Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: ALBATROS1234 on November 30, 2023, 08:45:51 am

Title: Need suggestions with adding Negative Feedback to and original design
Post by: ALBATROS1234 on November 30, 2023, 08:45:51 am

Good morning, I have just built an amp that will be residing in a 1959 Magnavox record player console cabinet. It is basically a tweed deluxe except instead of a 12AY7/12AX7 for V1 I have subbed a 6SJ7 and 6SF5 for the channel input stages. There is 1 input which effectively jumpers the channels permanently. There is a 6SL7 as the driver/PI into a standard pair of 6V6s feeding a 12" speaker. I would like to tame it down just a bit. I used a decade box and experimented with plate and kathode resistors as well as dropping some bottom out making the coupling caps smaller than standard Fender values. I subbed a 6SN7 for the driver/PI and it makes the volume a little lower but seems like it is missing something. It sounds really good, but I would like it to break up a little later. Fully cranked it screams and gets you into raw Marshallesque territory. I like the concept of balancing both volumes and tone to get a good dirty rhythm tone. Then maybe use a clean boost to get into leadville and vol knob it for clean. I experimented with Neg feedback and it didn't seem to effect it at all. I basically clipped in the decade box from a speaker terminal and tryed clipping to the Kathode of the driver 6SL7 it did nothing no matter what value resistor I used, I tryed clipping on the 6SJ7 and 6SF5 kathodes but still no real effect. I looked around a bit and I can't find an example of negative FB use in a PP amp using a cathodyne PI. I can find a SE champ NFB which doesn't apply here and NFB with parphase or LTP PI's. I even tryed to bypass the driver 1st stage of the 6SL7 but then it sounds limp. I am wondering about the PS resistors, Maybe I should try to drop more voltage. I thought NFB would help but maybe I need to switch to a different PI. It doesn't need too much I just would prefer a bit more headroom, it gets pretty loud and dirty quick. Any suggestions?

Title: Re: Need suggestions with adding Negative Feedback to and original design
Post by: sluckey on November 30, 2023, 10:35:34 am

You must remove the 22µF cathode bypass cap for the NFB to have any effect. Look at this Harvard schematic...

     https://el34world.com/charts/Schematics/files/Fender/Fender_harvard_5f10.pdf

Or look at Rob's NFB mod for a 5E3. Same as Harvard except with a switch...

     https://robrobinette.com/5e3_Modifications.htm#Switched_Negative_Feedback

Title: Re: Need suggestions with adding Negative Feedback to and original design
Post by: ALBATROS1234 on November 30, 2023, 03:32:29 pm

Thanks about to alligator clip it in after I remove the bypass cap. I was scratching my head on that one. I clipped in my decade box and didn't get why I couldn't get it to work. I have never had NFB on an amp build but I thought this situation it might help.

Title: Re: Need suggestions with adding Negative Feedback to and original design
Post by: ALBATROS1234 on November 30, 2023, 06:26:59 pm

Two things helped, I feel I need to mention:

1. I removed the bypass cap from the K.R.
2. I changed from the green wire to the black wire on the OT secondary.( this made a huge difference"

I think I will decide on a resistor that gets me into the territory I want and the add a toggle switch to defeat if the person who buys it wishes.

Thanks for the tip Sluckey.