DanielF Posted December 20, 2010 Share Posted December 20, 2010 My name is Daniel and I've been an Epi fan for a number of years now and I just bought my first EVJ a few weeks ago to use as a test/practice amp at my shop. After getting the amp I played it for a few hours and changed the tubes out for a set of JJ's which I have grown to like. I played the amp for a week or so and it fried my brand new JJ EL84 :( After breaking out the multimeter and doing some testing after re-installing the stock Sovtek EL84 I found the the power tube is biased way to hot for any of the modern production tubes. Which means I would never be able to run some of the much nicer NOS tubes. My queston with it being a cathode biased amp is what can I add or subtract in the circuit to get the bias levels to something safer. This thing sounds awsome I just can't afford to keep a steady supply of EL84's on hand. Any help guys would be greatly appreciated Link to comment Share on other sites More sharing options...
fuse Posted December 20, 2010 Share Posted December 20, 2010 What voltage reading do you have at the plate, and at the cathode with the chassis as reference? Link to comment Share on other sites More sharing options...
DanielF Posted December 23, 2010 Author Share Posted December 23, 2010 What voltage reading do you have at the plate, and at the cathode with the chassis as reference? 318VDC Anode to Cathode, 326VDC Cathode withthe chassis as reference. Thanks Link to comment Share on other sites More sharing options...
fuse Posted December 23, 2010 Share Posted December 23, 2010 Are you sure about that cat voltage? Even if it was 32.6 volts, it would still be high. If that is the number, try lifting one leg of the cat by-pass cap and see what you get. Best Link to comment Share on other sites More sharing options...
DanielF Posted December 23, 2010 Author Share Posted December 23, 2010 Are you sure about that cat voltage? Even if it was 32.6 volts, it would still be high. If that is the number, try lifting one leg of the cat by-pass cap and see what you get. Best I could be mistaken and probably am I'll recheck and repost. But I know that the plate volts is around 318VDC which is still too high for the EL84 which if I'm not mistaken has a 300 PV max right. How would I lower the plate volts all together to get it down around 285 to 300? Thanks for your patience. Link to comment Share on other sites More sharing options...
fuse Posted December 23, 2010 Share Posted December 23, 2010 You are fine at 318 on the plate. The schematic will probably show 350. Just do a double check on the cat voltage..... Best Link to comment Share on other sites More sharing options...
jefrs Posted January 19, 2011 Share Posted January 19, 2011 Problem with any cathode/self biased amp is that you are limited to the nearest "preferred" value resistor. But near-enough is good-enough, it's called self-bias because it is self limiting. You may find it is hot now but the next value may be too cold: Murphy's Law. Another thought, if the cathode voltage is too high, do check the solder connections and redo with pukka tin/lead fluxed. These lead-free PCB are terrible for cold joints (high resistance). Link to comment Share on other sites More sharing options...
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