pot sends 'some' of that signal to ground and some 'through to the amp'. I used light and dark of each color... it's a bit sloppy, but you should get the idea.

the type of cap used in theory 'might' impact the tone, but that's a hotly debated topic, but for sure some of the signal passes through it, and some goes to ground.

See if the crappy paint illustration helps of this bassman tone stack:

Treble: blue
Bass: green
mid: orange

signalpath.png

You should see, say with treble, that some of the signal goes through the cap, and on to the next stage, while some of it goes down to ground.

Hope that makes sense, I may even have the routing wrong, but that's the general idea of what's basically happening, you use the pot as a voltage divider to send some of the signal through, and some to earth.

~Phil

[xtian]

And just to clarify: "coupling caps" are used to block DC between amplification stages. We'd call these caps in the tone stack...wait for it..."tone caps".

[echuta13]

We'd call these caps in the tone stack...wait for it..."tone caps".

[sluckey]

Keep in mind that those three "tone caps" are also blocking dc.

[diddymix]

Oh I see! Great thanks, so depending on the pot some of the signal stays in the circuit.. well to me that says that the cap type will certainly be somewhat audible. Excellent drawing really helped me understand!!
Have any of you guys auditioned PIO caps, like the k40 Y type??. Any thoughts? I think you see them in older fenders..

[roberto]

On mid and high gain amps (from JCM800 and over) the tone stack is where, usually, the voltage swing is the widest, so where the caps are more stressed, so where their characteristics come our more.
Some producers (EG Engl) uses a cap before the tone stack to avoid DC on tone caps, and without that cap the sound is indeed different.

[pdf64]

On mid and high gain amps (from JCM800 and over) the tone stack is where, usually, the voltage swing is the widest, so where the caps are more stressed, so where their characteristics come our more....

It's worth noting that, to my knowledge, this is a hypothesis, rather than proven theory; and it might seem to goes against the importance some folks place on the particular cap type used for the instrument's tone control (where applicable), ie where the signal Vac is at/near its lowest?

...the voltage swing is the widest, so where the caps are more stressed, so where their characteristics come our more.
Some producers (EG Engl) uses a cap before the tone stack to avoid DC on tone caps, and without that cap the sound is indeed different.

With or without the dc blocking cap / polarising voltage, the Vac feeding the tone stack caps is surely barely affected?
Whereas the additional series capacitance of a dc blocking cap will reduce the effective cap values in the tone stack somewhat, which, to me, seems a more likely cause of a tonal change?

[roberto]

It's worth noting that, to my knowledge, this is a hypothesis, rather than proven theory;

Well, that capacitors are more prone to divert from specs when close to their limits, is not hypothesis. I also remember some tests done by caps producers, not dowsers.

and it might seem to goes against the importance some folks place on the particular cap type used for the instrument's tone control (where applicable), ie where the signal Vac is at/near its lowest?

It's something I've never tried myself directly, so I cannot comment, but alot of people hear differencies here also. This doesn't mean that there are no differences on the opposite conditions.

Whereas the additional series capacitance of a dc blocking cap will reduce the effective cap values in the tone stack somewhat, which, to me, seems a more likely cause of a tonal change?

Usually caps are between 100 n and 1uF, so their effect is not so big even at lowest guitar frequencies (19k4 on the former, 1k94 on the latter). If that is the case, why not just a 2k2 up to 22k resistor in series instead?

[Stevem]

It's my finding that once you split the signal up within a tone stack that for instance the quality of the cap used to feed the bass pot and the mid pot have far less of a effect on the sound then the quality of the cap for the treble pot.
This assumes that the afore mentioned bass and mid cap have low leakage, the ESR of them does not seem to come into play much at these frequencies.

On the other side of the coin when you are passing full frequency audio thru a cap like at the PI input, or the PI output coupling caps then everything about there quality can matter, especially what temperature they can stand up to when mounted near the output section of a amp.

[Ray Barbee]

Some producers (EG Engl) uses a cap before the tone stack to avoid DC on tone caps, and without that cap the sound is indeed different.

I've experimented with 1uf caps before the tone caps and could hear no difference. I have pretty sensitive ears. Some brands like diesel use a .22uf in the same spot, again, I just don't hear it.

[roberto]

Hi Ray,

I think that the debate "I hear it" vs "I don't hear it and my last hearing test shows flat from 20 Hz to 20 kHz" without any other comment is quite old and it is proven that brings nowere.
Oh well, to be fair it usually brings granitic minds to create walls and start flaming. It is not my case and I'm sure it is not yours.

Caps have usually huge tolerance compared to resistors, so even swapping two caps of the same brand can give a difference. This is well known.
Then there are circuits where it can be heard more, and some less. There are points of the circuit where it can be heard more and some less.
I noticed the difference when in extreme high gain (not my bread), way less in normal clean.

That said, I don't use the Engl/DieZel preEQ-cap in my amps.

[Ray Barbee]

Hi Ray,

I think that the debate "I hear it" vs "I don't hear it and my last hearing test shows flat from 20 Hz to 20 kHz" without any other comment is quite old and it is proven that brings nowere.
Oh well, to be fair it usually brings granitic minds to create walls and start flaming. It is not my case and I'm sure it is not yours.

Caps have usually huge tolerance compared to resistors, so even swapping two caps of the same brand can give a difference. This is well known.
Then there are circuits where it can be heard more, and some less. There are points of the circuit where it can be heard more and some less.
I noticed the difference when in extreme high gain (not my bread), way less in normal clean.

That said, I don't use the Engl/DieZel preEQ-cap in my amps.

Not sure you understood my point. Not claiming that cap type is inaudible in a tone circuit, I'd argue the exact opposite. Only that blocking dc before the tone caps with a cap large enough to pass all present audio frequencies is, in my experimentation, inaudible vs. not doing that, all other things held constant.

So why do that? Various reasons; you can use tone caps with lower voltage ratings, you can implement things like switching multiple values of midrange capacitance a little easier, etc.

[pdf64]

...

Whereas the additional series capacitance of a dc blocking cap will reduce the effective cap values in the tone stack somewhat, which, to me, seems a more likely cause of a tonal change?

Usually caps are between 100 n and 1uF, so their effect is not so big even at lowest guitar frequencies (19k4 on the former, 1k94 on the latter). If that is the case, why not just a 2k2 up to 22k resistor in series instead?

When a dc blocking cap is inserted between the tube and tonestack, the source impedance feeding the tone stack will now have a frequency dependent element. Agree that there may not be much of an effect if it's 1uF, but moreso with lower values.
And as the same tone stacks get used for bass amps, and as their bottom strings tuned down to D or even B becoming more common, the standard tonestack models / calculators may have errors.

With a dc blocking cap, the bass cap becomes kinda redundant, eg https://el34world.com/charts/Schematics ... ssv50w.pdf