What is the cap value here and what do I need?

[cap217]

This amp has me questioning everything and thank you for putting up with these questions but I am learning from you guys.


I attached the stock layout and a pic of a modded amp and a pic of my amp modded by pisonic. I am questioning the filter caps labeled c3 and c4. The stock layout shows 10nF values and the modded values show 100nF. My amp has a sozo 104k which I believe is 10nF in c4 but c3 is a tan cap or resistor with colors orange-orange-yellow-gold and I think that's 330k.

My question is that in every amp these 2 caps are the same kind and size. But in mine they are different. Why is that? Also, is this really 330k? If so, isn't that way off?

[Reeltarded]

First this helps.

http://www.justradios.com/uFnFpF.html

The 104k are .1uF/100nF. Ten times as big as stock which are shown to be .01uF silver micas in the scheme. Guys, is that some kind of smoothing scheme? I have no idea. Those aren't actually filter caps, per se; they must be like the .22uFs I am used to seeing in Marshalls?

The guy must really know what he's doing. I sure don't.

[Blindog]

First this helps.

http://www.justradios.com/uFnFpF.html

The 104k are .1uF/100nF. Ten times as big as stock which are shown to be .01uF silver micas in the scheme. Guys, is that some kind of smoothing scheme? I have no idea. Those aren't actually filter caps, per se; they must be like the .22uFs I am used to seeing in Marshalls?

The guy must really know what he's doing. I sure don't.

Looks like you got it right Miles. Found a thread at AX84 that discusses this

http://www.ax84.com/bbs/dm.php?thread=281497

The small caps bypassing the electrolytics are supposed to reduce ESR at higher frequencies...and one guy says that it can improve the slew rate, which improves the transient response of the amp. Others have said they have tried it but didn't notice anything remarkable.

As to the OP's question of what the value should be, some say to use a factor of 100, eg use a .47uf across a 47uf filter cap. If you read the thread you'll see that this issue is kind of contentious.

Good post though, I've never seen this before

Mark

[Jana]

I sometimes put a .022uf cap in parallel with the filter cap of the first gain stage (sometimes the 2nd stage also). It adds a bit of clarity and openness. Before doing that, however, changing the rectifier diodes to UF type is a good idea. (I'm not talking about vox amps in particular--referring to all amps)

[cap217]

Follow what your saying so far but why is my c3 a sozo cap and why is c4 not? It looks like a resistor but in every schematic and pic I see, these 2 caps are always the same and usually Mallory's or sozo's.

Should c4 be replaced?

[Blindog]

Follow what your saying so far but why is my c3 a sozo cap and why is c4 not? It looks like a resistor but in every schematic and pic I see, these 2 caps are always the same and usually Mallory's or sozo's.

Should c4 be replaced?

Okay, so the pic that you posted is your amp? In your op you mentioned that you attached a pic of a modified amp and a pic of your amp, modified by pisonic. Just want to be clear what we are looking at.

First off, The values of the first 2 filter caps (the large black ones that say 16uf) don't not match the values that are shown in the layout and schematic..the first filter cap should be 47uF 450V, and the second filter cap should be 22uF 450V. The schematic and layout also show that each of those filter caps are bypassed with a 10nF cap. The pic shows that both of your filter caps are 16uF 475V (that's fine, you can exceed the posted voltage rating), the first cap bypassed with a .1uf Sozo (could be Mallory 150 or any film cap you prefer...Sozo is fine) and the second has a 330k resistor (not the same as a capacitor) installed instead. It also looks like there is something else installed underneath the Sozo cap.

Do you know this "pisonic" person (or company)? If it were me, I would be contacting them and wanting to know why they strayed so far from the original schematic.

Mark

[JazzGuitarGimp]

I'm guessing the 330K resistor was installed as a bleeder resistor.

[Blindog]

I'm guessing the 330K resistor was installed as a bleeder resistor.

That was my guess as well...but why lower the filtering that much, especially the reservoir cap

Mark

[Reeltarded]

So the amp will act cranked at less than wide open prolly.. it also favors a lil assier lowend. Probably needs it with the bright contraption.

[JazzGuitarGimp]

Yup, the amp will be more spongy = not as stiff, with smaller caps.

[Masco]

16uF+16uF was the original value for the JMI AC-30's, sans bleeder resistor and 100n Sozo.

[Reeltarded]

And the last node is 8uF.

[Blindog]

Thanks for the explanations gentlemen

Now I'm wondering if the rest of the nodes have been changed in the op's amp...

[gingertube]

You will not notice any difference between 8uF and 16uF or 47uF for power supply nodes in the preamp. The Current draw is just too small to make any difference.

Some would also argue the requirement for the 100nF across the electrolytic. In "days of yore" electrolytics were not that good and their self resonant frequency (the frequency above which they behaved like inductors rather than capacitors) could be well down into the audio band. Adding the 100nF made sure you stll had good capacitive bypassing at higher frequencies

I usually do the main filter cap (470nF) and the input amplifier node (100nF). Also the Reverb Recovery Stage (100nF).

Due largerly to the damands made by switchmode power supplies electrolytics have improved dramatically in the last 20 years. When using modern electrolytics it could be argued that the extra 100nF is not required anymore but guaranteeing a clean B+ rail even to RF frequencies will always give some benefit. From a functional point of view this means guaranteeing that the B+ supply node is at AC 0V (AC Signal Ground) at all frequencies.

This becomes important to stop stray high frequency signals from "muddying up" your sound by creating intermodulation products. One of the source of high frequncy noise is power supply diode switching noise.
Diode switching noise has a high di/dt (rate of current change) which means that it has sharp edges which can couple magnetically, couple capacitively or radiate RF wise into any high impedance node or simply conduct along the B+ chain and NOT be filtered due to ESL (equivalent series inductance) of the electrolytic causing a high impedance to ground at high frequencies.
By all means retain those 100nF, 220nF or 470nF across the elctrolytics but also address the diode switching noise by changing all power supply diodes to ultrafast soft recovery.

Gingertube goes off at a tangent.
The above also gives you some hints when doing layout, particularly point to point wiring. If you have to "remote" a component on a long lead always choose low impedance wiring points, so cathode bias resistors and bypass caps are first choice to put remote from the tube socket. Anode stuff is usually next and grid stuff should be your last resort.

Cheers,
Ian