irritating "sheen" on "A" notes

[deci belle]

As I upgrade the components on a '66 single Showman, a frequency specific noise has now become prominent on "A" notes on the A and B strings of three different guitars (Fender and Gibson) on both channels. I hear it on C notes too, but not as disturbingly. Its a high frequency sheen that is most obvious on the neck pickup at lower volumes, but not much less on the bridge PU or at louder volumes.

I don't really know when I first started noticing it, but about the only thing I haven't replaced yet are the filter caps, which are are at least five years old (100µF/350v and 20µF/500v Spragues). And were never used for those five years. I haven't replaced the pwr tubes yet either.

I did add a filter stage (16µF/475v+20k), for the 5f6a normal channel.

I've changed the PI tube (at7). I've swapped the d130f for a d140f speaker.

Any ideas?

[Chuck H]

Strange thing for an amp to do. Have you tried a different speaker cabinet? I wonder if it's a mechanical vibration. Perhaps a microphonic preamp tube.

[mort]

fwiw, I've had a similar problem and it turned out to be a tube

[deci belle]

Thanks you guys! The thing that gets me is the sound is the same in both channels… I made a list of pre and pwr tubes to get. I'll diddle with the cab too and maybe I'll get lucky! If it goes away, I'll talk more. Thanks again!

[deci belle]

I ended up totally rewiring the fiberboard. And shielding at least 6' of wire. Got even nicer tubes. The PI had always had balanced PI caps and tube, and a lot of other stuff I tweaked for over a year and removed every OD-cap except in the clean channel.

Interestingly, I tried seven values from 47p to 250p between the PI plate Rs and found out that 47p was hardly better than no cap (it's what I'd put there per the schematic). But I'd tested with Polystyrene caps. I compared that 47p cap with a silver mica 47p and was surprised to feel that the silver mica was a little bit better sounding than the styrene. So the silver mica got it's old job back.

But nothing ever made any difference until I put in a set of .1 µF 630v Mundorf M-caps, matched— $9 total cost. I'd already tried matched Orange Drops and unmatched Mallorys (I didn't ever feel like trying the original molded blue caps)! But it went away noticeably and appreciably with the M-caps. Enough so…

The only other thing I did after that was to eliminate the few carbon comps that were still living in in one channel's cathode bypass section. It tested real good so I finally put it back in the head-cab and that noise still isn't a problem…

[Jazz P Bass]

How do you match a capacitor?
Hmmm.

[Chuck H]

Is it possible that a lead dress issue was corrected in the process of replacing those caps??

[deci belle]

Ooops! I mean they made sure the caps' values were the same. I could do it too, but then I'd have to buy a bunch, Jazz!

[deci belle]

No lead dress was altered at that stage, Chuck. I'd already done all I could think of in that department. I can still hear it, but the "sheen" isn't riding on top of those notes any more, so musically, it is not an issue for me …finally.

There are 4' of shielded wire carrying the high-gain signal going from the board (at the same point, where the vibrato circuit was) to the gain and MV pots (next to each other where the BF vibrato pots were). I'd even added an add'l 8uF filter cap right at the extra gain triode (to isolate it and to lower it's voltage).

I'd already used audio caps on the input stage of the PI (.0047 for the signal and a .1uF for the NFB), so I went ahead and tried the audio caps for the PI's output and it was enough to make a noticeable and acceptable difference.

I'm still considering stabbing in a nice, oversize multi-tap Bandmaster OT. The thought is that I may not be operating the amp in it's efficient sweet-spot (6L6 quad Showman), and running it at 40w will allow me to push it more at slightly less volumes. I don't need to push it for distortion (the 5f6 channel's extra gain stage has 70v on its plate and there's 325v on the three other triodes) and the ab763 channel (400v) has a nice edge from the ODs in the tone-stack.

I could have blamed the JBL D130 with its aluminum dome, but the sheen came through using a ToneTubby from another cab, so I knew it wasn't the speaker either. Ultimately it's probably the gain issue and capacitance gremlins I can't address yet due to my inexperience.

[Chuck H]

Ultimately it's probably the gain issue and capacitance gremlins I can't address yet due to my inexperience.

Those kinds of problems can be devilishly hard to troubleshoot. More often you just try things until something works as you have done. Even for experienced builders.

I just wonder what it was about the capacitors (I think your writing about the PI coupling caps?) that fixed the problem. There has been some renewed interest about capacitor differences lately but it hasn't been that long ago that any difference between film caps was considered moot. And if you dared to express that you could hear a difference you were dogpiled with insistances of scientifically proving your position. That's not me. If you say different caps fixed it and there was no alteration between when the amp had a problem and when it didn't other than different caps that's proof enough for me. I just wonder why because I think it could be valuable info for designers like me.

[deci belle]

The only thing I've heard described about anything specific is the speed of the signal through the cap (not necessarily fast, but maybe tuned somehow. To be honest, I don't know— I just wanted to get some basic entry-level audio caps for the PI since both channels exhibited the same problem. They were the smallest value offered by Mundorf). What I'd heard was what Sozo did in the development of their caps— they tuned the signal's speed through the cap by the thickness and width of the film?. Sozos aren't an "audio" cap, but that's just cuz they're supposed be what contributed to the particular guitar amp sound. But the technology is deliberate.

The sound is still there in those places, but it's not what it was. So there has been an improvement, and I'm glad. If I didn't know my amp and guitars intimately, I bet I couldn't tell the differences I was concentrating on— I'm not just listening, I've been playing at the same time too, for way over a year. But the sheen being there and not being there is pretty plain to my ear.

This only just happened, so I'll say something if it changes.

I felt if I really wanted to try, I could move the two pots for the high-gain channel to where the volume and treble pots are now, and shift everything else down the line— I'd eliminate three feet of hot signal wire, but I didn't want to do that.

[JoeM]

... What I'd heard was what Sozo did in the development of their caps— they tuned the signal's speed through the cap by the thickness and width of the film?. Sozos aren't an "audio" cap, but that's just cuz they're supposed be what contributed to the particular guitar amp sound. But the technology is deliberate...

Huh? The signal goes through the Sozos faster? It's a 'tuned' cap?

[Jazz P Bass]

"Tuned" electron flow.
Yeah!
Mojo Time.

[deci belle]

That speed thing was what the French caps called Solen said. Like skis, independent of design-types, there are good and bad. Solen, didn't perform so well in the reviews. If you want to talk like that, you two— be my guest.

If you have a BS in electronics, address those manufacturers— I don't care to hear your noise.

My amp problem is gone, thank you very much.

[JoeM]

Oh come on don't be ridiculous. Just because you believe all the BS we dont have to.