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EL84 Cathode bias push-pull oscillating

kanurys

Member
2013-08-31 9:25 pm
Hi there,

I'm working on an old Silvertone with two EL84's in push pull, sharing a single cathode bias resistor. When the output is increased to 3+ watts, 1KHz sine wave input, There is a big high frequency oscillation on the lower half of the wave-form. The oscillation stays there even if I swap the tubes. I've tried a known working phase inverter 12ax7 to rule that out. Any thoughts?

This is a Silvertone 73, I'll attach a schematic of the power section.[IMGDEAD]https://photos.app.goo.gl/aPar34tqLvnXXSRc9[/IMGDEAD]
 

kanurys

Member
2013-08-31 9:25 pm
Try also a snubber across the entire primary of the output traffo. Try with a resistor of 1.2 times the impedance plate to plate, and a cap of about 4700pF @ 1KV, Decrease the cap or increase it until no more oscillation appears.

Thanks for the suggestion. There is already an appropriate snubber across the primary. This is a 50kHz oscillation. It seems to be completely gone with a little bit more capacitance at the input to the phase inverter. If the problem persists I'll try to snubber cap, as well.
 

kanurys

Member
2013-08-31 9:25 pm
I can see in the pic, a cap of about 50pF (?) from first plate to gound. This cap places a HF rolloff that ensures reducing the gain in high frequencies. Please, check if this cap is OK, and eventually add a small cap (33pF) across it, to decrease still more the HF cut off frequency.

Thank you. That is exactly what fixed it. A larger value than 33pF did the trick. The speaker for this old console has no tweeter, so high frequency performance is not an issue. The existing 47pF cap was good, but not big enough for the variance in other parts of the circuit.