Geek wrote:Change the cathode resistor on the output tubes.
Got a better copy? Is that 400V or 480V on the anodes? If 480V, forget about 6L6 all together.
Cheers!
490 on anode and 495 on screen grid.
If you have regular 6L6, maximum on plate is 360 and screen grid is 270. Just can't use a regular 6L6.
The maximum of a 6L6GC is 500 plate and 500 screen. You'll have to keep the current way down to make it last at that voltage.
Removing C9 off the GZ34s should lower the B+ about 150 volts and make the 6L6 much happier. You can also lower C9 to something like 2uf to lower the B+ 70-80 volts, but use a high voltage 630-700V film cap. In any case you will have to increase the capacitance of the next filter cap past the choke to keep hum away.
Other thing you can do is lower the current. Find out what the voltage is at point "K", divide by 360 ohms to get current, divide current by 2 for each tube, subtract B+ from voltage at point "K" to get voltage drop of tube, then multiply by tube current by tube voltage to get tube power. 6L6GC should do fine with 28 to 30 watts. The EL37s should have been doing a little under 25 watts, probably should bias 6L6GCs there too.