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sibilance and coupling caps....

Posted by dls123 (D) on March 21, 2006 at 10:53:51

Hi folks,
I posted a thread about 10 days ago about sibilance and plate voltage. Got a couple of nice suggestions from two fellow inmates. I am thinking that I have the filament and plate voltages about where they should be now. To recap....

I have a wonderful custom push pull amp. Cathode bias, no feedback, AC filament. I generally use EL34's, but it also has a switch position to change the bias setting for 6550/KT88's. After enjoying it for about 3 years, the power transformer decided to die. The designer dug up another old stock transformer for me and did the labor for free. This transformer is huge - probably almost twice as large as the original - also old stock. Anyway, I just can't get the sound of the amp back the way I had it. It is a bit bright, and sibilance has reared its ugly head! I never had even a hint of sibilance previously. Nothing else in the system has changed. So here is what I have done so far:

Rolled all sorts of tubes from the stash with predictable changes to the tonal balance, but overall the amp is still a bit bright (even with OS Tungsol 6550’s) and has some sibilance I cannot get rid of.

Filament voltage is now 6.1 volts and B+ is 423 volts. I think they are fine.

I have not played at all with the coupling caps. Because of their mounting, and my general inexperience I cannot identify the brand and type. The second stage coupling caps (I think) are the larger blue ones in the picture (upper left). Writing is faint, but I think Philips is the brand. 275 volts, and I think it says 470n so my guess is that is 0.47 micro farad? I can get the specs from the amps designer. The small caps for the first stage – 12au7 – (bottom right) are mounted upside down so I can’t get the brand or value. They look like they may also be made by same manufacturer of the larger ones (Philips?). The brown ones are small Russian NOS PIO’s to sweeten the sound. They read 0.01mK which I assume are .01 mcro farad? No matter, not going to change them. My questions are these.

1. Anyone know what the small first stage caps are (blue, rectangular, bottom right)? Another Philips? What type are these caps – clearly not PIO…? Polyprop? Again, I can get the value from the amp’s designer. Can anyone speculate as to whether changing them will improve the sibilance problem? Are they lousy caps, or mid grade? Amp was wonderful before, but new power X-former although more powerful, has made it brighter. Would be happy to solder some new caps in, but would like recommendations. I don’t mind spending $10 - $20 a pop for the right caps. I need to try to get rid of sibilance, and maybe darken things VERY slightly. However, amp has tons of air and I don’t want to lose that. The speed is about medium and I don’t want to slow it down.

2. Or are the caps probably not going to tame the sibilance….or improve the amp? Barking up the wrong tree?

Any recommedations appreciated. Transformer is quiet, amp is dead quiet, tons of headroom, tons of air, huge soundstage (floor to ceiling and a foot or two wider than speakers, just now some sibilance... Sorry for the picture quality – no macro setting on my camera.

Cheers,
Don

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