Odd PRRI problem

[OaklandA]

OK, so I'm a few months into PRRI ownership and I'm loving the little amp but here lately it's developed a peculiar problem that I can't quite figure out. Before I haul it to a tech I thought I'd see what the resident amp gurus might think.

First, it has a couple of upgrades - a set of Tung Sol RI tubes and a Weber ceramic California 10".

The problem - about 2-3 weeks ago it developed a very high pitched ringing on certain frequencies. It sounds like someone running their finger around the rim of a crystal glass - high and piercing. Particulary the high "D" note (as in B string, third fret and higher) and various notes up the B string and to a slightly lessor degree on the high E string. It's the same pitch always, and quite ice pick piercing at volume. If I play, say, an open D chord and let it ring it will increase in intensity unless I dampen the strings. On my '72 Thinline it will build to feedback. It occurs, although not as bad, at low volumes too. You can hear it anyway.

I first thought it was a microphonic tube. So I did the tube substitute. Then I tried two complete other sets of tubes. No difference...in fact it got a little worse on the stock GT set of tubes.

Next suspect was cone cry (though it doesn't really sound like it to me). So I put the stock Jensen back in. Yikes...even more shrill....same sharp ringing tones on the same frequencies. Ran a Celestion G12-H off it....still hear it though not quite as bad. Bear in mind the Celestion was also in a separate cab.

Also, it does it on all three of my guitars...the '72 Thinline having humbuckers...so its not some generic single coil issue either.

No cab rattle.

So, by elimination I conclude it's NOT:

Guitars
Tubes
Speaker

Any thoughts?

I love this amp and would sure like to get it right again.

[The Bestron]

have you tried disconnecting the reverb tray? the problem might be there (although what you're talking about sounds like a problem in the tubes to me... if that's not the problem I'd take it to a tech)

[OaklandA]

Originally Posted by The Bestron

have you tried disconnecting the reverb tray? the problem might be there (although what you're talking about sounds like a problem in the tubes to me... if that's not the problem I'd take it to a tech)

Bestron...thanks for the tip, although disconnectiong the reverb tray didn't change things either.

If it were not for there being no difference on a complete tube substitution I'd be sure that it was a tube issue. Perplexing.

Thanks for your help.

[skeksis]

I've had what you describe happen with a few different amps with different causes...

First was a '60s Gibson GA-5T (little 15w 1x10" w/ trem). That was back before I knew anything about amps, and I pretty much gave up on it and sold the thing after awhile. It started at some point roughly when I changed out the tubes, but once started it never went away. Behavior was exactly as you describe, primarily notes on the un-wound strings up a few frets - no help for you, but I have sympathy. Different tubes, guitars, speakers, didn't matter. Worst part is that it's really a small componont of the sound being produced, but once you focus on it, it's all you hear.

Second was with a '66 PR, but it wasn't really the amp - instead I was playing a Tele with a volume kit... too many highs were getting through, ended up hurting my ears. Bright guitar, bright amp, it hurt. This isn't your problem, but somewhat similar.

Third was my 5F4 build, with a couple different "solutions": shortly after the build I'd get a squealy oscillation/feedback on certain notes... turns out that my OT had some leads reversed, so my negative feedback was functioning as a positive feedback. Once fixed, I'd still have some piercing highs on some notes - I have been able to tame it by using the presence knob. So yeah, that doesn't really help you either with your PR, but makes me think that there's a chance that it's in your negative feedback circuit?

Also, some highly resonant guitars get a lot of odd harmonics going on behind your fretted note, behind the nut, sometimes above the bridge (to the tailpiece), which show up as lame ghost notes... really confirm that you're not causing the problem in some bizarre way.

[Billm]

I've not heard of PRRIs being subject to oscillation in the phase inverter or output stage, but the symptoms fit. A bad OT could do it, too. So could a bad or leaking filter cap. The Illinois Capacitor caps that Fender uses are only fair in my estimation; look for leakage as shown here:



These are in a Blues Junior and I've seen enough of them to know that IC has a problem, or had one in a relatively recent production run.

[OaklandA]

Originally Posted by skeksis

Different tubes, guitars, speakers, didn't matter. Worst part is that it's really a small componont of the sound being produced, but once you focus on it, it's all you hear.

Exactly. I can play something just find and then I'll hit a D note and it stops me right in my tracks and then I just obsess over it.

The local Fender techs are known to take forever to get stuff off their bench. Plus I didn't have the guy I bought the amp from sign over his original receipt (if he had it) so warranty work will be hard to get. So I'm hoping I can figure it out.

Looks like the OT area might be the next place to start looking...

[OaklandA]

Originally Posted by Billm

I've not heard of PRRIs being subject to oscillation in the phase inverter or output stage, but the symptoms fit. A bad OT could do it, too. So could a bad or leaking filter cap. The Illinois Capacitor caps that Fender uses are only fair in my estimation; look for leakage as shown here:



These are in a Blues Junior and I've seen enough of them to know that IC has a problem, or had one in a relatively recent production run.

Bill, I was hoping you'd see this thread and I thought you might have some ideas. I saw your post on another PRRI thread about yours & Allen's new OT for this amp. I know that sometimes something in the OT or circuit can cause some oscillations.

Anyway, a little later I'll pull the chassis, drain the caps and take a look around for what you're showing there.

Thanks for the suggestions and response.

[OaklandA]

Had a look inside and there were no signs of a leaky cap nor anything else obviously wrong. As a matter of fact, the board looks very good. Though ultimately I prefer PTP, I think a lot of the negativity about PCB is overblown.

Of course a tech might have another opinion.

Thanks to everyone for the suggestions so far...looks like it might have to go to a tech.

One other question.....would a faulty bias cause this oscillation? I ask because when I installed the Tung Sols, the Weber Bias Rite I was using was acting up a little. It is possible that it is not biased correctly.

[strat a various]

Have you tapped all the tubes with a chop stick or pencil to see if one of them is microphonic? I've had a preamp tube go bad, but only ring sympathetically on one note. So far "A" and "D" have been the resonant frequencies on two different 12AX7s in two different amps. Easy enough to test.

[OaklandA]

Originally Posted by strat a various

Have you tapped all the tubes with a chop stick or pencil to see if one of them is microphonic? I've had a preamp tube go bad, but only ring sympathetically on one note. So far "A" and "D" have been the resonant frequencies on two different 12AX7s in two different amps. Easy enough to test.

Strat....thanks for the response.

Yes, I tapped the tubes with an unsharpened pencil. And, as mentioned earlier, replaced every tube both individually and as an entire set with the original GT's and the problem remained.

It does sound similar to a microphonic tube, but I don't see that it's likely. I am tending to think it may be perhaps a transformer problem or something else tube related though I don't know what.

[strat a various]

A friend of mine, a well known tech, had a Peavey tube amp with a high pitched oscillation that he had a heck of a time fixing. It was dog whistle high, not everybody could hear it, but it was visible on a scope. This guy I know spent hours tracing it, can't remember what the cause was, but it was easily fixed once he isolated it.
Is it used? If it's under warranty, I'd get Fender to fix it.

[Billm]

Yes, put the stock tubes back in and turn it in for warranty work. The service center won't care that you swapped speakers.

An oscilloscope or spectrum analyzer is the secret weapon for detecting oscillation--and for knowing when you've fixed it. The thin blue line at the top of this display is a 49KHz oscillation. The rest is normal output and harmonics in a sweep tone from 60Hz to 20KHz:

[OaklandA]

Thanks again, guys.

Unfortunately I am not the original owner and I do not have the original transferred receipt. Therefore I don't think I'll be able to get warranty covered work done.

I did send an email to the guy I bought it from and maybe he'll be able to find the receipt and transfer it to me. Otherwise I will bear the expense of the repair.

I'll update once I know more.

BillM - I may still contact you in the future about the OT upgrade.

[Billm]

Unless an OT swap magically makes it go away, you really need to hook it to some test equipment and create a signal that provokes the oscillation and then trace to see where it occurs.

You're running with the tube shields in place, right?

One thing you can try: re-orient the grid wires running to the tubes (they're all green) so that they're flat against the chassis and as far away as possible from one another and the blue plate wires. The two likeliest offenders, looking at the schematic, would be the blue running to pin 1 and the green running to pin 7 of V1. Also, try shielding the ribbon cable P15, between the volume and treble controls, with some aluminum or copper foil that's also in contact with the chassis.

This is the kind of thing where experienced techs wave their hands over the chassis, close to the components, and look at waveforms or listen to test tones, to see which components are "talking" to other. It looks like voodoo. :)

[OaklandA]

Originally Posted by Billm

Unless an OT swap magically makes it go away, you really need to hook it to some test equipment and create a signal that provokes the oscillation and then trace to see where it occurs.

You're running with the tube shields in place, right?

One thing you can try: re-orient the grid wires running to the tubes (they're all green) so that they're flat against the chassis and as far away as possible from one another and the blue plate wires. The two likeliest offenders, looking at the schematic, would be the blue running to pin 1 and the green running to pin 7 of V1. Also, try shielding the ribbon cable P15, between the volume and treble controls, with some aluminum or copper foil that's also in contact with the chassis.

This is the kind of thing where experienced techs wave their hands over the chassis, close to the components, and look at waveforms or listen to test tones, to see which components are "talking" to other. It looks like voodoo. :)

Bill...thanks for the advice. I've had a chance to try all of your suggestions and the ringing oscillation persists. The shielding of the ribbon cable gave the amp a warmer quality, though I prefer the tone without the shielding.

So, back to square one. I did get the OT I ordered from you that you and David Allen designed. So I'm hoping on a long-shot that once that's installed the problem might go away.

But as you have said this amp probably needs the voodoo of being hooked to a scope by a tech.