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Old 26th February 2007, 01:31 AM   #1
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Join Date: Jan 2005
Location: The emerald city, Seattle, WA.
Default KT-77 Blues

Hi Folks!

I recently read rave reviews on this site about the relatively new JJ KT-77 to replace my older (but still working) Electro-Harmonix EL34's, so I ordered a quad from the TubeDepot. What I understand is the KT-77 is electrically identical to the EL34, right? I figured I could just pop them in my amp and just have to tweak the bias setting just a tad and that's it.

I turned the bias pots fully counter-clockwise and connected my DMM across one of the 10 ohm cathode resistors, then I flipped the power switch. The TubeDepot gave these tubes a bias rating of 39 mA, so I figured each push-pull pair should be adjusted around 78 mA.

After power was applied, I noticed I could not make an adjustment...the reading was just all over the place. I'd get it around 75 mA and then it starts to climb uncontrollably. Then it starts to hum and motorboat. The music sounds distorted.

Here's the strange part. When I take the tubes out and flip the channels, the problem stays in the left channel. When I go back to my old EL34's, the problem disappears, there is nothing wrong with the amp.

Has anyone here had any problems with the KT-77's? My amp is a Dynaco ST-70 clone running in Ultra-Linear mode. It has separate 10K bias pots for each channel. Plate voltage is around 480 volts.

Thank you all in advance for your input,

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