Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: zfiles1701 on December 08, 2011, 01:12:50 pm

Title: What happens when you accidentally add too much negative feedback?
Post by: zfiles1701 on December 08, 2011, 01:12:50 pm

Added a 100 ohm pot and 5uf cap to the NFB on a hoffman reverb build.  Later removed that mod and instead of putting the 100 ohm to ground reisistor back in, I put a 100k in its place.  Yeah, ouch.  Anyway, replaced the 100k with a 100 ohm.  But, now the amp does not sound the same.  When I roll the volume up it does not have the same power (low), and it crackles above 6 or 7 on volume.  And, when going from 5 to 7 it hums more than usual.  Then the hum and volume drop off  a bit (not all the way) as I dial up to 12 (volume).  Swapped all tubes, replaced the TS caps, and still checking all values of other components.  Non reverb channel sounds fine.  Any Ideas?  Thanks in advance for any input.

Title: Re: What happens when you accidentally add too much negative feedback?
Post by: Platefire on December 08, 2011, 01:29:15 pm

I would re-solder those last connections on that resistor to ground. Maybe bad solder joint if everything was working fine before. Platefire

Title: Re: What happens when you accidentally add too much negative feedback?
Post by: zfiles1701 on December 08, 2011, 02:39:39 pm

Ground on that resistor is straight to the ground of the PS cap that supplies the PI.  Will check is again.  But what may have been zapped or fried by having the NFB voltage so high-probably full OT voltages?  Maybe the PS caps?  Have not checked them yet. 

Title: Re: What happens when you accidentally add too much negative feedback?
Post by: sluckey on December 08, 2011, 02:46:17 pm

Quote

But what may have been zapped or fried by having the NFB voltage so high-probably full OT voltages?  Maybe the PS caps?

You didn't zap anything if all you did is what you said above.

Title: Re: What happens when you accidentally add too much negative feedback?
Post by: zfiles1701 on December 08, 2011, 04:42:07 pm

Thanks for the info.  I must have a loose or bad joint somewhere.  Now the thing has full power, but buzzes bad at full volume on reverb channel when the lower notes are played, and a little on the nonRv channel.  I've had this happen where I reflowed all the joints and the prob was gone.  But all the joints looked good to begin with-never found out which one it was.

Title: Re: What happens when you accidentally add too much negative feedback?
Post by: HotBluePlates on December 08, 2011, 06:43:49 pm

Quote from: zfiles1701 on December 08, 2011, 01:12:50 pm

Added a 100 ohm pot and 5uf cap to the NFB on a hoffman reverb build.  Later removed that mod and instead of putting the 100 ohm to ground reisistor back in, I put a 100k in its place. 

What you describe here would not be more NFB, but much less.

Let's say you did apply too much negative feedback. Other than maybe causing the amp to squeal (which could happen depending on the amp/feedback design), there's only one other thing that could happen.

"Too much" NFB turns the circuit into a big cathode follower. A cathode follower has 100% NFB, and you can't get any more than that.

One thing that can't happen is blowing something up due to NFB. More likely is a solder problem, as everyone has said.