newlash09

Effect of negative feed back on tube power amps

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Hi all 

I recently bought a pair  of tube mono blocks which have a negative feed back control dail. I don't know what exactly it does, and there are no markings as to what values I can vary it inbetween. But I've been told it starts from zero. So I was wondering where should I start from. And if it helps, each amp also comes with a gain dail in the back to adjust the gain of the power amp. Is there a correlation between these two. Thanks.

P.s : obviously I should actually try it out and see. But Iam sadly sailing away. So thought I'd go back home armed with some knowledge before I start fiddling with the dials. Thanks for your time :)

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The gain control won't affect the NFB dial. I'd set the NFB to maximum, listen for a few days to get an idea, then try it on half way for a few more days, then adjust up or down as suits. Since it might affect more than the distortion of the amp there's no way of knowing what will work best for you.
Have fun and happy sailing!

Sent from my BLA-L09 using Tapatalk

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It will probably behave like a kind of tone control, but not tone in the conventional sense, maybe timbre, fun and educational thing to play with. You may find a preferred setting or find some recordings prefer one setting, and others prefer another..... Entering the mysterious zone of amplifier voicing.......

Edited by dave
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Super Wammer

For my IA MB845s I have gleaned the following from a review & the instructions. (the amps have a high/low sensitivity toggle switch) 

Low sensitivity = high feedback giving lower distortion, higher bandwidth & possibly tighter bass. 

High sensitivity = low feedback, giving higher distortion, lower bandwidth, increased spaciousness, deepening of sound stage and a tad extra warmth. 

I'm guessing the gain adjustment on your amps relate to the sensitivity switch on my IAs as the output volume changes when I switch between them. 

Hope this helps. 

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7 minutes ago, Lurch said:

For my IA MB845s I have gleaned the following from a review & the instructions. (the amps have a high/low sensitivity toggle switch) 

Low sensitivity = high feedback giving lower distortion, higher bandwidth & possibly tighter bass. 

High sensitivity = low feedback, giving higher distortion, lower bandwidth, increased spaciousness, deepening of sound stage and a tad extra warmth. 

I'm guessing the gain adjustment on your amps relate to the sensitivity switch on my IAs as the output volume changes when I switch between them. 

Hope this helps. 

Thanks all...

The thing is I will be biamping my speakers. The tube amps I have are called as TAD 1000 mono blocks , not to mistaken with that venerable Japanese TAD :)

So these tubes will be running just the midrange and up, and I have SS amps for the bass. So considering that bass is not important...how will adjusting negative feed back only effect the mids the treble

P.S : please note that there will be a active cross over In the chain. So the tube amps won't see the bass frequencies bellow 350 hz at all :)

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1 minute ago, steve 57 said:

I have read the 1st post correctly in that there is both a gain control and a feedback control

Cheers

Yes..sir .you are very much right. Both those dials still exist :)

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I would run it without negative feedback if at all possible. It will produce more distortion however should be more phase coherent. 

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On the crudest level turning up the feedback will lower the output impedance, so if the impedance curve of your speaker is relatively benign i.e. like a resistor the frequency balance will remain fairly stable, if however it is all over the place expect some degree of variation in frequency response, which may mask some of the more subtle effects. As you are making your own speakers(?) you have some control over that.

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Newlash, this is a very good site for explaining the history of valves and how they work. This particular page has a section on negative feedback 

https://www.effectrode.com/knowledge-base/steampunk-technology-the-inner-workings-of-vacuum-tube-buffers/

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5 hours ago, lostwin said:

Newlash, this is a very good site for explaining the history of valves and how they work. This particular page has a section on negative feedback 

https://www.effectrode.com/knowledge-base/steampunk-technology-the-inner-workings-of-vacuum-tube-buffers/

Thanks a ton Lostwin :)

will check out that site now. Thanks again :)

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1 hour ago, newlash09 said:

Thanks a ton Lostwin :)

will check out that site now. Thanks again :)

I appreciate the positive feedback!

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2 minutes ago, lostwin said:

I appreciate the positive feedback!

Most welcome Lostwin :)

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