# Ampeg SVT Classic Issues



## dcole (Oct 8, 2008)

Hi Guys,

I have an Ampeg SVT Classic in my shop right now and am experiencing the following symptoms with it:

1) After sufficient warm up, turning standby off causes a very loud popping/crackling sound.
2) When the amp is turned up, hard picking of the bass strings causes crackling sounds for the first few seconds of the note.

I looked at all the solder joints and I found they looked good. I looked at the filter capacitors next and found the cap used for the negative bias supply was bulged so I replaced it. All the other caps in the vicinity look fine. I have traced the problem down to the power amp boards as the symptoms were still present when the pre-amp board was disconnected.

Have you worked on any SVT Classics or experienced this type of problem yourself? Its hard to know where to go from in this amp as its got two rather large circuit boards that are difficult to test off of. There's a lot to be said about using turret boards to help out a service tech!

Thanks,

David Cole


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## Wild Bill (May 3, 2006)

dcole said:


> Hi Guys,
> 
> I have an Ampeg SVT Classic in my shop right now and am experiencing the following symptoms with it:
> 
> ...


I have found that sometimes the large power resistors between the filter cap power supply nodes can be suspects. They carry lots of current so they can get old and have a little bit of arcing inside.

This problem is common to many tube amps. One of the sneakiest culprits is the standby switch! It can get a bit carboned up inside. Easy to check if the noise is consistent. Just jumper it out and see if the noise goes away. If it does, replace the switch.

Wild Bill Busen Amps


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## dcole (Oct 8, 2008)

Thanks Wild Bill, I was trying to PM you on this one but it said your inbox is full. I tried jumping out the switch and the noise was still there. I don't have any of the power resistor values I need unfortunately to test those out so I'll have to try that one tomorrow.


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## Wild Bill (May 3, 2006)

dcole said:


> Thanks Wild Bill, I was trying to PM you on this one but it said your inbox is full. I tried jumping out the switch and the noise was still there. I don't have any of the power resistor values I need unfortunately to test those out so I'll have to try that one tomorrow.


You might pull preamp tubes along the signal path to see if you can isolate the noise to one power supply node. That will tell you the resistor feeding it from the start of the supply is the one at fault. Still, resistors are rather cheap compared to an hour of labour. Since they are the same age anyway and there are only a few of them it makes more sense to change them all, at least to me. After all, you already have everything opened up.

Wild Bill


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## Wild Bill (May 3, 2006)

dcole said:


> Thanks Wild Bill, I was trying to PM you on this one but it said your inbox is full. I tried jumping out the switch and the noise was still there. I don't have any of the power resistor values I need unfortunately to test those out so I'll have to try that one tomorrow.


Yeah, I am a bit lackadaisical about emptying out my mailbox but I don't like PMs for tech help anyway!

You see, I get a LOT of them! A lot of them tend to be repeats! I am busy enough trying to make a living so I think it more efficient to discuss tech problems in an open forum, rather than privately. This means that others might benefit as well, heading off those repeat questions from others with the same problem!

Even better, there are other techs on this board who are very experienced, often more than I am. Nobody knows everything! Except those internet gurus who want you to think they know it all and want you to buy over-priced or unnecessary stuff from them, of course! Posting in an open forum gives you more pros to draw from - meaning you are more likely to get a useful answer!

Wild Bill


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## dcole (Oct 8, 2008)

Thanks for the replies. I would imagine you would get a lot of PM's as you seem to know what your doing. I appreciate the help. I am hoping to get the schematic for this amp soon from the manufacture to help narrow things down. I'll try replacing all the power supply dropping resistors and see how that goes. 

Its sure annoying to troubleshoot issues like this in an amp of this size but my goal is to get the amp working for the owner.


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## dtsaudio (Apr 15, 2009)

It may not solve your problem, but re-solder the power tube sockets even if they look ok. There were two service bulletins released about fractured solder joints in these amps.


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## dcole (Oct 8, 2008)

I resoldered the sockets and have replaced the two big 5 watters in the power supply so I'm crossing my fingers that it works. Will report back later.


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## jb welder (Sep 14, 2010)

Schematics here: http://www.amparchives.com/album/Ampeg/Schematics/Ampeg SVT-CL Schematics/index.html

You said loud bass notes can make the problem occur, this implies vibration related. I'm guessing if you pound on it with your fist it will also crackle?
If so, it is a connection issue somewhere. Could be a dirty/oxidized tube socket, bad solder connection, loose connector, etc., even a bad tube.
If there are any smaller (preamp) tubes on that pwr amp board, you should resolder (and clean) their sockets also. And check the solder joints on any connectors on that board.


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## nonreverb (Sep 19, 2006)

First question above all else when servicing these amps is...what type of power tubes have you got in there and how old. I'd guess 75% of the problems with symptoms you describe can be attributed a bad or bad power tubes. Do they bias properly using the adjusters on the back?
Most new production tubes, especially Sovteks have difficulty with the 635+ volts on the plate of those amps...


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## dcole (Oct 8, 2008)

Wild Bill wins the award on this one. The 5 watt power resistors were arcing internally. I replaced them and the sound disappeared. Put them back in and it re-appeared so I put them back in and reflowed the solder on the pins of the tube sockets.

Thanks for the help all.

- - - Updated - - -



nonreverb said:


> First question above all else when servicing these amps is...what type of power tubes have you got in there and how old. I'd guess 75% of the problems with symptoms you describe can be attributed a bad or bad power tubes. Do they bias properly using the adjusters on the back?
> Most new production tubes, especially Sovteks have difficulty with the 635+ volts on the plate of those amps...


Do you have a suggestion for modern production tubes for these amps?


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## nonreverb (Sep 19, 2006)

There's very little choice unfortunately. Winged "C" is now out of production and have become expensive. The only real alternative is the JJ KT88's which seem to work well. The Sovteks just don't seem to hold up over the long term. I've wound up replacing them more often than not. I haven't tried the Chinese offerings but am not convinced they'd be any better in an application such as this.


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## dcole (Oct 8, 2008)

Cool. I was looking at the datasheets for a bunch of modern production 6550's. Most showed a max Ua of 600-680V where the JJ KT88 showed 800V. Most 6550's would be running at max Ua giving no room for error or voltage fluxuations so no wonder a few poop out now and again. It looks like the KT88's would be drop in replacements?


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## Wild Bill (May 3, 2006)

dcole said:


> It looks like the KT88's would be drop in replacements?


Yes, BUT!

The bias voltage may need to be tweaked a fair bit. Hopefully, there will be enough range from the adjustment pots. I would check the data sheets for the plate dissipation factor. The formula is Pd divided by the plate voltage, X 70%, or .7 gives the desired idle current for ONE tube! Since the SVT test points are for 3 tubes across a 1 ohm resistor then you would triple the desired reading.

Wild Bill


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## nonreverb (Sep 19, 2006)

They're pretty forgiving. The KT88 is a good replacement where 6550 is concerned as it's op. spec. is close. It was after all, Genalex's answer to the 6550...just more robust. The KT88 plate voltage rating is considerably higher There's a little difference in the bias setting but well within what the SVT can handle plus the new SVT classics have a decent bias range.. I've been using them in SVT's when customers budget will allow. Bottom line is the KT88's are a better option than what's currently available unless you're willing to shell out for the more rugged NOS GE 6550A's.


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## dcole (Oct 8, 2008)

I find this amp difficult compared to others to set bias. The 10 ohm cathode resistors are sandwiched in there and they give you those dummy lights on the back. I'll have to trace the resistors to the backside of the amp and try that way to measure voltage across them while the amp is running.


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## nonreverb (Sep 19, 2006)

The bias lights do a decent job...be careful in there! In all the years I've been doing this, there are two amps that I pay extra attention to safety when servicing. This one and the Fender PS400. Both can do serious harm or worse if not careful.


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## Wild Bill (May 3, 2006)

nonreverb said:


> They're pretty forgiving. The KT88 is a good replacement where 6550 is concerned as it's op. spec. is close. It was after all, Genalex's answer to the 6550...just more robust. The KT88 plate voltage rating is considerably higher There's a little difference in the bias setting but well within what the SVT can handle plus the new SVT classics have a decent bias range.. I've been using them in SVT's when customers budget will allow. Bottom line is the KT88's are a better option than what's currently available unless you're willing to shell out for the more rugged NOS GE 6550A's.


Thanks once again, NR! That info on the KT88 was new to me too!

I think I will follow your lead with SVTs. I have one in the shop right now. As soon as you turn the standby to on I can see a bit of arcing in the Russian 6550s. They have been working ok for a few years but now things have changed.

Wild Bill


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## nonreverb (Sep 19, 2006)

Yeah Bill, I learned my lesson with the OEM Sovteks. Not that they're a bad tube necessarily but they do at times, succumb to the SVT's brutal high voltage prematurely.
You should see what happens to them in a PS400 with 765V on the plate!.......'POOF'.



Wild Bill said:


> Thanks once again, NR! That info on the KT88 was new to me too!
> 
> I think I will follow your lead with SVTs. I have one in the shop right now. As soon as you turn the standby to on I can see a bit of arcing in the Russian 6550s. They have been working ok for a few years but now things have changed.
> 
> Wild Bill


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## WCGill (Mar 27, 2009)

I've done a few SVT re-tubes with the JJ KT88's, nice improvement in the tone of the amp. I'm not a huge JJ fan but they're probably the only game in town for KT88's, like NR says. Thankfully they've gotten it right with this bottle.


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