Ghost Notes - Use Scope to Track down ?

[HaroldBrooks]

I love my Gretsch 6162, but sometimes I hear faint ghost notes on the decay of high notes when the gain is pushed with a boost up front (top 3 strings above 12th fret). When playing with a band, they are inaudible.

I've used different guitars, changed and added filter capacitors, changed all tubes. Got better, and to be honest they are only audible when using horrific gain levels, but I am now overdue for buying and learning how to use a scope, so I ask anyway.

I am using old 6973 tubes in this amp, but they are balanced (at an idle) and both seem to be in very good condition and strong, as are all the 12AX7 tubes. I've swapped in new JJ 12ax7's and there's no difference.

Actually the problem is very, very small, and if there is no solution, no problem, but it's a slow day for me ! I boosted the gain of the amp by bypassing the weird tone circuit, and the distortion actually got much smoother in the process, but no real change In the occasional ghost note.

One more thing, there is still a residual 60 cycle hum (it's definitely 60 cycle not 120hz) I've tried to rid the amp of, and I've probably nocked the hum in half, but the power transformer on this amp is real big, and located pretty close to the PI & power tubes, and the output transformer, and directly below the second gain stage V4 tube on a sub chassis.

Can ghost notes be diagnosed with a scope ? or is total elimination a wizard's art ?

Thanks for your help !

Schematic (mostly correct, but only one channel on my 1964 version) :

http://www.oldfrets.com/Valco/Schema...retsch6162.pdf

[glebert]

Have you tried a different speaker? Just trying to rule out any external thing.

[nosaj]

Have you tried a different speaker? Just trying to rule out any external thing.

Ghost notes are usually a symptom of failing filter caps.

nosaj

[The Dude]

My guess is oscillation, which you should be able to see on a scope.

[The Dude]

Ghost notes are usually a symptom of failing filter caps.

nosaj

I agree. It usually is, but he already changed the filter caps.

[nosaj]

My guess is oscillation, which you should be able to see on a scope.

Which I learned today on how to see if an oscillator is oscillating. On a tube CB I have I wasn't sure if it was oscillating probing it directly stopped it. So it was suggested to take some thin insulated wire and do 6 wraps about thumb sized diameter clip one end to scope probe and the other end to the ground clip and hold it near the oscillator.
I can see it and the the frequency readout that it is oscillating at 31.4 mhz.

nosaj

[HaroldBrooks]

That is correct replaced all the big caps, and added some capacitance actually to the first node, from 20uf to 40uf.

But I've also changed things a bit, so I won't rule out not having enough capacitance at points. This amp is LOADED with big capacitors ! Two cans with three caps in each and a few 20uf independent ones for good measure. Not to mention a few 25uf cathode bypass caps for various reasons.

[HaroldBrooks]

Might be, you just reminded me that when I back off on one frequency with my parametric EQ, right around 550hz give or take, the ghost notes are diminished, and at super high gain is sounds like it's getting ready to feedback right in that area, but doesn't if I cover the strings, so it might amount to just that.

When I jumped around the weird tone control (Hexagonal on the schematic), it sounded much better, but I believe it also might have brought up some existing circuit or oscillation resonance, and not necessarily all good.

So a scope is in my future. Just wasn't sure if it would help, but it sounds like it might.

[nosaj]

Might be, you just reminded me that when I back off on one frequency with my parametric EQ, right around 550hz give or take, the ghost notes are diminished, and at super high gain is sounds like it's getting ready to feedback right in that area, but doesn't if I cover the strings, so it might amount to just that.

When I jumped around the weird tone control (Hexagonal on the schematic), it sounded much better, but I believe it also might have brought up some existing circuit or oscillation resonance, and not necessarily all good.

So a scope is in my future. Just wasn't sure if it would help, but it sounds like it might.

Let me know when your ready and I'll put you in touch with guy who sells calibrated scopes at great prices.
nosaj

[HaroldBrooks]

Yes, I did swap out the good Jensen C10Q s for Celestion Greenback (10") speakers. Same thing, but I read that could be the cause as well. Thanks.

[HaroldBrooks]

Thanks, will do. I have to hide the 'new' scope from the wife .

Just bought another old National amp a couple of weeks ago, and she wasn't happy, But You gotta do what you gotta do !

[nosaj]

Thanks, will do. I have to hide the 'new' scope from the wife .

Just bought another old National amp a couple of weeks ago, and she wasn't happy, But You gotta do what you gotta do !

Once you get enough stuff it all becomes that electronic junk to them. But if you want to stay married you'll have to find your balance, cause it's not hard to get fully immeresed in this rabbit hole.
nosaj

[The Dude]

When I was married and my wife would complain about my purchases, I'd just open up her shoe closet and remind her......

......but, that could be why I'm no longer married.

[Enzo]

OOh nononononono Never escalate.

[g1]

I'm not seeing a ground reference for the heater circuit? Is there a hum/balance pot or resistors somewhere? Or one side tied to ground or the power tube cathodes?