Fixing my Ampeg SVT - CL Crackling Test Suggestions

[jmone]

I'm having a crackling issue during our last few practices so I decided to take my head home last night.

Thinking back my best guess is it only happens when I first turn on the amp and goes away after some long period of time 10-20 minutes. I only hear it when making noise on my bass and don't think it's sensitive to any playing dynamics or frequencies.

I have a laundry list of test to try short of buying new tubes like new speakon cable, clearing out dust, using another head for the pre/poweramp to isolate the circuit, etc etc....

Apart from the obvious tests - what other diagnostics can I try? I should be able to get access to any electrical equipment I need. Could anyone direct me to more in depth resources?

 

[Old Garage-Bander]

Cheap and easy to try...
Tap on the tubes with a wooden pencil or something similar (doesn't have to be wood) see if you get crackling associated with the tapping.

 

[beans-on-toast]

Sounds like the amp needs a general servicing. Deoxit on all metal to metal contacts including tube teminals and sockets, jacks, jack shunts, cable connector contacts. The tube sockets whould be re-tensioned if necessary. The insde of the pots cleaned. Tubes checked.

There are many possibilities. There are easy things to try on your own:

1) Try inserting an instrument cable between the preamp out and the power amp in. If a jack shunt is oxidized it can cause a crackle, loss if signal, dull sound.

2) A bad tube. Swap out the small tubes one at a time, remember the preamp tubes are inside the top chassis. It needs to be pulled forward to gain access.

3) This can help narrow down where the issue lies. Take the pre output and feed that into another amp. If the noise is gone, the issue can be in the power amp. Take another pre and patch it into the power amp in. This tests the power amp and eliminates the preamp from the circuit.

 

[jmone]

...
1) Try inserting an instrument cable between the preamp out and the power amp in. If a jack shunt is oxidized it can cause a crackle, loss if signal, dull sound.

What would I expect when I put a cable between the two? I'm bypassing the main connection of the unit so I would test if there is a bad connection?

 

[Old Garage-Bander]

What would I expect when I put a cable between the two? I'm bypassing the main connection of the unit so I would test if there is a bad connection?

Sometimes just inserting the plugs will clear oxidation on the switching part of the jack. A little shot of De-oxit would go a long way to help, if that is the cause.

 

[beans-on-toast]

What would I expect when I put a cable between the two? I'm bypassing the main connection of the unit so I would test if there is a bad connection?

With no cable inserted, the signal flows through a shunt in the jack. The shunt is a switch, the contacts become oxidized with time. This impedes signal flow. When the instrument cable is connected, the shunt is opened, signal now flows out the preamp out jack, through the cable, into the power amp in. This bypassed the shunt connection. If it corrects the issue, the shunt needs to be cleaned. The solution is to partially insert a plug which will open the shunt and apply deoxit to clean the oxidization. This is a maintenance task that most amps require fom time to time.

I presented this test because it is a possibility and easy to test. You have to sweat the details and check out the possibilities. But because the issue goes away after the amp is on for a while, it appears to be a heat related issue. A bad contact is getting better when metal is heated and expands. An easy thing to check is a bad tube or a loose tube socket contact. There are other possibilities that a tech could check out.

 

[jmone]

With no cable inserted, the signal flows through a shunt in the jack. The shunt is a switch, the contacts become oxidized with time. This impedes signal flow. When the instrument cable is connected, the shunt is opened, signal now flows out the preamp out jack, through the cable, into the power amp in. This bypassed the shunt connection. If it corrects the issue, the shunt needs to be cleaned. The solution is to partially insert a plug which will open the shunt and apply deoxit to clean the oxidization. This is a maintenance task that most amps require fom time to time.

I presented this test because it is a possibility and easy to test. You have to sweat the details and check out the possibilities. But because the issue goes away after the amp is on for a while, it appears to be a heat related issue. A bad contact is getting better when metal is heated and expands. An easy thing to check is a bad tube or a loose tube socket contact. There are other possibilities that a tech could check out.

Thanks for the explanation. I would like to learn how to service these things myself to the best of my ability. I just needed some starting points. I don't have a great way of testing the tubes other than tapping them so that will have to suffice.

I'm going to grab some deoxit and do that regardless.

The only other thing I can think is maybe I got dust on the pre-amp tubes when I removed them. I had to re-solder the L.E.D. which fell out and took the tubes off so I could remove the board.

 

[agedhorse]

Thanks for the explanation. I would like to learn how to service these things myself to the best of my ability. I just needed some starting points. I don't have a great way of testing the tubes other than tapping them so that will have to suffice.

I'm going to grab some deoxit and do that regardless.

The only other thing I can think is maybe I got dust on the pre-amp tubes when I removed them. I had to re-solder the L.E.D. which fell out and took the tubes off so I could remove the board.

Have you considered learning how electronics work as a first step in learning how to service? There are many classes available, either on line, at trade schools and at community colleges.

Disclaimer, there are very dangerous, potentially lethal voltages present in the SVT amp. Using a pencil is not a good idea because graphite is a pretty great conductor at high voltages. There are things that you simply should not mess with, these are the kinds of things you learn as you understand how electronics works. It's not easy, but can be rewarding if you follow through.

 

[BadExample]

An SVT could be a very expensive way to learn electronics.

 

[jmone]

Have you considered learning how electronics work as a first step in learning how to service? There are many classes available, either on line, at trade schools and at community colleges.

Disclaimer, there are very dangerous, potentially lethal voltages present in the SVT amp. Using a pencil is not a good idea because graphite is a pretty great conductor at high voltages. There are things that you simply should not mess with, these are the kinds of things you learn as you understand how electronics works. It's not easy, but can be rewarding if you follow through.

I have an electrical engineering degree and built and designed my own little 10w tube amp. A lot of my knowledge isn't in the power electronics field (more signal processing) but I'm not blindly sticking my hands into high voltage circuit.

Also have years of prototyping and hobby electronics experience outside of school. Again smaller voltage stuff but I'm not clueless.

My intent of this post was to hear more technical and amp specific things. For example I didn't know about the shunt until Beans mentioned it.

 

[BadExample]

No idea how good these are, but a manual might help: Ampeg Schematics

 

[jmone]

No idea how good these are, but a manual might help: Ampeg Schematics

Cheers, this will be a good resource should I need it.

 

[agedhorse]

I have an electrical engineering degree and built and designed my own little 10w tube amp. A lot of my knowledge isn't in the power electronics field (more signal processing) but I'm not blindly sticking my hands into high voltage circuit.

Also have years of prototyping and hobby electronics experience outside of school. Again smaller voltage stuff but I'm not clueless.

My intent of this post was to hear more technical and amp specific things. For example I didn't know about the shunt until Beans mentioned it.

See, I was not aware of your background which is why it's important to include disclaimers since others are also reading and following along and might not have the same knowledge that you have.

 

[jmone]

See, I was not aware of your background which is why it's important to include disclaimers since others are also reading and following along and might not have the same knowledge that you have.

I will be more aware of this next time. Maybe this isn't the correct forum to be asking these questions? I understand most people shouldn't be poking their fingers in there and I respect your caution.

 

[beans-on-toast]

When I started out, natural selection weeded out the kids that shouldn't touch tube amps.

These days, a five year old kid tucked away alone in the basement, soldering iron in hand, experimenting with a tube radio wouldn't go over too well.

 

[cerrem]

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...

 

[Chris 'Wighat' Jordan]

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

 

[agedhorse]

I will be more aware of this next time. Maybe this isn't the correct forum to be asking these questions? I understand most people shouldn't be poking their fingers in there and I respect your caution.

Most folks that survive a finger poking incident never do that again

 

[ak56]

Most folks that survive a finger poking incident never do that again

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.

 

[Old Garage-Bander]

Have you considered learning how electronics work as a first step in learning how to service? There are many classes available, either on line, at trade schools and at community colleges.

Disclaimer, there are very dangerous, potentially lethal voltages present in the SVT amp. Using a pencil is not a good idea because graphite is a pretty great conductor at high voltages. There are things that you simply should not mess with, these are the kinds of things you learn as you understand how electronics works. It's not easy, but can be rewarding if you follow through.

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.