Applying Negative Feedback

[EETStudent]

Any advice on how to do this in a standard push-pull 6L6 output stage and a long-tail PI? I tried it, applying a 820ohm resistor from the OT secondary back to the PI and I got a HORRIBLE screeching sound. Louder than all get out. Any help would be nice

[tubeswell]

Look at the 6G3, 6G5, 6G5A, 6G9B, 6G16 or the 5F6A, ab165, aa763, ab763 (and many more besides)

[Enzo]

The feedback path you created was not negative, it was positive feedback - and that makes oscillation, the sound you heard.

"back to the PI" is not specific. That is about like saying I measured voltage on a pin of a tube. Without more specific information we can't tell what you did. Look at teh examples.

[EETStudent]

i basically took it from the BF Bandmaster, applying an 820ohm resistor from the green wire on the output jack back to the 22k tail resistor of the PI

[Tubis]

Switch the cables from the output tubes to the OT around so the feedback becomes negativ and it will work.

[EETStudent]

you mean reverse the OT leads?

[Tubis]

you mean reverse the OT leads?

to the tubes, yes, that will switch the phase 180 at the output/speaker/feedback to the pi and get it right.

[Steve Conner]

Assuming you're not trying to apply too much feedback, in which case it'll oscillate no matter which way you hook up the OT leads. 820 ohms implies a LOT of feedback.

[Tubis]

Assuming you're not trying to apply too much feedback, in which case it'll oscillate no matter which way you hook up the OT leads. 820 ohms implies a LOT of feedback.

820ohms over 100ohms if its a bandmaster pi. thats not a lot of feedback.

[Steve Conner]

Oh, the OP never mentioned that he took the whole PI from the Bandmaster. The feedback amount is set by the ratio of those resistors. 820 and 100 ohms suggests a closed-loop gain of about 9, which isn't so bad. If he used another PI with a larger resistor than 100 ohms in that position, the feedback could be a lot more.