Filter Capacitor Questions

[dujuarez]

I'm trying to use some capacitors I have on hand for filtering power supply, I want to use the following with the 100uf/350v in series:


2 - 100uf/350v      instead of   2 - 100uf/350v
2 - 47uf/500v                         3- 20uf/500v
1 - 22uf/500v

Is it ok to use this configuration? And also I read about doing cap job and using different values, since some of the values aren't available and that it can "tighten up the bass". If I can use these values, how will it effect the sound?

[eleventeen]

Too-big filter caps right after a rectifier tube can create a situation where inrush current on first-turn-on can cause arcing in the tube. That's IF you're using a tube rectifier. Regardless of how many ufds you wish to throw into the amp, the ecap right after the tube rectifier has a "max" rating if you look at the tube manuals and it's usually 30 to 40 ufd. Going from 30 to 40 means little. Going from 30 to 100 ufd is asking for trouble, though I see you wish to place them in series. In series, caps do not add; they act like R's in parallel: product/sum.


If you want to try such a thing, it would be my preference to blow up a cheap old 5Y3 instead of a nice new GZ34 or a vintage GZ34.


When you series connect these caps, it is essential that you place "balancing" resistors across each of the stacked-up caps. I never thought this was important, but it is. If you don't, somehow most of the B+ you apply to the stacked-up caps will appear across only one of them, and this is how you blow 350 v caps, by placing 425 volts on them out of a 450 v supply. 220K seems to be the usual value, usually want at least 1 watt but 2-3 watts is better. Go look at Twin Reverb schematic. This is especially true if you are using older, salvaged caps.


At some point these things take up too much room!

[HotBluePlates]

I'm trying to use some capacitors I have on hand for filtering power supply, I want to use the following with the 100uf/350v in series ...

I think your question was, "Can I use 2x 47uF and 1x 22uF caps, rather than the stock 3x 20uF caps?" Is that correct?

The answer is yes, you can.

... I read about doing cap job and using different values, since some of the values aren't available and that it can "tighten up the bass". If I can use these values, how will it effect the sound?

How will it sound? I dunno, it sometimes varies by specific amp-topology. Personally, I'd just try it an see what you think.

The amp will only "tighten up" if the amp circuit in question typically draws a lot of current, and was under-filtered to start with. That is, Small Filter Caps + Big Pulses of Current Draw (mainly from output tubes, with big signal) = Supply Voltage Sag.

Small filter caps store less charge, which is drained away more-easily by heavy current demands, resulting in the voltage at those caps dropping. But that supply voltage sag also is affected by the rectifier & power transformer's ability to re-charge the filter caps. Low-resistance rectifiers (bigger tube or solid-state) and low-resistance power transformers (big core and big-wire windings) enable faster re-charge of filter caps. Drain and recharge happen for different lengths of time, with the drain portion typically being much longer than recharge. And how low the voltage dips during drain is all about filter cap size and circuit current demands.

This is the long way to explain that bigger preamp filter caps (which is what I think you're asking about) probably won't translate to the solid-bass effect you've read about, because preamp stage current demand is quite small. I'd expect an audible sound/feel change to occur when raising filter cap values for the output tubes (plate and possibly screen), because that part of the circuit draws very much more current from the power supply and is more likely to exhibit voltage sag with small filter caps.

As Eleventeen notes, a too-big filter cap right after the rectifier, in the absence of significant PT winding resistance or added series resistance, can draw a big enough peak current during cap recharge to pop a rectifier.

[dujuarez]

Sorry, I forgot to mention that I'm trying to solid state rectification.

[eleventeen]

You still need the balancing resistors if you are stacking caps, placing them in series. I just learned this the hard way: I did what you are doing and stacked up two (I think they were) 350 volt caps for a temporary patch-together 450 volt supply. And, I actually soldered resistors across them BUT they were 470Ks I happened to have in the junkbox. They did work for a few days. I thought I was good to go, then I actually measured the volts across the caps in operation. Cap #1 had over 400 volts on it and cap #2 had 25-35 volts on it! I was very surprised. Cap #1 blew.


It MAY BE that once you get a real operating load on such a supply you are building that the voltages equalize. I would suspect they would. However, most people when building an amp, get the power supply working first, without the rest of the amp built, and check it out. And in that time period, you will probably not be drawing any load from the supply to speak of. That is what the bal resistors *help* to do. Without those equalizing resistors, slight differences in the caps will cause most of available volts to pile up on one and not be equally shared. This is where and how you can get an overvoltage condition, and said condition blew out one of my temporary caps in the above example.

[dujuarez]

Thanks, Eleventeen for the warning.