Is this the picture you speak of? I can see the 2 capacitors, just can't tell precisely what they're connected to.

Not that easy to find. Some links for the record:

cathode resistor bypass caps
cathode bypass

Also:

with photos

O.

Nevermind, I think I get it now.

Looking here, with the tube oriented so that the front panel is at 12 o clock, we are connecting the 4 o clock and 8 o clock pins to that central rail on opposite sides of that large resistor in the center? I'm just trying to reenact what PinkFloyd did.

I understand circuits as well as I was taught in one semester of college physics 2 years ago...not all that well, and certainly not at all in the sense of how amplifiers are designed...I couldn't tell you why a capacitor or a resistor should go in place x, y, or z.

Someone should really just build custom 336s all day. It's a good design and if you swap out the most odious parts inside it, fix the power switch, and perform this mod, you could take a $250 crapper and make it great. I say crapper with the highest reverence...the 336 is an amazing amp but it's more than rough around the edges.

Or you could buy a Woo. All the fun of Darkvoice with (likely) none of the serious QC problems.

I'd like to get my hands on a WA6 to try out, but I don't have the cash right now. Maybe someday...

I used 220uf caps on mine, worked great - now craiglester has it after I upgraded to WA6 and I won an EF1. My 336i was very special with HD600...

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I think you got it. The caps are cathode bypass caps connected in parallel with the cathode resistors, which must be the grey resistors in PinkFloyd's pic.

O.

Has anyone had success with this and the 336SE? -- Having trouble with mine; the bypass cap installation seems simple enough... 100-220uF in parallel with the cathode resistors; is this all it takes?

Yeah, just the 2 caps in parallel...make it look like the dude's picture I guess...

Wow. That worked. Like, really worked. Two Nichicon 220uF caps (I used axial vs radial)

- and now, ALL hum on ANY 68FG or 6SN7 is GONE from my 336se.

The only things I really have to ask are

1) Is this going to cause the amp to become less reliable down the line?

2) In other words -- why in the WORLD isn't this done at the factory? I can't see it being a cost saving issue. There must be a reason??

Anyways, so that folks may be assisted in doing this in the future, here are pics of my mod...

The electrical tape, don't mind it, just burnt the insulation there a bit while soldering so a slight precaution...

I tried to get some angles that show the connections well. This along with the previous images in this thread hopefully will be a help to people.

I did the Fitz mod for my 336, but it did not work for me. But I think I need to give it a try since the 100microF capacitators I used had been laying in a drawer the last 40 years .

I measured them and they seemd fine.

Is there any difference resultwise using 100 vs 220microF capacitators?? (for those who made it work)

I'm not sure; the 220uF nichicons seem to be very effective. From what I understand, increasing the capacitance (100uF -> 220uF) increases hum rejection but also increases gain.

Having a gain increase may be undesireable in some circumstances; For me, though, on my HD600s, it sounds rather good.

You may want to try again with better caps.

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I used the 220uf Nichicon when I did mine and it worked beautifully, and I was told if you used too small of a capacitance that you'd also get some bass roll-off and that 220uf was as low as you should go. I was told about gain changes by going higher, so I'm glad I didn't go higher as gain was enough as it was.

I don't see there being any long term harm with this mod.

I did the mod a couple of weeks ago using 220uF/25V Elna Silmic II caps. The amp still sounds good and is perfectly hum-free now. To tell the truth, I didn't have serious problems with hum even before the mod, except with one tube which had other noise issues as well. This faulty tube still hums slightly in the left channel, and naturally still has its other issues.

The faint hum that the amp had with some 6SN7GTs was a blessing in one way. It is now evident that one of my 6AS7Gs makes a tiny whistling sound in the right channel. This was effectively masked by the hum. The whistle is now more annoying during quiet passages than the hum ever was. A quiet 6AS7G solves this problem, of course.

I would like to be able to say something about how the actual sound quality of the amp changed with the caps installled, but I can't. I would need two amps for an adequate comparison. Anyway, if there is somebody who would like to try these Elna caps and report back, I'm ready to send you a pair, especially if you are willing to compare with something else like the Nichicon Muse KZ caps. (This supply is very limited.)