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Post by winggzero on Jan 22, 2012 0:30:16 GMT -5
Alright guys, lets see if anyones got an answer for this one. Tonight I opened up my head for the first time since Scott Had it in 03 to change and bias the power tubes. I took the plate voltage reading at 461 and proceeded to calculate my bias which should have been around 56. However when I plugged the probes in the bias was set at 73. NO ONE has been inside this head since Scott was in here so How am I supposed to bias this? At 73 it was biased for a class A amp... Is that how these are supposed to be biased? The amp has the MM transformer so maybe that changes somthing but Im stumped.
zero
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Post by briman on Jan 22, 2012 11:39:54 GMT -5
So your tubes haven't been changed since 03? All I can tell you is that I bought new tubes once on ebay and when I put them in and went to bias them...... one was at 18 mv and the other one was at around 72. It was defective obviously and was starting to red plate so I pulled it out right away. I am not an amp tech but I think tubes can pull too much when the get old or defective.
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Post by sasquatch on Jan 22, 2012 12:36:23 GMT -5
What type of tubes are in the amp? If they're EL34 type, then most the time your bias point should be in the 30-34mA range... depending on what sounds best. I think KT88's are set higher, like 55-56 mA or something like that... not sure cuz I don't use KT88's, but they're usually biased higher than EL34's. When you plugged the probes in and they read 73mA, was that with the new tubes or old tubes? If it was the new tubes, you may want to put the old ones in and check the bias just for reference. Even though bias can drift, 73 sure seams high!
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Post by winggzero on Jan 22, 2012 16:20:39 GMT -5
Ok before you judge me I guess I should let you know Ive been deployed seven of those years. So yeah they havent been changed. The amp has KT88's in it. The bias adjustment was almost dimed out.
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Post by winggzero on Jan 22, 2012 16:27:07 GMT -5
I checked the bias with the old tubes in it to getvan idea of where it was initialy set. I thought it was a tad bit high too!
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Post by briman on Jan 22, 2012 19:02:46 GMT -5
I was thinking wow, 8 years is a long time for a set of tubes lol. So basically yours were used for roughly a year I guess. If I was you, I'd buy another tube or return that one as it seems to be defective and not matched either. Sounds pretty much like the same situation I went through buying tubes from ebay. I ended up buying another set and everything was then ok.
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Post by winggzero on Jan 22, 2012 19:42:10 GMT -5
Its not the new tubes. I did the math and for some reason it was biased at 90% of max dissapation. Why it was maxed out like that Ill never know. I biased the new set at 75% For good measure. Im not sure when the old KT88'S died but all of the wierd mid harmonics are gone and Im much happier with the amp.
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