Bipolar cathode cap?

[Guus]

Hi Guys,

I've learned the effect of placing a electrolytic cap over a cathode resister of a tube. (a frequency dependent gain boost/low frequentie roll-off).

I find out that some amp builders use a 'bipolar' electrolytic capacitor instead of the regular ones as a cathode cap.

I can't figure out what the advantages are of using a bipolar electrolytic cap.
To test, I swapped between regular and bipolar and couldn't realy hear any tonal difference in the amp.
I'm confused, bipolar caps are more expensive, so there should be a (good)reason to use them.

Anyone knows why?

regards
Guus

[Blackburn]

I can't tell you how they differ from polarized caps in tone, but I can assure you it isn't a big deal. Just a cap doing what it does. Not sure where you've seen them or for how much, but you can get these for pennies on PartsConnexion. I got this green one there for probably around $.30 and it works great. I love green.

[JazzGuitarGimp]

IMO, usign bipolar caps for the cathode bypass is just misguided engineering. The standard (polarized) electroylytuc caps don't like to see reverse polarity. In other words, the voltage on the plus terminal must always be more posative than the voltage on the negative terminal, regardless of whether the voltage across the cap is AC or DC. A good example of this is the power supply filter cap; here, you have both a DC component as well as an AC component (ripple voltage). But the AC component sits on top of the DC component, and its magnatude is such that the positive terminal of the capacitor never comes close to crossing over the 0V line into negative voltage terratory. When an electrolytic capacitor is used to pass or couple an audio (AC) signal, where the polarity across the cap alternates between forward and reverse, a non-polarized capacitor is mandated. A good example of this is a passive speaker crossover network.

Looking at the cathode bypass cap, we see there us both a DC component and an AC component to the signal present across the cap (and I think this is where some designers get confused). But since the tube has no way to pull the cathode below ground, thus pulling the capacitor's posative terminal negative, there is no danger of reverse polarity across the bypass cap.

[JazzGuitarGimp]

That _is_ a wonderful shade of green!

[Blackburn]

Great info, Jazz. I haven't transitioned over to bipolar caps for regular use, I just pulled some from PcX to try. Very cheap and worked great in that Champion 600 rebuild. I can't really tell a difference in tone vs the Spragues I'm always using.

[JazzGuitarGimp]

A bipolar electrolytic cap is nothing more than two standard electrolytics connected in series, posative terminal to posative terminal. I suspect this is why there is no discernable difference in tone. When a circuit calls for a non-polarized cap, and you don't want to run to the store to get one, you can connect two lytics together to substitute for a non-polarized cap. The usual formula for two caps in series applies, of course. So two 50uF caps in series results in a single, 25uF cap.

[Blackburn]

Thanks for the tip! I didn't know that about these. I just grabbed what I had on hand for that Champion and didn't want to use the expensive Spragues.

[Cliff Schecht]

A bipolar electrolytic cap is nothing more than two standard electrolytics connected in series, posative terminal to posative terminal. I suspect this is why there is no discernable difference in tone. When a circuit calls for a non-polarized cap, and you don't want to run to the store to get one, you can connect two lytics together to substitute for a non-polarized cap. The usual formula for two caps in series applies, of course. So two 50uF caps in series results in a single, 25uF cap.

This. There is the notion that not having bleeding/balancing resistors will make it to where the voltage doesn't distribute evenly across the two ecaps but this isn't a problem unless you are handling voltages that are beyond the rating of the capacitors. That's where one cap will take the majority of the voltage and fail at some point.

[Guus]

Looking at the cathode bypass cap, we see there us both a DC component and an AC component to the signal present across the cap (and I think this is where some designers get confused). But since the tube has no way to pull the cathode below ground, thus pulling the capacitor's posative terminal negative, there is no danger of reverse polarity across the bypass cap.

Hi JazzGuitarGimp,
Thanks for the reply.

Just that, as you mentioned in quote, made me earlyer confused when I saw
the bipolar electrolytic caps in a German amp which I had for repair.
It set me to thinking and then also realising that Marshall uses oft (bipolar signal) caps as a cathode bypass. (680nF) I aspect Marshall chose a (signal) cap because you can't find a electrolytic cap that small.

So, all what I could think of is the goal of de the bypass cap for AC level, ie raising the level of a specific frequentie range passing through the tube. Ofcourse the cap doesn't contain any signal but it wil set the frequentie handling of the tube.

Could it be that the selected (lower) frequenties flows, via the bipolar, in a better/nicer way to ground?
(let's say in a more gradual or steeper way?)

regards,

Guus

[martin manning]

Guus, that 680n value is probably the reason... The value is too small for an aluminum electrolytic, expensive and large in size as a non-polar film. That doesn't mean somebody won't claim they can hear a difference...