Why the heck is there a feedback switch on the MG-Head OTL?
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phonejack

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Originally posted by Born2bwire
when you switch from transformer w/ feedback to w/o feedback, does the balance seem to shift to the right? My OTL output seems to be to the right some and I'm trying to find out if it's placebo or a slight fault with the amp because I can't figure out a logical reason why the feedback would correct the balance.

Here's what I noticed: Transformer input Feedback switch down or off (I presume) normal. with Switch up or "on" the volume decreases a lot.

OTL input: with feedback switch down (off-right?) sounds better to me . When I place the feedback up (on) I notice the left side shift to the right a bit but not as much as I notice the change of the proximity of the bass. It's almost as if someone threw a low cut of hi pass filter in the chain.

I'd like to know why this occurs. Does it actually suppose to be this way? I don't know, maybe some sources sound better this way. If not, why the hell is there a feedback switch?

I guess what I'm asking is "Why is there a feedback switch if it kills the sound (most people seem to never use it) and why or when would someone use it?

Thanks...

JAck
 
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andrzejpw

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From what I've read, the OTL mode improves as you get better tubes. I haven't heard an MG head with better tubes though. . .
 
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Born2bwire

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There are some shortcomings when you remove the feedback loop. The biggest being that the noise levels are brought up and the faults of the amp become apparent. There is a hum in the left tube that results from the raw AC power coming in close to the left power tube on its way to the power switch. You know, why can't you just move the switch to the back, not like it's a real reach. Joe elongated the chassis to alleviate this, but I still noticed it clear as day on the amp while I still had it.

I think that my problem with the amp was with the volume control in that, as you've noted, the gain greatly increases without the feedback loop and on very low volumes, the right channel was much louder than the left. Thus, I think that when I went to the higher gains of the non-feedback and OTL modes, I moved the volume control low enough to where I was getting into a region where it was having the balanced skewed but the feedback mode was loud enough to be outside of that region. I have to say that it sounds much better without feedback in terms of detail, but it really does bring out the noise problems with the amp and that's the reason why I reinstated the NFB loop on my original MG-Head DT.
 
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fredpb

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Using the right OTL jack, the feedback switch should not make a difference.

With the left jack, you use the transformer out. The UP position of the switch is "no feedback". The DOWN position is "feedback on".

The feedback is negative feedback and does two main things:
1) "Stabilizes the amp". Less prone to various evils like oscillations.
2)"reduces amplifier created nasties" like distortion or some noise.

Any circuit degrades the signal by removing or adding stuff. The negative feedback, when "in" (switch down) will reduce the output of the amplifer, but also just about eliminate many of the nasties created by the amp stages.

Theoretically, the purest sound should come with the feedback on, and transformer mode.

As far as levels and stuff changing, this could also be caused by tubes with mismatched characteristics, like current gain and such.
 
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phonejack

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Quote:

With the left jack, you use the transformer out. The UP position of the switch is "no feedback". The DOWN position is "feedback on".

Theoretically, the purest sound should come with the feedback on, and transformer mode. [/B]



Thanks for the help. I am one confused individual. I wish the manual was more clear.

I have 2 questions;
1) The writing ABOVE my switch states:
"On"
feedback

I thought by placing the switch up towards the word "On" placed the feedback on. Am I thinking backwards? Did they print this wrongly? I am just accustomed to placing switches in the direction that says "On" - so - is it defiantly "UP IN ON"?

2) Should the feedback switch ever be used in OTL mode and when if so?
 
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fredpb

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I don't know if the modified the direction of the switch. So the "lower volume" position of the switch is WITH feedback.

The feedback switch should have no effect on OTL. If it does, in your case, use what ever sounds better.

Keeps some spare tubes. I just had one 6blah go bad after only 6 months. It was a JJ Tesla.
 
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