irritating "sheen" on "A" notes

[deci belle]

Is that an irritating sheen I hear? Thanks for that too, Joe.

[Chuck H]

Is that an irritating sheen I hear? Thanks for that too, Joe.

I think deci belle is fitting right in here. FWIW I think your process was very methodical. Why the problem is gone IS hard to say. this is often the case WRT guitar amps and their design. Just look at all the wierd circuits in production amps. They're there because that's how the amp worked for the designer whether the circuits make sense or not. In the field of guitar amp design there is a whole bevy of variables that gum the mix like intentionally overdriving the tubes and trying to make them sound good doing it while dealing with proximity and grounding in a circuit capable of over 10,000X amplification!!! Sometimes if a proto works a designer will just leave it that way. Right down to partial circuit refinements left over in the proto stage because something could change if you take them out or idealize them. My point is that it's not always know how and science. Sometimes it's a happy accident that must be respected whether you know why or not if everything is to be repeatable.

As to what any cap review, manufacturer or distributor said, well, your not responsible for that and I appreciate you sharing it.

JoeM is just a little grumpy right now. He'll be alright in a day or two.

[JoeM]

....
JoeM is just a little grumpy right now. He'll be alright in a day or two.

????

I'm sorry but I just dont believe in claims like that. Leave it for the hi-fi guys. It doesnt have a damn thing to do with making music.

[Chuck H]

Well, to be clear, what db said was:

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.

There's even a hint of skepticism in there. db was only reciting what was learned in the research process. But the fact is that the new caps fixed the problem. I, for one, believe there was a problem and that the only change that fixed it was the caps. db's recitation of the process was fully satisfactory for me to believe that is what happened. For me the question is not whether db's experience was factual. But rather, how do we deal with these facts? The idea that a caps film thickness and overall mass could affect something is not that far fetched for me. Taken to extremes, do you think a cap with, say, 100 feet of four inch wide conductor for either terminal wrapped with an insulator of a thickness that resulted in a .022uf final capacitance would sound the same as a typical Mallory 150 400V .022uf cap??? Probably not. But why??? I'm not a capacitor designer. There are physics involved that are beyond my scope. The caps made a difference. We're not sure why but we do know what db read about the issue. And that's as much as db can be expected to do. Perhaps more. And I'm happy to know it because now I have at least some information about an obscure subject where before I had none. I know the caps did make a difference and I know what the ideology behind a particular cap designers product is. I'll take that and use the info for searches or combine it with any new info I hear and maybe I'll learn something I didn't know. Imagine.

[deci belle]

Dangs, I hadn't been here in a while!

????

Joe sez:
I'm sorry but I just dont believe in claims like that. Leave it for the hi-fi guys. It doesnt have a damn thing to do with making music.

The last time I made music I forgot my thoughts (that includes beliefs). It is not necessary to uphold or deny hair-splitting claims to experience what becomes self evident through a process of personal, and at times, desperate, exploration. My ideal in this context on this thread (my thread) is eliminating the sources producing an irritating artifact or artifacts. I'm not here (on this forum) to appear knowledgeable. I'm just here to explore. I'm too busy solving problems so I don't even want to waste time grooming arguments for or against beliefs, claims, audio/RF factions notwithstanding~ and I hope to stay far away from what is essentially and utterly lacking in freshness.

Joe, from now on, please avoid opportunities to use me to make yourself look good(?), because you don't need to solidify a social position on this forum in order to be appreciated. I know that.

Moving right along… I discovered that there were a few factors contributing to my attempts to mellow out my cranky bf Showman:

1. Multi-source oscillations due to the stock Fender lead-dress magnified and compounded by a high-gain circuit included in the 5f6a channel I put in place of the Vibrato channel that necessitated lots of extra wires going to and from gain and master volume pots.

Getting that under rein was a huge deal and I learned a lot in the process about gain-stages, power nodes and lead dress and conventional as well as unorthodox shielding techniques (most effective for the wires coming out of the cap-can and running across the length of the chassis). One of the old-timers suggested that I'd still not dealt with the source of the problem(s) tho'… rats!

2. High frequency oscillations… I'm still dealing with this now in the extra gain-stage triode (experimenting with input tube's plate R+cap values in parallel and between the treble>volume pots), and a 300Ω grid stopper on the regular 5f6 cathode-follower (I've smoothed things out enough that I'm going to try pulling the 5751 input preamp tube and replace it with an NOS Telefunken AX7).

3. Stabilizing the PI using (mostly) high-quality parts (but I found that carbon film resistors on the PI plates were smoother-sounding than metal film). The small-value silver-mica cap between the PI plates was quieter than the polystyrenes I tried. An NOS Mullard AT7 works better for me than a few others I tried in that position. A noticeable difference was discovered while daringly implementing two $5.00 non-guitar-amp hyped coupling caps in that section.

4. Plain ol' noise: All other resistors are either IRS 3w for power and little blue 1/2w metal film for everything else. Carbon comp were noisier compared to the metal films.

5. Speaker. I hope this is one of the last hold-outs. A 15" Jensen Neo/150 sounds so much better (smoother, more musical) than both the JBL d130F (60~6000Hz) and the JBL d140f (50~2500Hz) speakers that came with the 115 Showman tone-ring cab. I can fault the JBL's aluminum domes, but not the fact that these speakers are at least 46 years old). But I only tested the speaker, thanks the cool neighborhood guitar shop-owner, butI can't afford it yet… so i can't sleuth the circuit void of the JBL's harshness.

6. Not using the amp at its optimum volume (four 6L6s): I replaced the stock OT with one from a Tweed low-pwr twin so I can run just two pwr tubes (both OTs have 4k primaries): Now at least I can turn it up!!

[überfuzz]

Interesting project, apart from the noise, I guess it sounds quite delightful.

Did you scope the circuit at any point? Further, did you ever consider the layout, that it might cause the noise? I've done a some over ambitious projects and more then once the layout seemed to cause instability. Is it possible to post or show the layout here?

Cheers!

[deci belle]

Yes, it's getting there, überfuzz! I did check it out on a scope for about a week last year. The most action was between the PI and the power tubes, but that is the first time I'd used test equipment, so I can't say I had any idea what I was seeing.

Last week, the friendly neighborhood guitar shop owner took the time to critique my build and we tried out different speakers too. He liked the tone in the bass and midrange, but the treble was still harsh in certain places in both channels, probably due to oscillation, mechanical-resonance issues in the chassis and speaker as well as unresolved gain-stage/tone-stack subtleties I'm still digging into.

There were many instability issues with layout/lead-dress, power-node organization and component upgrades that I dealt with (and learned a bit) over the past two years as I've restored and modded this '66 Showman. Having finally replaced all the original pots last night seemed to make a difference in smoothing the tone. Shielding (wrapped w/ green wire) the filter-cap leads coming out of the cap-can and cutting across the circuit-board helped in a similar way.

I appreciate all the suggestions I've received for the various concerns I've posted concerning this project!!

Thank you for taking the time to look this over, überfuzz! Oh~ there are only two 6L6s in this amp now, so V7 and V8 are not part of any circuit.

[überfuzz]

Aha, so it's a rebuild/mod I thought it was a home-brew. Further I didn't realize you've been tinkering with this amp that long. Unfortunately the risk with that is that some issue was overlooked and you just refined the amp along the way. This without really fixing the problem. You should try to get some scope time. Possibly with some experienced dude at your side. That way you should be able to nail this one. (Doing this you should restart from square one, not ruling out anything.)

A comment regarding the pictures. It's not very common but I've seen people posting issues to end up stating that it was a ground issue due to some brass plate. Apparently some plates can corrode and cause bad groundings. Have you ever checked or resolder any of the grounds?

[deci belle]

überfuzz wrote:

You should try to get some scope time. Possibly with some experienced dude at your side. That way you should be able to nail this one

Ya! I like that!! In a perfect world (not yet in my little town), I should be able to do that… But what you've brought up is true. Going through the amp (I have all the test equipment I need to do that), and also doing a complete re-vamp of the ground circuit(s) is essential.

I recently scratch-built a 5E3 with an eye toward implementing the solid grounding techniques described by RG and the other builders and it works fabulously. I will eventually analyze this amp's grounding scheme based on what I learned and will rip out everything on the Showman's chassis (again) in order to employ the advanced techniques I used on the recent Deluxe build's layout.

überfuzz wrote:

Apparently some plates can corrode and cause bad groundings. Have you ever checked or resolder any of the grounds?

Yes, I have resoldered everything multiple times including the grounds, but the brass plate is just a squish-contact at the pots, and as you state, must undoubt-ably have some age-related corrosion compromising the brass plate's contact with the chassis.

My 5E3 build didn't even use the brass plate— so I'll be deciding what to do with the Showman's plate when I reconfigure its ground circuit soon, überfuzz …as there isn't much else to tweak on this chassis!! I'll be glad to take on the re-grounding scheme as a matter of course not only because it is very important for operational and tonal issues, but because I guess I do enjoy this kind of work.

I just bought two pounds of fiber-board so I'll probably make a new custom board-layout based on the OEM eyelet scheme to incorporate the necessary grounding refinements this amp deserves.

Thank you for those insights, überfuzz!!

[deci belle]

I removed the brass plate, cleaned it up and put some dielectric grease on it and made a new eyelet board too. I routed all the preamp grounds under the board and sent it out to the brass plate at one point. Every little bit helps, I guess.

The "sheen" died altogether though when I had the original d130f restored at DSR in Connecticut. They said the suspension had worn out and the HF sound I've been experiencing can come from a worn out speaker— especially the 15s.

[g1]

Looks like Chuck wins again (see post #2) . I try to look on such experiences as "schooling".
Glad you were able to sort it out.

Edit: Oops, I missed what you said in post #9 "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."
So I guess it was a combination of issues?

[52 Bill]

Deci Belle, I haven't been following your thread here, and if this has been covered before please direct me to the thread so I can read up on it, but can you explain the power supply design on this amp? I see that you have kept the series string of diodes, but there are resistors paralleling them and caps across some of them?

[deci belle]

Hi g-one, ya— like you say, a combination of issues were conspiring in this "master-class" situation, and I got a lot of help from everyone here.

Lead dress and power node organization helped with oscillation, then tuning the PI and stumbling onto those caps that a couple of guys that ought to know better couldn't handle because they were more interested in rehashing the "golden ear" trip when I was only interested in not hearing the caps!

Next, along with some very nice tubes, I installed iron from a 40w Fender so I could actually drive the system harder. Finally, from the advice of the guitar-shop owner in town who suggested a new speaker (we tested his Jensen Neo-150 15"), I had the original JBL restored. Now I don't have to compensate for that speaker issue any more as I work on the amp's voicing on the two channels: 5f6a and 6g14a which shared the same PI w/ presence design.

Hi Bill! RG covered it in his pwr section mods thread. Don't use the caps+resistors (my idea). They work to protect the diodes (in the SS rectifier application), but as Steve pointed out, the caps suck the higher frequencies (when used in the pwr-tube safety mod). So I took the caps and rs out. I'll try to show the link…
http://music-electronics-forum.com/t30086/