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Help!! The Evil Amp Strikes Again!

9/28/1999 1:08 AM
Psycho Bass Guy	Help!! The Evil Amp Strikes Again! Ok, I'm counting on you guys for a little help here. My DMM is on the fritz so I can't take any measurements, but I have a Trace Elliot VR400 tube power amp that is blowing tubes after about 5 minutes. It cherry plates some of them and eventually smokes one. The amp was working before it got shipped to me, and UPS dropped it. Even though it was shipped in an ATA approved flightcase, it broke all the power tubes inside it. I opened it up, cleaned out the glass and took a look-see at the circuitry. It's basically an SVT with only one tube 12ax7 pregain stage (no tone controls),two 12ax7 drivers,a higher bias and none of the adjustment points. There are two bias test points on the chassis, but I'll have to borrow a DMM. The amp was designed around KT88's but came to me with 6550's.It had been rebiased to 6550 and had been running on them for a few months. I am thinking something has the bias way, way too high as it smoked both 6550's and KT88's. How could a drop cause this? Am I right in thinking along the lines of over bias? There are no noises, but there are a set of LED's on the front which appear burnt out. These are connected to the standby switch and heater indicators. The amp sounds ungodly good before it gets the tubes too hot.(sort of a hi-fi sounding SVT)there were no visible cracked solder joints(under magnification) and no damage other than a loose ribbon cable connecting the inputs to the power section. These are kind of rare and it is very well built, but I don't like having to replace too many tubes. The initial replacement tubes were new Svetlana 6550's Same as what it had before being dropped) and it killed them very quickly. The one that died had tested sort of iffy anyway so I next tried a much more rugged set of Jolida rebranded Chinese KT100's (which are just select KT88's I know, the China thing, but these tubes have been very good to me.) It cherry plated only one of these and the one that blew was looking to be operating normally. What's the verdict guys? I don't have a schem and can't even get a manual due to its short production history. Can you think of anything else to look for when I rebias it? Thanks in advance. Psycho

9/28/1999 1:08 AM

Psycho Bass Guy

Help!! The Evil Amp Strikes Again!

Ok, I'm counting on you guys for a little help here. My DMM is on the fritz so I can't take any measurements, but I have a Trace Elliot VR400 tube power amp that is blowing tubes after about 5 minutes.
It cherry plates some of them and eventually smokes one. The amp was working before it got shipped to me, and UPS dropped it. Even though it was shipped in an ATA approved flightcase, it broke all the power tubes inside it. I opened it up, cleaned out the glass and took a look-see at the circuitry.
It's basically an SVT with only one tube 12ax7 pregain stage (no tone controls),two 12ax7 drivers,a higher bias and none of the adjustment points. There are two bias test points on the chassis, but I'll have to borrow a DMM. The amp was designed around KT88's but came to me with 6550's.It had been rebiased to 6550 and had been running on them for a few months. I am thinking something has the bias way, way too high as it smoked both 6550's and KT88's. How could a drop cause this? Am I right in thinking along the lines of over bias? There are no noises, but there are a set of LED's on the front which appear burnt out. These are connected to the standby switch and heater indicators. The amp sounds ungodly good before it gets the tubes too hot.(sort of a hi-fi sounding SVT)there were no visible cracked solder joints(under magnification) and no damage other than a loose ribbon cable connecting the inputs to the power section.
These are kind of rare and it is very well built, but I don't like having to replace too many tubes. The initial replacement tubes were new Svetlana 6550's Same as what it had before being dropped) and it killed them very quickly. The one that died had tested sort of iffy anyway so I next tried a much more rugged set of Jolida rebranded Chinese KT100's (which are just select KT88's I know, the China thing, but these tubes have been very good to me.) It cherry plated only one of these and the one that blew was looking to be operating normally.
What's the verdict guys? I don't have a schem and can't even get a manual due to its short production history. Can you think of anything else to look for when I rebias it? Thanks in advance.
Psycho

9/28/1999 1:30 AM
Psycho Bass Guy	the Amp got dropped and don't work right no more. Update: The tubes didn't get smoked according to the Mighty Mite. It smoked the supply fuse (10a 250v) and saved the tubes. I still think the bias is way too high. I am also going to check the filter caps. They are 6x(400v 220mfd)but I really don't think they are the problem, but this is too weird. Anybody else have similar problems after a transport tumble?

9/28/1999 1:30 AM

Psycho Bass Guy

the Amp got dropped and don't work right no more.

Update: The tubes didn't get smoked according to the Mighty Mite. It smoked the supply fuse (10a 250v) and saved the tubes. I still think the bias is way too high. I am also going to check the filter caps. They are 6x(400v 220mfd)but I really don't think they are the problem, but this is too weird. Anybody else have similar problems after a transport tumble?

9/28/1999 2:29 AM
Don Symes	Where wires go thru holes, are any nicked? At these voltages, just a thin spot could leak enough to be a problem. Next thought would be a bend at/near a transformer mount that makes enough of a crease to just touch an ill-trimmed component lead on the back of the circuit board. The thrust here is that something mechanical could be boning you electrically.

9/28/1999 4:41 AM
Psycho Bass Guy	The wiring looks good. I had to resolder all the major connections to the faceplate, so I checked it thoroughly. The bend to the unit is in the rack ears on the front which doesn't affect the interior chassis in any way. It has a three-piece steel frame which really does a great job of isolating everything but the faceplate. (Although the power tubes were mounted inside and they bought it.) This amp is even more point to point than my old SVT with a super clean and logical layout. My only complaint is a small ribbon cable which has obviously been fixed before (and not very well). I question the logic in using a ribbon cable in a high volume bass amp between a major structural juncture. Another cool plus is a three way power level selector switch which looks like it turns on the tubes in pairs for more crunch at lower volume. Since this is a sort of variable standby, it may be part of the problem as it is just a pot-type switch. It still shouldn't cause an over bias or blown fuse and at worst would open up and run all six tubes continuously or be intermittent and cause audible popping. I can see that bias current is applied as only two tubes at a time by the switch and the amp itself super quiet, so I think it is ok. There are also two separate switches for standby and run on the front panel with corresponding LED's(which don't work but are getting voltage). All the trannie connections were unhurt, but there was an absolute shitload of glass everywhere. I know glass is an insulator, but is there any way a small piece could be goosing the bias? I vacuumed out the chassis and don't think there is any left, but you never know. It cherryplated the tubes in about one minute and it was always the same sockets that would cherryplate.(bad socket?) This is baffling without the DMM, but I always try to shoot the thing in my head first. I am really bugged by this one.

9/28/1999 4:41 AM

Psycho Bass Guy

The wiring looks good. I had to resolder all the major connections to the faceplate, so I checked it thoroughly. The bend to the unit is in the rack ears on the front which doesn't affect the interior chassis in any way.
It has a three-piece steel frame which really does a great job of isolating everything but the faceplate. (Although the power tubes were mounted inside and they bought it.) This amp is even more point to point than my old SVT with a super clean and logical layout.
My only complaint is a small ribbon cable which has obviously been fixed before (and not very well). I question the logic in using a ribbon cable in a high volume bass amp between a major structural juncture.
Another cool plus is a three way power level selector switch which looks like it turns on the tubes in pairs for more crunch at lower volume. Since this is a sort of variable standby, it may be part of the problem as it is just a pot-type switch. It still shouldn't cause an over bias or blown fuse and at worst would open up and run all six tubes continuously or be intermittent and cause audible popping. I can see that bias current is applied as only two tubes at a time by the switch and the amp itself super quiet, so I think it is ok.
There are also two separate switches for standby and run on the front panel with corresponding LED's(which don't work but are getting voltage).
All the trannie connections were unhurt, but there was an absolute shitload of glass everywhere. I know glass is an insulator, but is there any way a small piece could be goosing the bias? I vacuumed out the chassis and don't think there is any left, but you never know.
It cherryplated the tubes in about one minute and it was always the same sockets that would cherryplate.(bad socket?)
This is baffling without the DMM, but I always try to shoot the thing in my head first. I am really bugged by this one.

9/28/1999 5:46 AM
Carlo	Hi PBG, What's wrong with the DMM? Perhaps it's just a bad fuse. (in the DMM)

9/28/1999 10:56 AM
Psycho Bass Guy	DMM My cat knocked it off the bench onto the concrete floor and it won't hold ranges anymore. I can fix it, but I want to see if there is any other way to shoot the amp first.

9/28/1999 9:07 AM
A.S	Re: the Amp got dropped and don't work right no more. I fixed a Fender a couple of weeks ago and it had a filter cap in the bias circuit leaking to ground this caused the voltage to rise towards 0 V, gradually and eventually blowing the fuses. You might wanna check for similar problems because this problem started when the amp was knocked over. A.S

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