Most pushpull cathode biased EL84 amplifiers I have seen used cathode resistors of > 200 ohms on a 300V supply. A 150 ohm resistor with 21V across it equates to 140mA which is excessive for a pair and is equivalent to over 20W dissipation each.. They are rated at 12W dissipation, although in triode connection you may get away with as much as 14 - 15W.
It may be that your plate voltage is substantially higher than the 300V indicated in your schematic, this would result in the problems you are observing.
Normal cathode bias voltages are typically around 12-14V which leads me to suspect high plate voltage again.
Check the coupling capacitors for leakage, if you see more than a couple of hundred mV across the grid resistor they are leaky and should be replaced. (Do this with the tubes removed so that grid current does not confuse the result.)
Try changing that 150 ohm resistor to about 250 to 300 after verifying that the coupling caps are ok.
The SCA35 I owned had 7189's installed in it from the factory and they are a little more rugged than the EL84/6BQ5, they bias slightly differently than the EL84 as well, although the SCA35 and ST35 do definitely over run them.
Kevin
It may be that your plate voltage is substantially higher than the 300V indicated in your schematic, this would result in the problems you are observing.
Normal cathode bias voltages are typically around 12-14V which leads me to suspect high plate voltage again.
Check the coupling capacitors for leakage, if you see more than a couple of hundred mV across the grid resistor they are leaky and should be replaced. (Do this with the tubes removed so that grid current does not confuse the result.)
Try changing that 150 ohm resistor to about 250 to 300 after verifying that the coupling caps are ok.
The SCA35 I owned had 7189's installed in it from the factory and they are a little more rugged than the EL84/6BQ5, they bias slightly differently than the EL84 as well, although the SCA35 and ST35 do definitely over run them.
Kevin
Hi SY,
Yes I have seen those datasheets, my experience with EL84 has been different. In a lot of cases those values seem to work well in situations where the plate voltages are quite low, say 250V. In many other instances the recommended values have the tubes running well beyond their static ratings. Vintage ones may behave differently, but truthfully I haven't seen much difference.
Kevin
Yes I have seen those datasheets, my experience with EL84 has been different. In a lot of cases those values seem to work well in situations where the plate voltages are quite low, say 250V. In many other instances the recommended values have the tubes running well beyond their static ratings. Vintage ones may behave differently, but truthfully I haven't seen much difference.
Kevin
Well, you bring up a point which is pretty much in accord with what I'm on about. Is there a problem with the amp or is it out of spec tubes? If the former, it's straightforward to fix. If the latter, Danko ought to ask his tube vendor for a refund rather than have to redesign his amp to accommodate defective parts- it IS possible to buy in-spec EL84s.
The JJs I've been using seem to conform pretty well to the original spec. I can't comment on other new production, but I do note that Morgan Jones built 5 different stereo pairs of the Bevois Valley amp with 270 ohm separate cathode resistors and 300V Vak.
The JJs I've been using seem to conform pretty well to the original spec. I can't comment on other new production, but I do note that Morgan Jones built 5 different stereo pairs of the Bevois Valley amp with 270 ohm separate cathode resistors and 300V Vak.
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