Guitar Tube Amp

Step 25 Local NFB Option, V0.4

Late Sept., 2008

While reading about the weird additions to the Fender Bassman (Ver AB165), I noticed a comment about a local Negative FeedBack loop on one of the gain stages. In theory, this should add some compression to the amp. NFB will also extend the frequency response of the amp.

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NOTE: Just so we're clear:
Negative Feedback results in less of the high frequency feedback squeal usually associated with guitar amps (that's "positive feedback.") Negative is good, in this case.

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So I tried it on the second preamp stage. I really like it! Of course, a NFB (negative feedback) loop will subtract a bit of gain from the amp--hence it's "optional" nature.

For a bit of a twist, I ran the feedback signal back through the tone stack. Note that the "presence" control still works with the NFB, but has less effect on the sound.

Any value from 100K to 680K will probably yield results...pick one that suits you. Rule-of-thumb: the lower the resistance in the loop, the more feedback and the more signal loss. So lower values like100K will have more effect on the tone, but make the amp quieter and less overdriven. it's certain possible to have a switching option, also.

NOTE: I did add a switch for the NFB loop, and it works great. Switched in, the sound is definitely sweet and lush. Switched out, the punchy, raw sound returns.

The cab back photo shows the switch, along side the screen supply switch. The switch has now been reassigned for different uses--twice. Maybe a relay / footpedal setup in the future?

Comments

[cnludwig]

Cool project. I like how you used otherwise unwanted parts (power tubes and transformer) and created a design around it.

[gmoon]

:-)

[eridan]

Very interesting. Thanx a lot!