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Post by ivorytower on Jun 6, 2010 23:45:55 GMT -7
I have a KT-45 with all original tubes (very low usage). It sounded fairly disappointing (bland, uninspiring tone). Since I know DR. Z amps to usually sound nothing less than magical, I felt I had a problem with it. An Amp Tech friend of mind suggested I check the biasing. Sure enough, it was almost 30 volts under the spec voltage for a KT-45 with the solid state rectifier (should be at 480 volts and was at 452!).
I was psyched to see the situation since it should be an easy fix: crank the Biasing trim pot til we get it to voltage. Unfortunately, at the top of the travel, it still only got it to 475.
First question then is: Should this be happening? Can I/should I change out the resistor to a different value to allow proper bias voltage?
Last issue: When I did this (partial) re-biasing, the amp definitely got growl-ier and had more of that "Z" magic (thank goodness) but then the sound seemed a bit erradic - it was cutting in and out. Could that be behavior due to re-biasing incorrectly?
Again: all original (and almost new) tubes. Solid State rectifier, correct impedance load on the system.
Any help would be appreciated.
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Post by timford on Jun 7, 2010 5:57:29 GMT -7
I'd ring or email Dr Z directly and get an answer/suggestions from the horses mouth.
I managed to cook a resistor at 2am in the morning. Called Dr Z and got Brent on the line and got my questions answered quick smart.
Now that is service! (The advantage being that I'm on the other side of the globe time wise.......)
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Post by timford on Jun 7, 2010 6:05:46 GMT -7
Maybe try another rectifier - even a tube rectifier (that should read around 440-455 or so from memory).
I prefer the tube rectifier and overall love this amp to death. Sounds like there is something wrong with it, so I'd give it a good go before dumping it.
Maybe shoot this link to Myles in the 'ask the experts' section also. Myles is awesome.
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Post by ivorytower on Jul 4, 2010 12:44:18 GMT -7
Ok, this issue can be closed but I wanted to post the results if anyone else is interesed. First, it turns out that the tubes were not original at all and the guy that had it before me had the wrong tubes in it. It had he more common EL-34s and it calls for E34LS (which is a 30 watt tube where the common EL-34 is a 25 watt tube). So I changed these and put in a tube rectifier and it biases fine. I also replaced the EF-86 with a NOS Tesla and replaced the Phase invertor with a groove tubes MPI and the thing sounds amazing. Now THAT'S the Dr. Z sound i was going for. Issue closed.
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Post by timford on Jul 4, 2010 18:06:26 GMT -7
Great to hear. Now its time to start gloating about the tones
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Post by submersible on Jul 6, 2010 11:19:31 GMT -7
It had he more common EL-34s and it calls for E34LS (which is a 30 watt tube where the common EL-34 is a 25 watt tube). Where did you find those? I wonder if they are a different tube than they used to be (you can't be sure with GT) or if they are no longer readily available. In any event, they apparently aren't what you would find in a KT-45 if you were to buy one from the factory. Here's what Dr. Z recommends: KT 45: V1: NOS EF86, V2: Sovtek 12AX7LPS, V3/4: JJ E34L (matched pair), V5 Solid State plug in or Sovtek 5AR4 I went with a pair of JJ's as recommend and had them biased to spec by my tech. I won't know until tomorrow if my headroom issue was resolved, though (can't crank it until I'm in the practice space!)
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Post by timford on Jul 6, 2010 19:00:58 GMT -7
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