MK III bias
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| Posted on March 16, 2021 at 18:30:35 | |
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Posts: 2
Location: Midwest Joined: March 15, 2021 |
Hi, I have a pair of MK IIIs that have been in mothballs for a number of years. They worked well the last time I tried them (years ago), but one no longer functioned when I hooked them up again recently. I traced the problem to a shorted can cap (30/20/20/20mfd). I replaced this cap on both amps with new 30/20/20/20 550V caps from Dynakit Inc. I also replaced the (2)50mfd caps on both, installed new diodes, the 11.2 ohm bias resistor,and the 6.8K resistor across the capacitor (someone had put 11.2K in). These had EL34 output tubes and 5U4GB rectifier when I got them, and still do. They do have 6AN8As on the input stage. Both sound distorted now when hooking them up. Trying to set the bias to approximately 1.5V at the bias socket, I am only reading ~0.25V at half rotation of the bias pot, and 0.58V at full rotation. Question : even with the EL34s and 5U4GB, should I be able to set the bias V higher? I am hesitant to buy/replace with new KT88s and GZ-34 if something else is causing this issue. Thanks! and any advice would be appreciated.
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