SVT Troubles

[Barbaric.Eric]

Hey gang,
I've been having some issues with my '04 SVT CL and I was looking for some advise from the top brains here on TB before taking it into the shop, just in case these are common problems with simple fixes (can't hurt to dream) and money is tight right now.

So here's the rundown: when the amp is switched off of Standby (after ample warm up time), there is loud continuous crackling coming from the cab with both Volume and Gain pots at zero position. It sounds like a loud buzzing hum, with random crackling amidst. I have tried several patch cables in both inputs which all seem to be fine, I have tried both speakon and regular speaker cables in the back which are also good. I have experienced this at our (usually trustworthy) jamspace, at home and at a respected live venue which should rule out dirty power. Tubes appear to be the original Russian-made Sovtek 6550WE (circa 2004 I assume) and the bias is bang on. Also, and this is most bizarre, the only thing that appears to affect the buzzing is tapping just above Ultra Lo/Hi switches, between the Gain and Bass pots (there's another tube in there isn't there?). The tapping changes the crackling and hum but does not make it disappear. I have popped it open just to check that the connections are tight and solders are strong and was only able to find one somewhat loose ribbon connector, which didn't change a thing once I seated it properly again.

After anything between 5 to 30 seconds it will dissipate and it good to play for the rest of the session. However I have noticed a lack of oomph since this began, and at times I can hear wavering volume levels. Also, right after the crackling disappears, I have noticed a bit of crunch on the top end, as if it's redlining the gain, but the Gain pot is set very low.

Does this ring a bell for anyone, or point towards a certain part(s) of the amplifier that may be at fault? Just looking to tap into some of your knowledge of amps and their parts, any advice is welcome.

Cheers,
E

 

[beans-on-toast]

Yes, it sounds like a bad tube.

The crackle could be a loose tube contact. The tube sockets should be checked to see that the terminals require retensioning. The fluctating volume is also a sign of a bad tube. I would start with the preamp tubes, they are in the upper chassis. Could also be that the tube socket contacts need a cleaning.

 

[Barbaric.Eric]

Thank you for the quick reply @beans-on-toast, is it possible for a pretube to fail without any visual signs? I've heard to take a look for slight burn or char marks on the glass but wasn't able to locate any on mine. I assume so but it doesn't hurt to check. And best bet for a tube socket cleaning would be a little spritz of Deoxit or is there a specific method for cleaning sockets?

 

[bobyoung53]

Thank you for the quick reply @beans-on-toast, is it possible for a pretube to fail without any visual signs? I've heard to take a look for slight burn or char marks on the glass but wasn't able to locate any on mine. I assume so but it doesn't hurt to check. And best bet for a tube socket cleaning would be a little spritz of Deoxit or is there a specific method for cleaning sockets?

I don't think you'll see anything with a bad preamp tube, I never have anyway, sometimes the worst looking tubes sound the best, they get dirty after some years especially with the fan blowing on them. Deoxit is the best I can think of for tube sockets.

 

[Barbaric.Eric]

Great, that's good to know. Thanks @bobyoung53! I've never used Deoxit before but am I correct in assuming it's a matter of apply, let dry, reinstall old or new tube?

 

[bobyoung53]

Great, that's good to know. Thanks @bobyoung53! I've never used Deoxit before but am I correct in assuming it's a matter of apply, let dry, reinstall old or new tube?

Yes and don't spray a whole lot of it and wiggle the tube around a little after you put it in, could be as beans said just a loose tube, happened to me when an SVT head fell in the airport a few months ago, a preamp tube was practically out of the socket and the head was dead until it was reinserted.

 

[Barbaric.Eric]

Interesting, thanks a tonne you guys. This is exactly the kind of thing I'd like to rule out before taking it to a tech. Plus, I love learning a bit along the way. Much appreciated!

 

[beans-on-toast]

Thank you for the quick reply @beans-on-toast, is it possible for a pretube to fail without any visual signs? I've heard to take a look for slight burn or char marks on the glass but wasn't able to locate any on mine. I assume so but it doesn't hurt to check. And best bet for a tube socket cleaning would be a little spritz of Deoxit or is there a specific method for cleaning sockets?

As mentioned yes. When tubes get old, they wear down, like a car tire, and the performance changes. You wouldn’t see this by looking at it. When a tube fails internally, it can burn up, sometimes the glass envelope even cracks and you would see that.

Don't spritz Deoxit onto a socket. It can get into places where it shouldn't be and cause a problem. Apply a little to a toothpick and rub it onto the contact. I use an interdental brush with deoxit on it to scrub the metal, they available from a pharmacy. When cleaning the socket contacts, you have to be careful so as to not permanently stretch them open. Otherwise, they will need to be retensioned to pinch them closed.

Here is a link that you should check out: How Do I Know When to Change My Tubes?

NOTE: Even with the amp unplugged you can get a bad shock from the charged power supply capacitors. Be very careful, do not touch anything that you shouldn't. A tech would discharge the capacitors befor working on the amp.

 

[JMacBass65]

Yes, it sounds like a bad tube.

THe crackel could be a loose tube contact. The tube sockets should be checked to see that the terminals require retensioning. The fluctating volume is also a sign of a bad tube. I would start with the preamp tubes, they are in the upper chassis. Could also be that the tube socket contacts need a cleaning.

Thank you for the quick reply @beans-on-toast, is it possible for a pretube to fail without any visual signs? I've heard to take a look for slight burn or char marks on the glass but wasn't able to locate any on mine. I assume so but it doesn't hurt to check. And best bet for a tube socket cleaning would be a little spritz of Deoxit or is there a specific method for cleaning sockets?

A couple of years ago I had nearly the exact symptoms you describe. Mine is a 1976 SVT, so no idea if they differ much.

Anyway, @JimmyM tipped me off to a particular preamp tube (a weird one, and the only one used in the amp) shared by both channels, said that that particular tube was prone to failure.

As it turned out, I actually had a spare lying around (it was likely the original, that for some reason I swapped out). Anyway, I dropped it in and......problem solved.

@beans-on-toast has given you good guidance as well, he knows his stuff. I have learned a lot just reading his posts on tube related things.

If I can find my original thread, I'll link to it.

 

[JMacBass65]

A couple of years ago I had nearly the exact symptoms you describe. Mine is a 1976 SVT, so no idea if they differ much.

Anyway, @JimmyM tipped me off to a particular preamp tube (a weird one, and the only one used in the amp) shared by both channels, said that that particular tube was prone to failure.

As it turned out, I actually had a spare lying around (it was likely the original, that for some reason I swapped out). Anyway, I dropped it in and......problem solved.

@beans-on-toast has given you good guidance as well, he knows his stuff. I have learned a lot just reading his posts on tube related things.

If I can find my original thread, I'll link to it.

Find the post from a few years ago, have a read and maybe you will find its the same issue.

BTW, it was @Mark Reccord who tipped me off to the specific tube, as it turns out. But Jimmy and Beans were helpful too, Beans including schematics too!

'76 SVT - Issue w/ preamp? power amp? can't tell?

 

[Wasnex]

A couple of years ago I had nearly the exact symptoms you describe. Mine is a 1976 SVT, so no idea if they differ much.

Anyway, @JimmyM tipped me off to a particular preamp tube (a weird one, and the only one used in the amp) shared by both channels, said that that particular tube was prone to failure.

As it turned out, I actually had a spare lying around (it was likely the original, that for some reason I swapped out). Anyway, I dropped it in and......problem solved.

@beans-on-toast has given you good guidance as well, he knows his stuff. I have learned a lot just reading his posts on tube related things.

If I can find my original thread, I'll link to it.

The unique (problematic) tube that mixes the channels in the vintage SVTs is a 6C4. I don't believe the CL uses this tube.

 

[Wasnex]

Yes, it sounds like a bad tube.

THe crackel could be a loose tube contact. The tube sockets should be checked to see that the terminals require retensioning. The fluctating volume is also a sign of a bad tube. I would start with the preamp tubes, they are in the upper chassis. Could also be that the tube socket contacts need a cleaning.

Beans,

Here's my thoughts...feel free to correct me.

Gain and Master are down and bias is stable so hopefully the problem is V2 in the preamp or the phase inverter. If the bias was unstable, I would probably suspect a driver or output tube. Of course, stable bias does not totally eliminate a bad driver or output tube.

I would probably pull V2 and turn the amp on and check for noise. The second triode of V2 comes after the Master Volume so it can still inject noise to the power amp with the master volume down. If the noise remains, the next tube in the signal path is the phase inverter; this is the only 12AX7 in the bottom of the amp. Turn the amp off and pull the phase inverter. If the amp is still noisy with the phase inverter out of the circuit, I would turn the amp off and let the 6550s cool down. Remove and reseat the two 12AU7s in the bottom of the amp a couple of times and also remove and reseat all of the 6550 output tubes. If all of this does not correct the malfunction, I would probably recommend taking the amp to a tech.

I don't advise pulling the 12AU7s from the amp and turning it on is this does not provide the same sort of test. Pulling the 12AU7s should effectively bias the output tubes off. In order to safely troubleshoot the 12AU7s at home you would need to: 1. Max out the bias so the amp is running cold, 2. turn the amp off and replace one of the 12AU7s with a new tube, 3. turn the amp on and reset the bias to the proper operating range, 4. listen for noise, and 5 repeat steps 1 through 4 for as necessary for the other 12AU7.

 

[rtslinger]

Sounds like a tube I would take it to a tech myself unless you know how to discharge the capacitors, I have found out the hard way the inside of a tube amp isn't anything to play with even after you unplug it good luck

 

[beans-on-toast]

Beans,

Here's my thoughts...feel free to correct me.

Gain and Master are down and bias is stable so hopefully the problem is V2 in the preamp or the phase inverter. If the bias was unstable, I would probably suspect a driver or output tube. Of course, stable bias does not totally eliminate a bad driver or output tube.

I would probably pull V2 and turn the amp on and check for noise. The second triode of V2 comes after the Master Volume so it can still inject noise to the power amp with the master volume down. If the noise remains, the next tube in the signal path is the phase inverter; this is the only 12AX7 in the bottom of the amp. Turn the amp off and pull the phase inverter. If the amp is still noisy with the phase inverter out of the circuit, I would turn the amp off and let the 6550s cool down. Remove and reseat the two 12AU7s in the bottom of the amp a couple of times and also remove and reseat all of the 6550 output tubes. If all of this does not correct the malfunction, I would probably recommend taking the amp to a tech.

I don't advise pulling the 12AU7s from the amp and turning it on is this does not provide the same sort of test. Pulling the 12AU7s should effectively bias the output tubes off. In order to safely troubleshoot the 12AU7s at home you would need to: 1. Max out the bias so the amp is running cold, 2. turn the amp off and replace one of the 12AU7s with a new tube, 3. turn the amp on and reset the bias to the proper operating range, 4. listen for noise, and 5 repeat steps 1 through 4 for as necessary for the other 12AU7.

The logic of the approach is good. I would change the order of attack.

I would first isolate the preamp and power amp by trying each with another amp to identify where the problem resides. Take the pre out into another amp and test it. Then take a pre from another amp and feed that into the power amp. This could help determine if the problem resides in the pre or the power amp.

More often than not, this issue is likely a simple one, down to a tube. If the amp hasn’t been serviced in a while, it never hurts to start with basic maintenance. Deoxit the pots and all metal-to-metal contacts, and check the tube socket tension. This levels the playing field for the next level of troubleshooting.

You can start with test equipment readings but without going to a tech, you can first swap out the tubes with known good ones and see. By doing so, one tube at a time, it can help identify if it is one tube that is causing the issue.

Experience tells me that this issue is more likely to be a small signal tube, most likely in the preamp.

Since the 12Ax7 and two 12AU7 tubes are in the bottom chassis and easiest to get at, swap them out starting with the 12AX7 and see. If that doesn’t help, proceed to the preamp. There I would start swapping with the first tube in the signal path and proceed down the signal chain.

 

[JMacBass65]

The unique (problematic) tube that mixes the channels in the vintage SVTs is a 6C4. I don't believe the CL uses this tube.

Yes, that's the one. No idea if the OPs SVT uses this tube though.

I would first isolate the preamp and power amp by trying each with another amp to identify where the problem resides. Take the pre out into another amp and test it. Then take a pre from another amp and feed that into the power amp. This could help determine if the problem resides in the pre or the power amp.

This is what I did, and it pointed me in the direction of the preamp. From there, the wise men of TB pointed me in direction of the 6C4 tube.

 

[JimmyM]

Yes, that's the one. No idea if the OPs SVT uses this tube thoug

This is what I did, and it pointed me in the direction of the preamp. From there, the wise men of TB pointed me in direction of the 6C4 tube.

Right, but the SVT-CL doesn't use the 6C4. They went by the wayside in the 80's.

 

[BogeyBass]

Possible bad tube or one that has worked loose and not fully seated. Cathode to heater shorts can be random. Sometimes obvious on cold start. And taps n bangs can make it go away or start again.

Or solder connection is going bad somewhere. And intermittent bad/good like socket or large component stressing the leads.

If it is bad tube then the bad one needs replacing. You can tap the tubes and listen for metalic pings. Thats usually loose plates. Cathode to heater short is harder to find. Wont ping and sometimes noise stops at warm up and on off cycles wont make it happen again. Could be lucky and be the one near the area your tapping.

Sucks to shot gun replace all the pre tubes. But if there all ancient factory tubes it woulnt hurt. Almost worth it cause its pain in butt to open up a svt case. Likewise its good time to inspect and touch up the preamp board while your there.