Negative Feed back-Valve amps?

Marty Shields. · 7th January 2004

Hi There, peoples.
Iv'e built this push pull amp for the lounge room,and thaught i'd introduce some negative feedback.Ive designed the beast using a standard scmitt phase changer with a long tail pair.I thaught i'd take the FB from one side of the pair back to output, now, heres the question;Whats the best way to vary the attenuation?Can I wind my transformer with seperate switched tappings on a seperate fb winding, or should i just switch it thru a few differentresistors and caps? How do they do the ones at MANLEY?
Id be greatfull for your thaughts.
KIND REGARDS,
Marty Shields.
Tasmania aussie.

dale116dot7 · 8th January 2004

I'm not from Manley but here's my two bits:

I assume you're talking about an audio power amplifier, not a preamp. I guess things are similar in a preamp anyways.

My tube amps are a bit different - unbypassed cathode on the first stage, second stage is a split load phase inverter. Anyways, what I do to get the feedback is vary the resistance from the output terminal back to the cathode of the first stage. The cathode resistor is always small (maybe 2.2k) and the feedback resistor is always large (like maybe 150k) so the DC current of the first stage is no issue to run through the output transformer. But because of stability, I run the amplifier at a fixed gain and attenuate the input signal, I don't vary the feedback because of phase shifts through the output transformer.

One caution - since the amplifier is AC coupled through a transformer, you need to watch the response carefully with a scope. I missed this step and oh boy did it oscillate. Too much phase shift in the transformer coupled with too much negative feedback results in too much positive feedback at 150 kHz. Now I check the feedback stability by feeding in a small square wave and looking at the response across a dummy load and also across a real speaker.

Good luck!

-Dale

Marty Shields. · 9th January 2004

Thanks Dale,
I'll Certainly keep an eye on the crow, Good point about the phase shift.
kind regards,
Marty

7th January 2004	#1
Marty Shields. Gear Head Join Date: Jan 2004 Location: Tasmania, Aus't Posts: 66 Thread Starter	Negative Feed back-Valve amps? Hi There, peoples. Iv'e built this push pull amp for the lounge room,and thaught i'd introduce some negative feedback.Ive designed the beast using a standard scmitt phase changer with a long tail pair.I thaught i'd take the FB from one side of the pair back to output, now, heres the question;Whats the best way to vary the attenuation?Can I wind my transformer with seperate switched tappings on a seperate fb winding, or should i just switch it thru a few differentresistors and caps? How do they do the ones at MANLEY? Id be greatfull for your thaughts. KIND REGARDS, Marty Shields. Tasmania aussie.

9th January 2004	#3
Marty Shields. Gear Head Join Date: Jan 2004 Location: Tasmania, Aus't Posts: 66 Thread Starter	Feed back: valve amplifiers. Thanks Dale, I'll Certainly keep an eye on the crow, Good point about the phase shift. kind regards, Marty

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8th January 2004	#2
dale116dot7 Lives for gear Join Date: Dec 2003 Location: Calgary, Alberta Posts: 815	I'm not from Manley but here's my two bits: I assume you're talking about an audio power amplifier, not a preamp. I guess things are similar in a preamp anyways. My tube amps are a bit different - unbypassed cathode on the first stage, second stage is a split load phase inverter. Anyways, what I do to get the feedback is vary the resistance from the output terminal back to the cathode of the first stage. The cathode resistor is always small (maybe 2.2k) and the feedback resistor is always large (like maybe 150k) so the DC current of the first stage is no issue to run through the output transformer. But because of stability, I run the amplifier at a fixed gain and attenuate the input signal, I don't vary the feedback because of phase shifts through the output transformer. One caution - since the amplifier is AC coupled through a transformer, you need to watch the response carefully with a scope. I missed this step and oh boy did it oscillate. Too much phase shift in the transformer coupled with too much negative feedback results in too much positive feedback at 150 kHz. Now I check the feedback stability by feeding in a small square wave and looking at the response across a dummy load and also across a real speaker. Good luck! -Dale