Fixing my Ampeg SVT - CL Crackling Test Suggestions

[jmone]

If you use DeOxit, make sure to clean it up very well.. Personally would never use that stuff on anything other then cleaning potentiometers...
For the the tube sockets, use a contact cleaner that dries with NO RESIDUE....
The red de-oxit leaves a conductive residue that can make arcing and more noises on the high voltage tube sockets.....
Modern EE degree in DSP is not always relevant with respect to tube amps...
Highest probability is that its a pre-amp tube or pre-amp tube socket needs cleaning...

Thanks, I leaning towards the preamp socket being the issue too. Only thing that would explain why it goes away after awhile (heats up).

In terms of schooling, my degree is general EE. Took a bit of everything including analog electronics but most of my courses fell under DSP. Now I work in automation... point I'm making is I don't know everything (otherwise I wouldn't have posted) but I'm comfortable enough to try and wouldn't mind learning.

I'm having a similar issue. Can you post a sound clip?

Sure thing. I won't be able to test it until tomorrow but I'll get a clip and note anything I find.

When I was in High School, back in the dark ages, in our electronics class, we breadboarded tube circuits, using high-voltage bench supplies for power. I wish I was there to see it, but apparently our instructor made the ASSUMPTION one time that the supply was off, and reached in to check a connection in the circuit.

The story is that he landed on top of the adjacent file cabinets.

Haha I've watched my professor do the exact same thing!

I've also been at work and watched a certified electrician lick the end of a wire to test if I was connected. I mean he knew it was only 24V but that's not a gamble I'm willing to take...

Stupidity is everywhere.

 

[Old Garage-Bander]

Okay thanks, is this something a general hardware store would have or should I look online?

In terms of schooling, my degree is general EE. Took a bit of everything including analog electronic but most of my courses fell under DSP. Now I work in automation... point I'm making is I don't know everything (otherwise I wouldn't have posted) but I'm comfortable enough try and wouldn't mind learning.

I leaning towards the preamp socket being the issue. Only thing that would explain why it goes away after awhile.



Sure thing. I won't be able to test it until tomorrow but I'll get a clip and note anything I find.



Haha I've watched my professor in the modern age do the exact same thing! Pretty funny until it happens to you.

I've also been at work and watched a certified electrician lick the end of a wire to test if I was connected. I mean he knew it was only 24V but that's not a gamble I'm willing to take...

Stupidity is everywhere.

Stupidity yes. High School Electronics class and capacitor tossing.
But even from that foolishness, we learned to never trust that cap wasn't charged up.

 

[agedhorse]

The most dangerous aspect of getting a serious shock is not electrocution but collateral damage from what you bounce off of, or what heavy chassis may fall on top of you. That was my experience in 1979, it was a large Harman Kardon power amp from the late 1950's, I remember it like it was yesterday. I went over backwards and the chassis fortunately missed me, but not by much. I was bruised but survived without major disfigurement or concussion.

 

[BadExample]

Stupidity yes. High School Electronics class and capacitor tossing.
But even from that foolishness, we learned to never trust that cap wasn't charged up.

Bench booby traps can be fun... as long as you don't work for me, in which case it's only fun for me.

 

[Old Garage-Bander]

The most dangerous aspect of getting a serious shock is not electrocution but collateral damage from what you bounce off of, or what heavy chassis may fall on top of you. That was my experience in 1979, it was a large Harman Kardon power amp from the late 1950's, I remember it like it was yesterday. I went over backwards and the chassis fortunately missed me, but not by much. I was bruised but survived without major disfigurement or concussion.

No kiddin! In tech school I got on the wrong end of an 800 volt power supply. Knocked me off my bench stool, and squarely on my ass. The worst collateral damage, in this case was to my pride. Took a lot a guff from my classmates.

 

[beans-on-toast]

Go to a hardware store and buy a ¼" hardwood dowel. You can cut it to desired lengths, put one end in a pencil sharpener and use that to poke and tap inside the chassis. You can buy different diameter dowels, I have one turned into a mini baseball bat. Good for tapping on anything microphonic. A wooden drumstick works as well for tapping.

 

[agedhorse]

No kiddin! In tech school I got on the wrong end of an 800 volt power supply. Knocked me off my bench stool, and squarely on my ass. The worst collateral damage, in this case was to my pride. Took a lot a guff from my classmates.

In my case I could have landed on some units waiting to go on the bench, I'm lucky I didn't get really hurt.

 

[Gideon352]

I'm sure you're aware (given your EE degree), but it could definitely be a bad resistor.

 

[agedhorse]

I'm sure you're aware (given your EE degree), but it could definitely be a bad capacitor.

As an engineer, he's probably aware that it's unlikely to be a "bad capacitor" (or other internet myths).

WHY do you think it's a bad capacitor? I'm curious how you arrived at this conclusion considering the symptoms presented.

 

[Engle]

Thanks, I leaning towards the preamp socket being the issue too. Only thing that would explain why it goes away after awhile (heats up).

In terms of schooling, my degree is general EE. Took a bit of everything including analog electronics but most of my courses fell under DSP. Now I work in automation... point I'm making is I don't know everything (otherwise I wouldn't have posted) but I'm comfortable enough to try and wouldn't mind learning.



Sure thing. I won't be able to test it until tomorrow but I'll get a clip and note anything I find.



Haha I've watched my professor do the exact same thing!

I've also been at work and watched a certified electrician lick the end of a wire to test if I was connected. I mean he knew it was only 24V but that's not a gamble I'm willing to take...

Stupidity is everywhere.

No kidding! A certified electrician should have a volt tick with him.

 

[Gideon352]

As an engineer, he's probably aware that it's unlikely to be a "bad capacitor" (or other internet myths).

WHY do you think it's a bad capacitor? I'm curious how you arrived at this conclusion considering the symptoms presented.

Hahaha! Post edited. Thanks.


I had one of these years ago that was doing what the op described.
Turns out it was a bad resistor.

I'd grab a multimeter and the schematic, some libations and start testing out parts.

 

[jmone]

Go to a hardware store and buy a ¼" hardwood dowel. You can cut it to desired lengths, put one end in a pencil sharpener and use that to poke and tap inside the chassis. You can buy different diameter dowels, I have one turned into a mini baseball bat. Good for tapping on anything microphonic.


View attachment 1130097

Nice!!! Gonna get me one of those!

Hahaha! Post edited. Thanks.


I had one of these years ago that was doing what the op described.
Turns out it was a bad resistor.

I'd grab a multimeter and the schematic, some libations and start testing out parts.

It may very well be - but on a higher level I want to isolate where the problem exists and understand ways of testing that. Measuring across every resistors arbitrarily doesn't seem like the best use of time but I may conclude that is the issue somewhere in one of these circuits.

I'm going to first try and reproduce the issue myself at my own place to remove the possibility of electrical noise from my rehearsal space. Honestly it's just some dudes basement. Ill then start messing around with my other amp head to see if the issue is pre/power amp. Regardless I'll clean up with some deoxit and an equivalent no residue leaving cleaner for the tube sockets.

Stay tuned...

Appreciate the advice and all the funny stories

 

[agedhorse]

Nice!!! Gonna get me one of those!



It may very well be - but on a higher level I want to isolate where the problem exists and understand ways of testing that. Measuring across every resistors arbitrarily doesn't seem like the best use of time but I may conclude that is the issue somewhere in one of these circuits.

I'm going to first try and reproduce the issue myself at my own place to remove the possibility of electrical noise from my rehearsal space. Honestly it's just some dudes basement. Ill then start messing around with my other amp head to see if the issue is pre/power amp. Regardless I'll clean up with some deoxit and an equivalent no residue leaving cleaner for the tube sockets.

Stay tuned...

Appreciate the advice and all the funny stories

Correct, measuring a thermally intermittent resistor (if it even is a resistor, which is reasonably unlikely) is tough even if you have lottery winning luck. Not impossible, but I wouldn't want to bill a customer for that kind of labor

It's a LOT easier fixing the problem if you can clearly identify the cause. Likewise, if you can't identify the cause the repair is going to be that much more difficult.

A Q-Tip and a little DeOxit on the pins of the tube is a safe and effective way to improve contact conductivity and reliability. Spraying on the sockets... not so much as it can lead to arcing and carbon tracking from voltage breakdown.

 

[Old Garage-Bander]

Hahaha! Post edited. Thanks.


I had one of these years ago that was doing what the op described.
Turns out it was a bad resistor.

I'd grab a multimeter and the schematic, some libations and start testing out parts.

That makes my "poke it with a pencil" sound like a much better idea.

A resistor is even less likely to "go bad" than a capacitor.
They can change resistance, but usually that happens when they are overheated.
The overheating is often caused by something else being wrong.

You usually have to lift one side of a component to test it.
That's a lot of unsoldering and re-soldering.
Better to put that multi-meter to work checking voltages first.

But hey! Let's get some sharp sticks and a few beers and see how it all turns out.

 

[agedhorse]

Can we all run with sharp sticks? It's all fun and games until somebody loses and eye

 

[Old Garage-Bander]

Can we all run with sharp sticks? It's all fun and games until somebody loses and eye

All this time I thought this imojie was winking.

 

[SirMjac28]

This is me without Beans and agedhorse

 

[beans-on-toast]

This is me without Beans and agedhorse

Who hasn't done that! You start off by licking a 9 volt battery, then move up to more dangerous forms of discovery.

 

[Gideon352]

That makes my "poke it with a pencil" sound like a much better idea.

A resistor is even less likely to "go bad" than a capacitor.
They can change resistance, but usually that happens when they are overheated.
The overheating is often caused by something else being wrong.

You usually have to lift one side of a component to test it.
That's a lot of unsoldering and re-soldering.
Better to put that multi-meter to work checking voltages first.

But hey! Let's get some sharp sticks and a few beers and see how it all turns out.

Your "poke it with a pencil" idea is one way to go about it. I wouldn't go about it that way, but to each his own.

A resistor is bad when it's value drifts way beyond it's specified value (which was the case when I had this happen in an old CL I used to own).
What caused it? Probably a unicorn since it's so rare.

This particular (cathode) resistor was causing one of the output tubes to audibly crackle (due to voltage spikes). No fault detected by the comparator though.

I only brought this up as something for the op (and anyone else who is into internet myths) to be aware of.
Yes, resistors are a pain to check. I'd check the tubes first, of course.
But if you're doing this as a learning project it might be fun (and you might solve a more-rare-than-a-charybdis problem).

Grab your pencils and carry on!

 

[J-Bassomatic]

Of course, you are correct about the graphite in a pencil.
Should there be any stray high voltage on the envelope of the tube, it could be carried through the pencil.
I was going for a common household item, that the every-man might have available in that one kitchen drawer with all of the other non-kitchen stuff. But yes, not the best choice. DON'T GO STICKING PENCILS INTO YOUR AMP!

A pencil would have been less conductive than going in with a metal screwdriver or a butter knife.
Even if those items have plastic or wooden handles it would not be a good idea.

Of course I wouldn't expect a properly designed and properly operating musical instrument amp, with "user serviceable" parts (tubes) to have high voltage above the chassis and around the tube. That is unless the tube had a plate cap. If the cap itself, or the wire insulation were old and cracked, you might pick up a shock.
Of course if the amp is not operating properly, then all bets are off and extra precautions should apply as in find a qualified technician.

I've worked around multi-thousand watt radio transmitters with tube finals and never got myself into trouble.
Yet there was one time when I was prototyping an an antenna matching circuit for a 100 watt solid state radio, and I got a nasty RF burn.

Lesson... just because it's low power, doesn't mean it can't get you.

Get one of these maybe?
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