Screen dropping resistor value in 5F1 Champ>

[Mark]

I've kept a bit of a running blog on the Mercury Magnets Fender Champ 600 kit as nothing exists on it in forums when I did searches. There a few pitfalls to be avoided too with this kit.

https://tubeamparchive.com/viewtopic.ph ... 9&start=15

I have been quite happy how the Champ 600 (fully dimed) sounds when used with my Les Paul and a Celestion Blue speaker (mounted in my 18 watt Marshall clone).

Yesterday I wanted to demonstrate to friend how great the amp sounds. To my horror smoke poured out of the amp and it ceased to work.

The smoke was due to the 100 ohm centre tap resistors for the heaters, the problem was the screen and the heaters now measured 300 ohm, never good of course. the 1K screen dropping resistor is also somewhat blackened.

This brings me onto my question, (the B+ for this amp is 360VDC) in your experience is 1K too small a value for a screen dropping resistor?
This value was used in later silver faced Champs, though they aren't capable of delivering the distortion this Champ is capable of.

The Champ 5F1 uses a 10K which I've found to be a real good value in my 5F1 clone. I did use a 6L6 in it for a while (output transformer replaced and amp rebiased for the 6L6) and I found it blew output tubes
I upped the screen dropping resistor to 10K and the problem was solved.

What are your thoughts and what have you experienced with the screen grid dropping resistor?

Thanks

P/S: the bold print is the actual question and not all the waffle.

[Stevem]

You have blown out 6L6's in that amp!?
What the heck was it pulling for cathode and screen current?
I can not see why anything more than a 2K resistor should be needed with a 6V6 that's got 360 volts on the plate and it's not pull more than 42ma.

Those 100 ohm hum balance resistors go south many times when the outout tube shorts out screen to heater.
When you have the amp fitted with a V6 and beat the hell out of it in a dark room does the plate structure get a dull purple spot on it?

[Mark]

Steve, Thanks for replying to my post, I do appreciate that, though I think I should take some measurements and post the results, this should be a bit more meaningful instead of asking people to relate their experiences with single ended amps.

Oh yeah, I have blown several 6L6's in a Champ, the screen grid dissipation was exceeded.

Here are the 6V6 specs.

http://www.thetubestore.com/lib/thetube ... GT-STR.pdf

The screen grid dissipation is 2.2 watts, if the screens pull over 6mA the dissipation is exceeded.

http://www.drtube.com/datasheets/6l6gc-sed1996.pdf

5 watts in the case of a 6L6.

[martin manning]

Mark what is your operating point (Va, Ia, Vg2) and OT primary impedance?