Tube amp oscillates when I connect negative feedback - Page 1
Forum section: electronics / projects

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: January 15, 2022, 02:36:04 pm
content: what am I missing here? this IS negative feedback what I'm adding right? I even reversed the wires going to the OPT just in case the schematic ofc it was wrong, and no, that oscillates even more...if I drive a signal into the cathode, the plate doesnt invert right?, so it should all be negative feedback... why is it oscillating?, turning the bass control ( the tone stack ) doesnt change it, or not enough to make me think it's the cause...without the negative feedback the amp works fine

author: TimFox
author_rank: Super Contributor
timestamp: January 15, 2022, 03:02:23 pm
content: Ordinarily, one doesn't include the tone stack within the overall feedback loop. The negative feedback for the power amplifier is normally applied from the loudspeaker to the input tube of the power section, after the tone stack.Tone controls introduce substantial phase shift. What frequency is your oscillation with the tone stack at "neutral"?

author: TurboTom
author_rank: Super Contributor
timestamp: January 15, 2022, 03:10:17 pm
content: Exactly -- the feedback loop is meant to linearize the gain of your amplifier. This would, if it is working as supposed, even nullify the effect of your tone control. Place a resistor in the cathode path of the second triode, reverse the output winding of the output transformer and attach the feedback line to the cathode of the second triode. Then the feedback should work okay.

author: CaptDon
author_rank: Super Contributor
timestamp: January 15, 2022, 03:38:53 pm
content: B.T.W., grid leak bias is a poor man's parts saving idea (reminds me of a Mad Man Muntz design). Calculate and use a cathode resistor and bypass capacitor on that second triode and bring your negative feedback also to the second triode. Use just enough negative feedback to linearize at the power levels and frequencies of interest. Often to much negative feedback can both result in unacceptable low gain overall and also create oscillations at ultrasonic frequencies where the amplifier has significant phase delay to make your negative feedback appear as positive feedback.It can create very low frequency oscillations also called 'motorboating'. Typically 2hz to 5hz range.

author: CaptDon
author_rank: Super Contributor
timestamp: January 15, 2022, 04:00:58 pm
content: With cathode bias you still need the grid resistor to ground and the coupling capacitor. Typically 100kohm and .47uf PLUS a cathode resistor (220 to 470 typical) and a cathode bypass capacitor. Grid leak saves two parts, the cathode resistor and bypass cap.

author: Terry Bites
author_rank: Super Contributor
timestamp: January 15, 2022, 05:29:46 pm
content: Therse' is no compensation in the FB loop- try a few pF say 6.8p, across the 1Meg.You dont need DC feedback so a few 10's ofn in series with the 1Meg won't hurt.Speakers are a reactive load.

author: floobydust
author_rank: Super Contributor
timestamp: January 15, 2022, 08:03:23 pm
content: Why is the EL84 screen to B+? OUCH

author: CaptDon
author_rank: Super Contributor
timestamp: January 16, 2022, 03:27:09 am
content: He tied the screen to B+ to get a visual indication of when the plate swings substantially below the screen. You get the nice shower of internal sparks as the screen melts down and leaves town, followed by the neat blue glow of some out-gassing from the exploded innards of expensive tubes or 'valves' in EL84 land. Ultra-linear connection would have been nice or even just a screen resistor for protection. I guess at 270vdc B+ it actually may never draw enough screen current to self destruct, but it sure is a crappy circuit!! Mad Man Muntz returns to the scene of the crime. "I buy 'em retail and sell 'em wholesale!! It's more fun that way!!!" Muntz was a character!!! I worked on a lot of his stuff, pity the fool who bought it!!!