Silvertone 1484 stray note

[acatasus]

I use my Silvertone 1484 for harp. It is a powerful amp for large venues. I have discovered a peculiarity. When playing a "C" harp, second hole draw bend, which gives me an F below middle C, the amp gives me a very clear second tone along with it- middle C. It is the only place this happens. I have changed mics. It happens with all of them. I have adjusted volume and tone to no avail. I have played through other amps and it doesn't happen. Only my Silvertone.

The amp has been retubed and recapped. It has no reverb- The tank has been removed. It sounds great... except for that one note.

I am waiting for a shipment of tubes- I am planning to replace the two 12AX7's with 12AU7's in search for a nicer breakup at lower volume settings. Might this help? What about tube dampers to prevent microphonics?

[ahamay79]

Those amps are a mess of wires. You can try star grounding some of the filter cap ground wires and running them back toward the power transformer. It may also be the transformers causing the ghost notes.

[acatasus]

Stray Note

My 1484 is not as big a mess as people say. Maybe the tech that worked on it neatened things up or perhaps I'm just more tolerant than most.

In any case, it looks to me that the little bit of grounding you suggest has been done. And yet the stray note persists. My new tubes have arrived. I'll be installing them Saturday. Let's see...

[pdf64]

Might be cone cry. Try running it through a different speaker. Peter.

[acatasus]

Cone cry!

Cone cry- I like it. It certainly would be the simple solution. Now I'll have to work out a way to test it.

Kind of weird, though, how it ONLY happens on F below middle C. I guess it could happen.

[Enzo]

All that means is whatever the problem is, that particular frequency resonates. It could be a mechanical resonance in the speaker cone - that cry. It could be a resonance in a microphonic component, who knows??

[Mars Amp Repair]

Also,
Are you certain that the amp isn't distorting over a certain wattage demand? You might just be hearing amp distortion that the particular note seems to exacerbate. I'd run it up on a load & sweep through with a tone generator & look for those side tones you're hearing.

Also It could be a weird parasitic oscillation, too. This amp originally has no screen dropping resistors. I noticed in one of these I worked on that only old tubes would not oscillate. New tubes oscillated like crazy. I had to install screen limiting resistors (470 ohm) to rid the amp of the oscillation.

glen

You might be dealing with that issue. glen

[acatasus]

Distortion?

I'm not saying that's not possible, but if this amp has a fault from my perspective, it is that it is too clean- not enough breakup. That's why I'm putting in those lower gain preamp tubes. It doesn't sound like distortion. All notes play nice and clean. When I get to that F, it sounds like someone is standing next to me with an electronic tone generating machine. That C is clean- electronic sounding- no overtones. Just a pure C. It doesn't feel like it comes out of what I'm playing. It sounds independent and separate. No where else in the entire range, from a low F to a high G is there any distortion to speak of. It all sounds very clean- Too clean if anything. In fact, even when I get that ghost sound, the F sounds very clean. It is just that that C sounds at the same time.

I'll post again when I change those tubes and test it again.

I want to thank everyone for their input. I am looking into all of it, one at a time, until I have it mastered.

[acatasus]

Could it be as simple as this?

I haven't had a chance to check this out at full volume yet. I got back to Connecticut (where my amp is) late enough so that I had to run my test at a lower volume in deference to my neighbors, but here is my narrative and my preliminary results. Does this sound unlikely?

I discovered my phantom tone issue just as I had to leave my Connecticut home to get back to my workaday life. I checked out whatever occurred to me very quickly before I had to leave (Just half a step in front of a snowstorm, by the way. Life can be crazy at times, can't it.).

When I got home I started my posting on my problem. You ever so generous folk were kind enough to give me all sorts of suggestions.

When I came back, the one easy check I could do was to simply eyeball the speakers more carefully. I had kind of looked up at them through the space below the half-back panel. I removed the panel and looked at what seemed to me to be unexpectedly clean and lovely Jensen speakers in what appeared to be pristine condition.

Then it occurred to me to try to wiggle each mounting stud. Surprise, surprise, surprise! Many of the ones that were hidden by the back panel were extremely loose. One was so loose that there was an eighth inch gap between the nut and the speaker flange.

I went to work tightening all the nuts that needed it, to be found almost exclusively on the left-hand speaker. I thought it was essential but I didn't think it would solve my problem. I thought that the more likely result of a loose mounting would be buzzing and rattling, but it does seem to have taken that sound away. Of course, we'll see better in the morning when I test it at higher volumes.

I suppose that the particular note that triggered the ghost tone was just the right frequency to set off a sympathetic vibration, but I never dreamt it could create such a clean, electronic-sounding tone. As I said before, I would have thought a buzz or rattle.

Again, I would like to thank all my responders. Pdf64 and Enzo, in particular, set me looking at the speakers for answers, but everyone's post contributed towards my careful observation.

I will wail away on my amp a bit later and will post my results, but I am fairly sure I have found the problem. Now I can start working on world peace!

[pdf64]

Don't tighten the speaker nuts too tight. Just 1/4 turn max after finger tight. Use a spring washer if wanted to stop then vibtrating loose again. Or a little nail varnish on the exposed screw thread.
A possible problem with old cabinets is that the baffle can warp, especially if the mounting method doesn't fasten it on all 4 sides, so tweed style is better than BF style, other things being equal. Which they aren't as BF baffles are generally thicker than tweed baffles, probably to compensate for the above issue.
So if the baffle is warped, tightening the speakers on to it can twist the speaker frame and cause voice coil rub.
Anyway, here's hoping you've fixed it. Peter.

[acatasus]

hurrah!

The deed is done. Mission accomplished! (At least one was, anyway.) Thank you pdf64 (Peter). Actually, the baffle is remarkably flat. I think it is a double layer of masonite- Not what I would normally think of as highest quality stuff, but it seems to have held up beautifully.
There are many more than four attachment points. Every hole save one on each speaker is an attachment point- I guess about 7 or 9 points. I used the change a flat tire method- Tightening a little at a time in a star pattern. I couldn't possible overtighten even if I wanted- Lack of access prevents that. Later on I'll buy some lock washers and maybe use nail polish as well. I've been known to wear suspenders AND a belt.

Anyway, my amp is now honking! Thanks! I can't wait to try my new tubes! I wasn't going to but I think I'll bring it to tonight's jam session. My wife says I'm like a little boy. I respond, "what do you mean,'like?'"