Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: timmyjoe on February 20, 2022, 09:32:02 am
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I would like to go from the normal 5E3 gritty goodness to the Negative Feedback Mod (NFB) with a footswitch. However, Rob Robinette's footswitch mod actually only toggles between no 22uf bypass cap and NFB. Which of the following would be best way to do this? If possible.
1. Would the NF mess with the 22uf bypass cap? In other words, could I just leave the bypass cap in the circuit and feed in the NF on and off with a foot switch?
2. Could I use a stereo cable as the foot switch cord toggle between the NFB and bypass cap? Seems like the long run doesn't hurt the NFB, but not sure if it would affect the Bypass cap.
3. Relay? I know I'm going to have to learn how to do this one day, but not sure if it is really necessary at this point.
Attached is Rob's NFB footswitch.
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Rob uses a three position toggle switch that gives three modes of operation. You can't have the NFB and bypass cap connected at the same time because the cap would just kill the NFB signal.
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Have you tried the feedback mod yet, with a toggle switch as shown? I like it, does what you expect negative feedback to do, but can’t imagine wanting to switch it on and off during a gig.
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JM2CW. The NFB loop in BF Princeton has full boost with NFB and is easily adaptable to the 5E3. The driver stage is the same, the PI is the same and the OT is practically/technically the same - 8k plate-to-plate 15W. (The B+ voltages are a bit different, but no need to split hairs). If you short the 47R you have a full boost 5E3 driver stage. Very quiet to implement, and if you want to be fancy, you can use a relay for the switch. I have done this and it works well IMO.
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Yeah, JB.
Really like the NFB three position switch in the diagram. Just want to be able to go from normal to NFB during a song.
Wow Tubewell. I'm going to have to sit down and study on that diagram.
I think I'll try the stereo cable first, option 2. That way people will know its a dumb idea before doing it themselves. :think1: Seems to me that if you want a no cap no NFB then you can just unplug the cord an have that sound too.