which are the decoupling caps?

[cooldude666]

Hello, my sunn 200s is motor boating, going to replace the decoupling caps in the preamp coming from the B+ (as per geofex's advice), problem is I'm new to this whole world and don't really know what's what. How do I tell which caps these are?

thank you!

[Jazz P Bass]

Unfortunately, the power supply schematic is missing.
But on your copy, the caps will be attached to the points labelled "A" & "B".

[cooldude666]

here's a better schematic,
is it that 12pf cap connecting to the phase inverter tube?

[Enzo]

Lower left in the second posting is the power supply. The B+ (high voltage) supply has filter caps in a row to the left of the standby switch. The first one, by the switch is a filter cap. The next three have a letter over them, A,B,C. We'd probably call the cap at A a filter cap too. The ones at B and C are also filter caps, but their main importance is as decoupling caps. If they are not working well, then the preamp stages can interact with the power amp, causing motor boats.

"Filter cap" and "decoupling cap" are descritions of their function in the circuit. They are not different kinds of capacitors.

The 12pf cap is in parallel with the 270k resistor, the plate load of the pentode. But due to the direct coupling of the pentode and the PI triode, yes, it is connected to the PI. But it is mainly there for what it does with the pentode.

[cooldude666]

Ah, thank you! I thought that all of them were filter caps. Like I said- I'm still learning!
That said, i replaced those caps about a year ago, could they be bad already? The filter and decoupling caps (that whole section) is contained in a multi-section can cap. Could this be symptomatic of a larger issue? I read http://music-electronics-forum.com/t24781/ and it seems to make sense that higher line voltage would be overloading them. Are soundmasterg's suggestions sound for my 200s? It seems to have the same can cap- what about his recommendation of a solid state rectifier? I'd like to keep my tube rectifier, I dig the sag.

[Enzo]

They are all filter caps, but by the time we get to the second capacitor, there is next to no ripple left to filter, so the remaining filter caps are mainly there to isolate the stages - in other words to decouple them from the other stages. You could say that the decoupling amounts to filtering out any traces of signal that would have gotten into the B+ supply.

Just get a single cap that meets the spec, I have adrawer full of 22uf/500v ones I'd probably chose. Clip the negative end to ground at the can cap. Now clip the positive end to the C point in the supply. Your new cap is now parallel to that section of the can cap. ANy change? Try the test on the B node next, any difference? If tacking on that parallel caps helps, then that means your can cap is not doing the job for whatever reason.

[cooldude666]

the gz34 rectifier tube appears to have a maximum filter capacitance of 60uF.
That post I linked to recommends 2 80uF 350v caps (for a total of 40uF/700v), 2 40uF 350C, 20uF 500v, and a 20uF 450v. These are all less than the max 60uF. Is it a good idea to install this? What impact will this have on sound if any?

[52 Bill]

the gz34 rectifier tube appears to have a maximum filter capacitance of 60uF.
That post I linked to recommends 2 80uF 350v caps (for a total of 40uF/700v), 2 40uF 350C, 20uF 500v, and a 20uF 450v. These are all less than the max 60uF. Is it a good idea to install this? What impact will this have on sound if any?

The 60uF maximum is how much you can connect directly to the output of the rectifier tube, not the total capacitance that is in the entire amp's circuitry.

What Enzo was suggesting was to temporarily parallel the new cap across the existing ones to find out if any of the existing ones were no longer performing correctly in circuit. If you add a cap across the existing one and the problem goes away, then you need to replace that cap. If the problem remains, then that cap is probably okay.