Loud high pitched noise

[pda041376]

I have been playing the amp, put an hour on it this morning. Move the amp to changed speaker in cabinet it was on. Got the speaker changed, fired the amp up and got a full volume sort of high pitched constant drone as soon as flipped off of standby. Turned gain and volume down trod again, same result. I figured maybe the speaker was the issue but hooked cabinet to a different amp and it works perfectly. Changed amp to speaker cord to the one used with the working amp, same thing: loud constant tone, not a simple hum or buzz, sounds like a test tone gone wrong, as soon as flipped off of standby.

Flipped chassis over and do not see any immediately visible issues with speaker output jack solder joints.

The speaker that was changed was an 8 ohm Jensen and replaced with an 8 ohm eminence.

Any suggestions as to where to focus?

[dude]

Switch the OT plates wires on power tubes, ? Put the other speaker back in first and try.

[pda041376]

I'll definitely try the old speaker when I get back to house. Not sure why plate wires would need moved?

[shooter]

why plate wires would need moved

since there's NO schematic or amp information, "we're" assuming a PP amp with NFB.  IF the wires are "wrong"  then you are feeding back positive - in phase signal, which send the amp into infinite gain

[pda041376]

It is PP with NFB, I was asking why switch wires on tubes if amp was wiring an hour prior.

In any case, tried old speaker and the noise is not there, triee with new again and it is there. Again like a test tone but at full volume.

As far as amp info, kld JCM25 kit, not the pcb variant. Has worked fine for a couple months. What would cause one speaker (mod 12/50 8ohm) to not cause the issue and another (eminence gov 75 w 8 ohm) to cause it? Could it be the issue was always there and the mod just wasn't affected? Set positive to positive and neg to neg when replacing.

[pda041376]

And just tried swapping the pos/neg speaker connection, still gets the crazy sound but still not crazy sound on old speaker... At a loss as to how speakers of same resistance can cause this.

[pda041376]

And just tried the 4 ohm jack to see if it might be speaker is mislabeled. Different sound, very high pitched squeal as opposed to test tone. And yes the old, cheap speaker still works as it always did. Dumbfounded

[shooter]

this happens with NO input signal?
IF SO, pull V1, change?
IF NO, pull V2, any change?
IF NO.......
stop before the PI tube



disconnect NFB, any change?

[pda041376]

Correct, no input, volume 0, gain 0 and yet wails like a banshee after flipping off standby. Not instantly, maybe 3 to 5 seconds then wailing starts.

Will pull tubes in order and report back but stumped as to why, even right now, I can hook old speaker back up and it works the same as it did this morning prior to speaker change.

[pda041376]

Further test, hooked up to another speaker outside the cabinet (celestion g12 v type, 70w, 8 ohm) and ko nose there. Going to try to jumper past the input jack on cabinet and try eminence direct that way.

[pda041376]

So odd... Clipped wires, direct connected to the eminence and still does it. Other amps work with the eminence and other speakers work with this amp. Guess this amp just does not like the eminence, period. I don't understand it but guess I'll just have to use a different speaker in this cab.

[pda041376]

Or not. As soon as try to play anything any speaker goes crazy feedback sounding, high pitched whine with no other sound coming through. This kills me since I know it was working absolutely fine this morning.

[pda041376]

this happens with NO input signal?
IF SO, pull V1, change?
IF NO, pull V2, any change?
IF NO.......
stop before the PI tube



disconnect NFB, any change?

Unreal, changed power tubes to so seriously old Sylvanias instead of the 2 month old groove tubes and it works. WTF?

[pda041376]

Scratch that. *worked* for about 5 minutes then the test tone came back. Blah.

[shooter]

going back to my planet, enjoy 

[pda041376]

going back to my planet, enjoy 

Thanks for chase points, breaking out iron tomorrow to hit joints an DC NFB.

[pdf64]

And just tried swapping the pos/neg speaker connection, still gets the crazy sound but still not crazy sound on old speaker... At a loss as to how speakers of same resistance can cause this.

The stability of your amp seems to be marginal, ie with some conditions it’s stable, but change the conditions slightly and it oscillates.
Note that the impedance - frequency chart of all speakers differs. Do a bit of research by obtaining it for the speakers in question. You may need to request it, or it may be published on the manufacturer’s website. The higher the impedance peak at bass resonance, the higher the amplifier gain at that point.
The relevance being that when connected to speakers with a smaller bass resonance peak, the amp may not have quite enough gain to oscillate, whereas when connected to a speaker with a bigger bass resonance peak, it does.
So the feedback loop may be positive rather than negative.

[eggbert]

Are you using shielded wire for the grids?  Happened to me once. The shielded was bad/ or caused oscillation and created bad squeal. I replaced the grid wires with solid core 600v wire and it worked fine. Might not be your issue but worth a try. 

[mresistor]

Are you using shielded wire for the grids? 

Without a picture who knows..

[pda041376]

Well, swapped v3 and spot-checked/reflowed some solder joints. Quieter now than it ever was. Decided I needed to know actual cause so popped old v3 back in and sure enough the noise came back. Chalk it up to a bad tube. Thanks all.

[dude]

    Chalk it up to a bad tube. Thanks all.

Hum...? usually checking tubes is one of the first things done, saves a lot of unnecessary diagnosing. 

[pda041376]

Agreed, still learning. I know I should have checked the easiest thing first but was perplexed because it was working earlier that day and after I changed out a speaker in the cab it went wonky. Was thinking the speaker might have been a factor but know better now to always check tubes when freak stuff happens.