Need help; I have "dog leg" shaped waveform on my amp

andjur78 · 3rd April 2007, 06:37 AM

Can anyone help with a good procedure for tuning my guitar amp?

The amp runs 4 x EL34 tubes in push pull and each are running 33mA each tube at the moment but on the oscilloscope there is a "dog leg" shaped sine wave thru the zero crossing (8 OHM load).
Should I increase the bias current on each tube in turn? (There are adjustments pots for each tube)

ray_moth · 3rd April 2007, 08:23 AM

Looks like it. This is likely to be crossover distortion, caused by one tube ceasing to conduct before the other has started conducting fully, a common problem with Class B or very lean Class AB. Try increasing the current in small increments, to find the point where the kink disappears. (If it doesn't disappear, then the problem is something else!)

3rd April 2007, 06:37 AM	#1
andjur78 diyAudio Member Join Date: Mar 2005 Location: Aussie	Need help ahve "dog leg" shaped waveform on my amp Can anyone help with a good procedure for tuning my guitar amp? The amp runs 4 x EL34 tubes in push pull and each are running 33mA each tube at the moment but on the oscilloscope there is a "dog leg" shaped sine wave thru the zero crossing (8 OHM load). Should I increase the bias current on each tube in turn? (There are adjustments pots for each tube)

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3rd April 2007, 08:23 AM	#2
ray_moth diyAudio Moderator Emeritus Join Date: Jan 2004 Location: Jakarta	Looks like it. This is likely to be crossover distortion, caused by one tube ceasing to conduct before the other has started conducting fully, a common problem with Class B or very lean Class AB. Try increasing the current in small increments, to find the point where the kink disappears. (If it doesn't disappear, then the problem is something else!)

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