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Hi again I built a KB85 a few years ago and I was running EL37's and all is great, I changed the tubes with 6l6's and found that the plates had a red glow at the center. Could someone tell me what to do to reduce the voltage/current so I could run 6l6's and kt66's.
Here is a copy of the schematics.
There is not much room left in the case

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!

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.

What power transformer did you use, does it have plenty of filament current? The reason is that a 6CA7 or 6550 tube would be the way to go here, standard 6L6's will die in short order and you will be pushing a GC to it's limits!

Try using 5R4 in the power supply instead of 5AR4 that should drop the B+ to about 450 volts , and help out a lot .

Thanks guys, I forgot to say that the tubes I was using were National 6l6gc's Russian made
The power transformer was purposely built for this amp when I was living in Australia and has more than enough grunt for the filaments.

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.

Will a film cap in the place of C9 be ok instead of a electrolytic?
Would an increase of 20uF be ok at position H or would I need more?

I will put the amp on the bench in the next couple of days and meter all the voltages at all points and I will re post the schematics.

Made up my mined and made an order for a quad of "C" 6l6gc's.
No changes to the amp.