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quantum · 23rd Jan 2006, 9:25 pm

If you've managed to get stability with an ECL80 with a cathode bypass cap of only 47uf, you've done very well indeed - normally, because of the different bias required by the triode and pentode parts of the valve, to ensure stability you need a much higher value. If you look at circuits which employ ECL80s as audio amps (such as in TV receivers and gram amps such as the Champion Rev-ler no. 805 or Radio Rev-ler no. 820) values such as 250-500uf are common. When I've used an ECL80 in the past I've employed grid bias for the triode part of the ECL80, so the bias resistor is 330 ohms and the triode grid is biased by a 10 meg resistor connected to the top of this resistor (not the ground side), and I've always used a high value cathode bias cap, 470 - 1000uf or whatever I have handy. The triode grid then connects through a small cap (0.005uf) to a the wiper of the volume control.

I once cheated with an ECL80 amp - I was making a low powered stereo set up, and I used the triode of one valve driving the pentode of the other, with 330 ohm resistors and low value bypass caps, without any trouble of instability, as the two sections of the amp were not in the same valve envelope!

23rd Jan 2006, 9:25 pm	#17
quantum Retired Dormant Member Join Date: Mar 2003 Location: Birmingham, West Midlands, UK. Posts: 785	Re: Motorboating If you've managed to get stability with an ECL80 with a cathode bypass cap of only 47uf, you've done very well indeed - normally, because of the different bias required by the triode and pentode parts of the valve, to ensure stability you need a much higher value. If you look at circuits which employ ECL80s as audio amps (such as in TV receivers and gram amps such as the Champion Rev-ler no. 805 or Radio Rev-ler no. 820) values such as 250-500uf are common. When I've used an ECL80 in the past I've employed grid bias for the triode part of the ECL80, so the bias resistor is 330 ohms and the triode grid is biased by a 10 meg resistor connected to the top of this resistor (not the ground side), and I've always used a high value cathode bias cap, 470 - 1000uf or whatever I have handy. The triode grid then connects through a small cap (0.005uf) to a the wiper of the volume control. I once cheated with an ECL80 amp - I was making a low powered stereo set up, and I used the triode of one valve driving the pentode of the other, with 330 ohm resistors and low value bypass caps, without any trouble of instability, as the two sections of the amp were not in the same valve envelope!