SVT CL Issues

[jaythebaron]

Hi,

I'm new to the board, and appreciate everyone's input in advance!
I'm going to take a stab at explaining my problem all while trying not to be too wordy.

A couple of years ago, 50 minutes into the gig, my 2004 SVT CL stops and working the green and red LED start flashing. I turned it off, gave it a sec and turned it back on and it worked for another half a song and went to green and red flashing. When I turned it back off and on again I noticed the power tubes flashed blue and it made a slight popping sound and could not get any sound out of it.

After finally getting around to metering things I discovered one of the R7 grid resistor on the power tube board was blown. Did the tube blow and take out the resistor, or did the resistor blow and take out the tube? Below are links to pics of the values I got. The schematics show that I need a 220 ohm 1/2 watt 5% tolerance resistor. I have ordered these from two different companies and they both sent me 2% tolerance resistors. I'm giving it one last shot and am ordering from Mouser since they claim they have the 5% in stock.

Just out of curiosity, if I changed all of the grid resistors to the 220 ohm 1/2 W 2% resistors that I currently have, would that be a problem? Below are links to pics of the values and boards that I measured with a multimeter.

After metering the power tube PCB, I found the R7 resistor (pictured below) gave me no reading on the meter. It is the one next to the bottom left tube socket with a blue arrow pointing to it along with the "tested by" sticker. Don't mind my toes...

http://s1383.photobucket.com/user/ja...tml?sort=3&o=0

And here are the recorded values for this board. Does anyone see any issues other than R7?

http://s1383.photobucket.com/user/ja...tml?sort=3&o=6

And here are the values for the other boards.
http://s1383.photobucket.com/user/ja...tml?sort=3&o=2

http://s1383.photobucket.com/user/ja...tml?sort=3&o=1

http://s1383.photobucket.com/user/ja...tml?sort=3&o=5

http://s1383.photobucket.com/user/ja...tml?sort=3&o=4

http://s1383.photobucket.com/user/ja...tml?sort=3&o=3


Any insight you guys could give me would be greatly appreciated.

Thanks in advance

 

[RoadRanger]

Most likely the tube took out the resistor - fairly common. There's nothing wrong with replacing the blown one with a 2%, anything 5% or less is fine . Obviously replace that tube also before powering it up again. Unfortunately a tube tester can't detect a cheaparse carp tube that arcs over at a lower than spec voltage . The old tube might even work for a while until it takes out the resistor again anywhere from instantly to several years from now - it's a carpshoot .

 

[beans-on-toast]

Concerning the tolerance, +/- 2% is better than +/- 5%, it has a tighter tolerance; 1% is even better. Flameproof is important. I like to but a bunch and measure and match them.

By the way, the resistor that you are referring to is a screen resistor. It is possible that low voltage at the gig caused excessive currents which blew your tube and it took the resistor out with it. The specified resistors are rated for ½ watt. This allows the resistor to act like a fuse and blow as it did. This protects other things.

When mounting the resistor, offset it an eighth of an inch off the board. This helps protect the board when the resistor blows.

I couldn't view any of your images.

 

[B-string]

The pics won't work for me either. I also over-order and hand match. Flame-proof metal-oxide is the best way to go for those screen resistors.
Sounds like you need at least one tube that closely matches the others. You very well may have blown a HV fuse as well after continuing to try using the amp.

 

[jaythebaron]

Thanks for clarifying the screen resistors. Should I replace all of them with the 2% so they would be mostly matched?

B-String what/where is HV fuse? I have the head completely taken apart. All of the fuses I found were still fine.

Sorry about the images, I'll work on them this evening. Thanks for the replies!

 

[BassmanPaul]

Jay you have me terrified. Have you experience working in very high voltage equipment?? There are lethal voltages in that amp!

 

[B-string]

Yes replace and match all the screen resistors.

I thought I remembered a High Voltage (HV) fuse, looking at the schematic it just has a high voltage lock-out relay.

 

[beans-on-toast]

Should I replace all of them with the 2% so they would be mostly matched?

I would to keep things consistent. You are in there anyway.

We addressed the issue without the images although it is always good to provide a full picture of the issue.

As I recall, there are two internal fuses, F1, and F2, both on the primary side of the transformers. There are six protection 1N4007 diodes in front of the output transformer, perhaps this is what B-string was referring to. These are intended to filter spikes from the power tubes. This can occur when a tubes goes. It is a good idea to check these diodes or simply replace them if you are unsure. The diodes are strung together in series, three on a side of each power tube triplet from HV to ground.

 

[B-string]

I would to keep things consistent. You are in there anyway.

We addressed the issue without the images although it is always good to provide a full picture of the issue.

As I recall, there are two internal fuses, F1, and F2, both on the primary side of the transformers. There are six protection 1N4007 diodes in front of the output transformer, perhaps this is what B-string was referring to. These are intended to filter spikes from the power tubes. This can occur when a tubes goes. It is a good idea to check these diodes or simply replace them if you are unsure. The diodes are strung together in series, three on a side of each power tube triplet from HV to ground.

Nope, it was a brain fart . Good point on the clamp diodes though.

 

[RoadRanger]

Thanks for clarifying the screen resistors. Should I replace all of them with the 2% so they would be mostly matched?

I'd not bother unless you're OCD?

 

[beans-on-toast]

I'd not bother unless you're OCD?

If the amp is buttoned up without changing the other screen resistors, then you may have repeated thoughts that you should have done it which could lead to OCD.

Better safe than sorry.

 

[B-string]

I think it no more OCD than buying "matched" tubes, that will be unmatched if the screen dissipation varies due to screen supply.

 

[RoadRanger]

If the amp is buttoned up without changing the other screen resistors, then you may have repeated thoughts that you should have done it which could lead to OCD. Better safe than sorry.

They only blow when a tube goes - maybe you should replace all the tubes, resistors and capacitors too while you're in there ?

Of course, if any resistors look toasted or capacitors look swollen it might be a good idea to change them .

BTW when yous OCD guys have your amp completely re-tubed when a single power tube bites it what do you think happens to the old-but-good tubes ?

 

[beans-on-toast]

I think it no more OCD than buying "matched" tubes, that will be unmatched if the screen dissipation varies due to screen supply.

Your degree of OCD dictates how matched your tubes should be.

 

[beans-on-toast]

They only blow when a tube goes - maybe you should replace all the tubes, resistors and capacitors too while you're in there ?

Of course, if any resistors look toasted or capacitors look swollen it might be a good idea to change them .

BTW when yous OCD guys have your amp completely re-tubed when a single power tube bites it what do you think happens to the old-but-good tubes ?

I match my own tubes so they go into the old-but-good tube tub.

Resistors might not be blown but they could have been stressed from use. Blown screen resistors are commonly found in these amps that have had tube issues. Sometimes tubes don't blow but they are pushed too hard which can take a toll with the resistors. It is more of a preventative maintenance issue to change them. I'd also want them all matched to optimize performance from a matched tube set as B-string pointed out. You don't have to be nuts about matching though. If it costs a couple of extra bucks to be thorough, why not.

Not now, but shoddy soldering was part of the problem at one time. Heat was another issue that caused problems. There was a customer service bulletin that SLM issued dealing with fractures on power board socket solder joints due to mechanical stress. Sockets are mounted on standoffs now to prevent this. Worth checking all solder joints while the power board is out.

 

[jaythebaron]

http://s1383.photobucket.com/user/jaythebaron/media/Power_zps0b2b9ca9.jpg.html?sort=3&o=0

Please let me know if the above link works or not. If you hover your cursor over the picture, it will show arrows to advance to the next picture.
Thanks for all of the discussion, it really helps a novice like myself and yes I am staying clear of the transformers. I'll probably go ahead and change out the resistors since it's relatively easy and I'll take your advice and leave some space between the board and resistors.

I retubed it a while back with Tung Sols and never really cared for them. I think when I do retube I'll go back to the Sovtek WE's. I do have a question regarding tube matching though. I bought this last set of tubes from thetubestore.com. They write a reference number on their tubes that deals with the matching. Would it be ok to just buy one tube if they can still match it? I still plan to retube, but for troubleshooting sake I'd rather blow these older tubes rather than new Sovteks.

From what I can tell all the diodes look ok, but I'll leave to you guys to tell me otherwise (see picture). I haven't noticed any swollen or odd looking caps and have no way of testing capacitance.

Thanks again folks

 

[beans-on-toast]

Yes the photobucket link is working. It looks like two of the resistors have already been changed, the beige ones. They may not be flameproof but it is hard to tell without knowing what they are. The other ones look typical of the flameproof ones that you see in these amps.

The diodes should be tested with a volt-ohm meter. There are different ways to test a diode. Some meters have a diode test function. See if there's a diode symbol on the meter that you can select. Your manual will explain how to do it.

Often there are two reference numbers on the tubes, a cathode current and a transconductance. But each reseller has their own system. If you give whatever is on the sticker to thetubestore people, and tell them what amp it is for, they will be able to sell you one tube that matches. Alternatively, you can send them your tubes to test and select one that is the best match for your set. Unless you are local, it will probably cost too much to do this with shipping.

If your warranty on the tube is still in effect, they should send you a new one. If you haven't used the amp a lot and the warranty has expired, if you ask politely, they may send you a new tube to replace the blown one. Worth asking them.

 

[jaythebaron]

I bought the amp new in 2004 and it's never been in the shop since I've had. Could it be possible that these resistors are factory installed?

I did a diode test with my multimeter. See the results here. http://s1383.photobucket.com/user/jaythebaron/media/photo1re_zpsc8009fad.jpg.html?sort=3&o=6

Do you guys see anything that looks unusual? I do plan to contact the tube store. They were helpful in the past although my warranty has long expired.

 

[beans-on-toast]

I bought the amp new in 2004 and it's never been in the shop since I've had. Could it be possible that these resistors are factory installed?

Anything is possible. It is odd that two are different.

The diode readings look ok, each shows a voltage. If there was a short or they were open, there would be a problem.

BTW, the missing diodes are supposed to be missing. In the original design there was a diode across each screen resistor. They are not necessary. There is a place for them for, I assume historical reasons, but they are not installed at the factory.

Let us know how it goes with the tube store.