Is the Manley amp fixed bias or cathode bias? If fixed you must adjust the bias current so the EL34's do not exceed their plate current limit. A multimeter is needed. Ask Manley for the procedure. You can do it yourself.
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Manley and other tube amp companies should always provide information for proper bias adjustment.They do.
Tube amps do need this done when output tubes are changed.A cathode-biased tube amplifier may also require periodic re-biasing as the output tubes age.
What everyone else said. Also, if you bought the amps used with used tubes installed, it may not be unusual at all to have lost two tubes over a period of 1.5 years. The fact that you do not use the amps so much is neither here nor there. Tubes are most stressed during turn on and warm up.. Just like an electric lightbulb, they are most apt to blow at start up. Especially if you have never checked and adjusted bias during your period of ownership. As tubes age, the bias must be periodically adjusted. However, the tendency during the aging process is to lose transconductance, which means that as the bias stays the same the current through the tube will go down. That per se would not stress the tube; it would cause the sound to grow dull and lifeless.
What lewm said. Tubes are just plain unpredictable. New or used, biased or not, they can just blow, or last forever, and I have seen it all. I've had three brand new Preferred Series tubes from thetubestore.com blow within weeks, and I've had the most inexpensive JJ tubes run so many years they very gradually became dull just like lewm said. Only the change was so gradual over so long I didn't even notice until the new tubes went in and then WOW! (Sorry for the hyperbole, wow!)
OP here is the real question, was the tube failure in the same position? If not, move on and understand, valves fail. If it was in the same position check the bias per position that ONE is to hot.. 300ma is not plus anything, that is MAX. That is every valve? or per bank.. 300 sound HIGH per valve to me.. Anyway LOWER is better for now and accurate is even more important..
Make sure your using a DMM, not an analog meter, they are not close enough.. Every year they are suppose to be calibrated if they are being used as a TOOL, for looks that’s different. Who cares.. Blowing tubes.. MAKE SURE... the meter is good and you're at least bias 300 or LOWER not higher..
Surging ON is a problem too.. Make sure you have a 120 volt maintainer for start up, no voltage dips. Soft start or pre start really helps too..(stand by switch)
I have the older Macs that are 117 vac MAX they don’t like 120vac. They will actually run too hot.. but on a Variac at 110vac.. perfect... Those Manley aren’t that old... are they?
I have VTL 300 deluxe (Manley design) they like 110-115. Above that they run HOT. 110 I can almost tolerate them in the summer.. but I can adjust the bias, the Macs I can’t.. ALL fixed..
Fruit for thought OP..
Good comments from all the above.
If failure was at same position, it could be random but also a cause for some concern.
Also monitor the bias more frequently than Manley suggests to check that your amps can hold it there. Safer not to exceed the value whatever the sound, a bit lower the better for tube life. Also a question to the seller of what caps exactly were changed and why.
Tubes can go funny and not created equal. For me two failures in 1,5 years with no constant use is at the limit of unexpected.
I dont use an amp that uses EL34s but I have retubed many. If you keep blowing tubes you might try old stock EL34s, perhaps some less expensive types like Matsushitas. I have heard that the longevity of the new production tubes aint what it used to be. I dont carry new production so I cant speak from actual experience.
Russ quoted the bias instructions. "300mV across a 10-ohm resistor". If he is correct, that would give a plate current of 30mA. I just checked some EL34 data sheets on-line, and they all confirm what Russ said, that 30mA is very conservative. Most give a range of plate current from 35mA to 70mA. The point about whether the tube that fails is always in the same position is a very important one. If that is the case, watch the replacement tube carefully. Is it glowing more "red" than the other tubes? If so, then you may have a leaky coupling capacitor (the capacitor that feeds the control grid signal from the front end). If that cap leaks positive DC to the grid, that makes the (negative) grid bias voltage go less negative, which turns that tube ON harder and harder. Moreover, over time, the leak builds in magnitude such that the bias voltage becomes less and less negative and then... blooey. The tube goes cherry red, sometimes also blue, and dies. The good thing is that fixing this problem is easy; replace the coupling capacitor. I especially suspect paper in oil capacitors of leaking (DC voltage, not necessarily oil).