NFB resistor

cmoore
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NFB resistor

« on: February 14, 2009, 04:28:38 PM »

I know the value Nik uses for the NFB is 100K. But what value resistor are you guys ending up with for the NFB?
Thank You


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M Fowler
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Re: NFB resistor

« Reply #1 on: March 25, 2009, 04:25:16 AM »

For less gain use a 68k or you can just try different tubes like the 12at7, 12au7, 5751 rather than the 12ax7/7025/ecc83 variant with gain of 100.

the 5751 is my favorite in either v1 or v2 position and run a matched 12ax7 in v3 the phase inverter position (PI). By using matched NOS el34 you also get a different tone. Remember this amp also allows you to run 6v6 as well as the el34.

The caps in the tone stack like the 250pf and .1 can also be changed to .330 and .33 for more bass response.

Mark


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cmoore
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Re: NFB resistor

« Reply #2 on: March 25, 2009, 06:28:01 PM »

M Fowler,
Thanks for the help. I finished with a 51k resistor at the NFB position. I also played around with the lead dress a little. I separated the plate load wires that were tied to the NFB wire which helped a bit at volume.
Thanks Again


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M Fowler
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Re: NFB resistor

« Reply #3 on: March 30, 2009, 04:59:01 PM »

Hired Hand,

I know you read ampgarage.com too as I think I have seen you there. Did you read about leaving the 100K resistor alone and switch between the impedance selector 16, 8 and 4 ohm to achieve the same results?

Mark


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cmoore
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Re: NFB resistor

« Reply #4 on: March 31, 2009, 07:08:44 PM »

Quote from: M Fowler on March 30, 2009, 04:59:01 PM

Hey Mark -
I do read TAG sometimes. But if i seem familiar to you it is probably because you have helped me out here before. I guess if I were to use the speaker jack as the NFB source, I would be increasing or decreasing the voltage to the NFB resistor any time I went to 4 or 16 ohms? I assume at 8 ohms the NFB would be functioning as designed? But to answer your question... no.
I should have been a little more forthcoming with my original post. I was having a lot of feedback and oscillation troubles with this amp. It took quite a bit of fooling around with lead dress and component changes to tame the troubles. It seems that I now have the amp pretty stable, for a TrainWreck type of amp. I can imagine what Ken Fischer must have gone through. Amazing he did not give up at some point. I realize he had much more ability/knowledge with electronics than I do, but he had no guide with him. I see the PI looks like a "Plexi" but I imagine the rest of the amp was mostly KF tinkering with his ideas. Anyway.......
Thanks for your help again.


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