champ with loud squeal volume down

[unclerny]

Hi Guys, I have the oddest thing yet.  I have an AA764 champ in the shop that has a very loud squeal with the volume turned down.  If I turn the volume up past 3-4 it goes away and appears to work normally.  maybe a bit of sizzle but very little.  All the voltages appear good and when turned up I hear no power supply buzzing or coupling noise.

only when the volume is turned down does it squeal.  If I pull the 12ax7 there's no squeal.  I tried three so it's not the tubes.  I pulled the volume pot return form the grid of the second triod then tried to connect a ground to it and it squeals.  It still has all it's original caps but I've never seen this sort of failure.

Yes I've tried new tubes through out.  I'm thinking power supply/ tube bias.

opinions?

Tim Wagoner

[Willabe]

Since you have changed all the tubes with known good tubes and it's still acting up and it has all the original filter caps/K bypass caps, I'd change all the e-caps and see what happens. They need to be changed anyway.

If that doesn't fix it I'd test the coupling caps next for dcv leakage. It's a champ so that wont take long.


           Brad      

[stratele52]

No mod on this Champ ? What about wiring ? Any picture

Look with a 'scope if this signal is coming from 12AX7 pin 1 or pin 7 or pin 6 . You'll know where to look

[HotBluePlates]

...  I have an AA764 champ in the shop that has a very loud squeal with the volume turned down.  If I turn the volume up past 3-4 it goes away and appears to work normally.  maybe a bit of sizzle but very little.  ...

Look with a 'scope ...

+1.

Sounds like a parasitic oscillation. A scope would help you pin down whee it's happening. Also, "sizzle" could be an ultrasonic oscillation (like 100kHz) riding on top of a clean signal, which then sounds buzzy/raspy.

[terminalgs]

I'd change all the e-caps and see what happens.

+1.

I suspect the 20uf/450V that the two 100K plate resistors connect to..

[Platefire]

I had a DIY type champ act like that once and the problem turned out to be one of the resistors from grid to ground was not connected to ground. Platefire

[sluckey]

With the amp squealing, put a grounded gator clip lead directly on pin 7 of the 12AX7. Does the squeal stop?

[jjasilli]

1.   "I have an AA764 champ in the shop"  Is this the whole truth and nothing but the truth?    AA764 could be Champ or VibroChamp.  Is there a trem circuit?

2.  Listening amp diagnostics may help.  

• Pull the preamp tube.  Does the power amp squeal by itself with no signal?  Feed signal to the power amp from an outside source, bypassing the internal preamp.
• Feed the internal signal to an outside power amp (listening amp).  Any squeal?

[unclerny]

I guess I forgot to set notify,hmm.  Thanks you guys for all the suggestions.

I will start at the top.  As to power caps, there's no visible or audible thing to note that leads me to believe the multi-cap is the issue.  All voltages measure dead on what I'd expect.  I did change out the cathode caps, there are three.  One was clearly bad, on the 6v6 cathode.

sluckey: a ground will make it squeal.

jjasilli:  it doesn't squeal with the preamp tube out, dropping a signal onto the power amp sounds normal.  The preamp signal to another amp sounds normal also.  it's like an interaction between the stages.  I tried lifting lots of leads and sometimes the squeal goes away but at the cost of performance like gain loss.

okay, more thoughts.  Owner gave me a hammond 1750c OT he had new from a project he never did.  Tried that.  The squeal goes away!  BUT now the notes fizzle out rather fast instead of clean note decay like we'd expect.  I'm going after the 470 2w on the cathode of 6v6 first although it measures good.  IMO this amp has a history of eating 6v6 tubes, just gut feeling.

I'm going to fire up my scope today I hadn't really thought about an oscillation that high.  I do have a multi-cap for it I'm just trying not the change a part that is hard to undo later.  I suppose I could bridge in singles to prove one bad section.  That wouldn't be too bad.

Thank you guys, Tim

I'll get back

[unclerny]

definitely not the 470 2w

[stratele52]

definitely not the 470 2w

+1

[stratele52]

No mod on this Champ ? What about wiring ? Any picture

Look with a 'scope if this signal is coming from 12AX7 pin 1 or pin 7 or pin 6 . You'll know where to look

No answer   

Mods ? Pictures ? Wiring ?

[unclerny]

sorry about that, it's completely stock, I cut out the death cap.

I'm overlooking something but damned if I see it.

Tim

[stratele52]

sorry about that, it's completely stock, 
 
Tim

Sure of that ?  You have lots of  experience on repair amps ?

[PRR]

Squeals when turned to zero.... I'd be examining the grounding real-real close.

There isn't much to a Champ, but a minor flaw in ground can throw output signals back to input stages and cause squeal.

[stratele52]

Squeals when turned to zero.... I'd be examining the grounding real-real close.

There isn't much to a Champ, but a minor flaw in ground can throw output signals back to input stages and cause squeal.

+1  but how can a ground "move" if no  mods ? If it original ?

[unclerny]

today I swapped out each power cap, one at a time with no results.  Amp still fizzles.  tried several resisters after measuring everything.  Nothing seems to improve this amp.

Tim

[unclerny]

Thanks guys, examined every ground very closely and I don't see a problem, all good and tight.  I lifted the .02 from the 6v6 grid and dropped that into the front of another amp.  It's clean, excellent in fact. 

I also lifted the speaker jack off the chassis just to be sure that there wasn't something just barely touching to cause the problem and no change.

put the scope on the power supply and it is not carrying any signal on top of it.  I didn't think there would be after swapping power caps earlier.

I'm down to the octal?  it looks great but I suppose it could be bad.  Pretty weak.

Tim

[unclerny]

here's a pic but there's not much to see.

[sluckey]

There should be a brass plate behind the pots and jacks on the front panel. It's used as a ground plane or buss. There should be about 3 ground wires from the board that are soldered to that brass plate. The brass plate is famous for loosing it's good chassis connection and causing weird problems such as you have.

[unclerny]

sorry Sluckey, no brass plate in this one.

[SILVERGUN]

conductive board?

[stratele52]

Why don't you put heater with two wires , remove the pins to the chassis ? This will quiet the amp .  

This won't fix the amp's squeal .

Wire dress; very poor , THIS could be your squeal but not sure. Clean it .
Lead dress is one of the first thing I do when I receive amp , always a ( great?)  improvement.

[eleventeen]

The amp looks fine, and you are there and I am not, so take my far-fetched suggestions accordingly.

The output transformer secondary (speaker side) is grounded via the ground contact on the RCA speaker output jack. I'm not sure I like that. That ground contact is via two rather smallish pieces of metal on the chassis side (underside) of the RCA jack, as mounted. And that connection is dependent upon the pressure exerted by the two mounting screws for that jack, and those screws cannot be overtightened or the bakelite would crack. If those front contacts are corroded, you could have a high resistance in the transformer secondary causing you some grief by reflecting intermittently back into the amp. This is a cousin to the suggestions already given you that you could have a grounding problem on the pots or the input jacks but it has the additional capability of creating a reflective signal back into the amp. Yes, it's a weird idea, but all the eyes on this forum aren't seeing anything out of the ordinary, so maybe it's something unseen/unseeable.

Because the amp looks right and you have checked the normal stuff, it starts to smell like there's maybe something corroded making a hinky connection or maybe you have a cold but otherwise decent-looking solder connection somewhere.

And by the way, you could check this without disturbing anything by connecting a jumper from the black OT wire to a good source of ground and see if it affects your squeal. 

[punkykatt]

Maybe its the picture, but the solder joint does not look healthy on pin 7 of the 12AX7?

[eleventeen]

There are definitely a couple of questionable looking solder joints in the input section, and in general. Many of them look granular, crystalline, they don't look that great.

Also...the input jack has that 1 MB resistor to ground which is part of V1 first stage grid bias. Funny things can happen in 45 year old amps when the "hot" contact on the input jack gets a little corroded. Maybe that needs a little bit of clean up.

[eleventeen]

Any news from the repair dep't?