Power tubes not drawing current

redplater · 14th February 2011, 11:54 PM

I'm building an Eiclone monoblock stereo. It's an El34 ultralinear amp. Strangest thing. The tubes are not drawing any current. I've got like 570 vdc on the plates and -75 on the grids. It's not bad tubes....I've substituted two pairs and the same results. The tubes are drawing like 1 ma of current. The plates should be about 90 volts less. I'm baffled!

Thanks!
David

mctavish · 15th February 2011, 12:10 AM

With the grids at -75v your tubes are in cutoff and cannot possibly pass much current at all.

As you decrease the grids to closer to ~ -30v the tubes will start to conduct and your plate voltage will decrease due to the current draw. This amp is fixed bias?

14th February 2011, 11:54 PM	#1
redplater diyAudio Member Join Date: Nov 2010	Power tubes not drawing current I'm building an Eiclone monoblock stereo. It's an El34 ultralinear amp. Strangest thing. The tubes are not drawing any current. I've got like 570 vdc on the plates and -75 on the grids. It's not bad tubes....I've substituted two pairs and the same results. The tubes are drawing like 1 ma of current. The plates should be about 90 volts less. I'm baffled! Thanks! David

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15th February 2011, 12:10 AM	#2
mctavish diyAudio Member Join Date: Feb 2008 Location: Niederösterreich	With the grids at -75v your tubes are in cutoff and cannot possibly pass much current at all. As you decrease the grids to closer to ~ -30v the tubes will start to conduct and your plate voltage will decrease due to the current draw. This amp is fixed bias?

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