Tube Amps / Music Electronics
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|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 Bass Guy the Amp got dropped and don't work right no more. -- 9/28/1999 1:30 AM|