nfb resistor question

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harris51
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nfb resistor question

Post by harris51 »

I have a jtm45 with radio spares ot running a 8 ohm load I have a 27 k resistor in there now would a 47 k make the amp a little more gainy?
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Colossal
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Re: nfb resistor question

Post by Colossal »

Yes. For more fun and gain, leave the 27k fixed and insert a 100-250kB pot in series. Adjust to taste.
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Re: nfb resistor question

Post by Stevem »

Just note that you will be driving the output tubes harder so they will clip sooner, as such you may also want to bias them a tad cooler depending on what your ears tell ya!
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roberto
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Re: nfb resistor question

Post by roberto »

As for the pot in series on the NFB, I prefer 100k lin or 250k log.
This way you have more control on the whole range.
LPC78wino
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Re: nfb resistor question

Post by LPC78wino »

Stevem wrote:Just note that you will be driving the output tubes harder so they will clip sooner, as such you may also want to bias them a tad cooler depending on what your ears tell ya!
So when a higher value NFB resistor is replaced making the amp a little more gainy, resulting in clipping sooner; will drive the output tubes harder ?
I also see, if one had a high plate voltage, that biasing a little cooler will prolong tube life, but more importantly dial in a tone that one is satisfied with.
If one has a 47k upped to a 100k, then I would be conscious of biasing a tad under (to taste of course) and have the option of the 3 taps. (something like that ?)
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gui_tarzan
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Re: nfb resistor question

Post by gui_tarzan »

Do you have pics of the outside and inside? I just finished updating a '64 JTM45 and you're right, it does need a little more gain than stock but boy are they sweet.
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Reeltarded
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Re: nfb resistor question

Post by Reeltarded »

First thing I do on a 45 is swap a 470k divider pair in place of the 270ks before V2. Mostly about bringing the 'chest' out a little.
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TimmyP1955
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Re: nfb resistor question

Post by TimmyP1955 »

Reeltarded wrote:First thing I do on a 45 is swap a 470k divider pair in place of the 270ks before V2. Mostly about bringing the 'chest' out a little.
Doesn't that make the bright input even brighter?
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Re: nfb resistor question

Post by Roe »

Colossal wrote:Yes. For more fun and gain, leave the 27k fixed and insert a 100-250kB pot in series. Adjust to taste.
yes, 22k + 250k pot into 16ohms secondary
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rp
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Re: nfb resistor question

Post by rp »

This is a real 45, not a reissue or clone? +1 Please post good picts.
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