cathode cap question

[Tdale]

On the Matchless DC30, the first 12AX7 is wired in paralell. The cathodes share a 1.5K resistor, and there is a 25uF/25V bypass cap. But there is also a 0.1uF bypass cap in addition.... What is the point of this cap?

To me it looks like the only thing that happenes is that the total bypass cap value is 25.01 uF in stead of only 25uF.... ?

Tommy

[dehughes]

On the Matchless DC30, the first 12AX7 is wired in paralell. The cathodes share a 1.5K resistor, and there is a 25uF/25V bypass cap. But there is also a 0.1uF bypass cap in addition.... What is the point of this cap?

To me it looks like the only thing that happenes is that the total bypass cap value is 25.01 uF in stead of only 25uF.... ?

Tommy

Wouldn't you then get an even greater boost in the frequencies covered by the .1uf cap, versus just an overall boost via the 25uf cap? Or am I missing something...?

[Tdale]

Doesn't caps let high frequencies through. The smaller the value, the less bass is let through.

The frequencies that goes through the 0.1 uF, are also going through the 25uF..aren't they?

Tommy

[Mickey_C]

Is there a switch that adds the 25uf or something?

Because the difference between 25uf and 25.1uf would be inaudible.

I would suspect something more like the .1uf, and then a 25uf provided via switch as a switchable fatness boost.

Because the difference between .1uf and 25uf, when switched is HUGE. The .1uf boosts the highs... and the 25uf kicks up the whole range (including the highs), so it makes sense to switch it in/out maybe via a push/pull.

A link to a schematic would be helpful.

mickey

[Sonny ReVerb]

This is way over my head, but basically the idea in paralleling an electrolytic with a smaller value film cap is to overlap the high-impedance (resonance) point of the "bad" cap by a low impedance from the second cap. I've seen this more in power supply filtering.

Here's a quote from a website that ran some tests on the topic:

...the ability of the larger capacitor to supply instantaneous current demands is not impaired, so the idea of using a small cap ("they have a higher self-resonant frequency") in parallel with a big cap is essentially nonsense - more capacitance equals more energy storage.

Here's the link to the full page: http://sound.westhost.com/articles/capacitors.htm

[drz400]

This is way over my head, but basically the idea in paralleling an electrolytic with a smaller value film cap is to overlap the high-impedance (resonance) point of the "bad" cap by a low impedance from the second cap. I've seen this more in power supply filtering.

Here's a quote from a website that ran some tests on the topic:

...the ability of the larger capacitor to supply instantaneous current demands is not impaired, so the idea of using a small cap ("they have a higher self-resonant frequency") in parallel with a big cap is essentially nonsense - more capacitance equals more energy storage.

Here's the link to the full page: http://sound.westhost.com/articles/capacitors.htm

exactly....Old audiophile trick, but I dont think it means anything used in a cathode.
Only coupling situations and power supplys. There are many things in the Badcat stuff that dont make a bunch of sense. Seems like some guys are beating them up about reliability as well at thegearpage.

[mhuss]

This is an old audiophile (phool?) trick. All electrolytic capacitors start to get higher in impedance as the frequency gets higher (at some dog-hearing-only frequency). The small value bypass ensures that the impedance of the combination stays low up to close to RF.

Also, you'll sometimes see this done in a power supply to help keep RFI off the B+/B- lines.

This makes sense in a ultra high-end hifi system where you're trying to get every possible transient and nuance, but in a guitar amp, in a cathode circuit, it's way overkill.

--mark h

[krash]

I agree it's overkill in a guitar amp cathode, especially considering anything over about 7kHz is of almost no import for the rest of the circuit.

However, electrolytic caps (and all caps, to an extent), by virtue of their construction, are primarily inductive at some frequency above their self resonance. So you would normally put a bypass cap across them in order to make sure the circuit remains primarily capacitive (low impedance) at higher frequencies. However putting a .1uF cap across a 25uF is kind of out of the ordinary. Normally you'd step down in decades like put a 2.2uF and then maybe a .22uF, depending on the selection of caps and their construction.

For radio frequency stuff or computer power supplies, this is very important stuff. For guitar amps, not worth the trouble.

My bigger suspicion is this is a band-aid to cure some oscillation.