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FDP Forum / Amp Mods, Repairs, and Projects / Negative Feed Back Mod



Bobby Roadrunner
Feb 5th, 2018 01:54 PM   Edit   Profile  

Hi Everyone: I have a 1972 Fender Twin Reverb, that I reworked as an AB763 and it is working great. What I would like to do is to put a capacitor in line with the 820 feedback resistor, which will aid in the bass harmonics. I had read somewhere that you can take a capacitor and either put it in series or in parallel with the negative feedback resistor so only the high frequency portion of the feedback comes from the speaker jack. sort of a fixed resonance control. Also, I'm not sure of what sized capacitor to use. It looks like a some of the pot based resonance controls use a .0047uf non polarized cap. Also, does the resistor size matter relative to the capacitor? Any suggestions would be great, thanks again.



Bobby Roadrunner
Feb 6th, 2018 07:38 PM   Edit   Profile  

Ok I took out he 820 ohm negative feedback resistor and put in a 22k. The amp really opened up a lot, extremely touch sensitive. The noise level came up a bit. it’s now noticeable with at idle. I’m going to slowly back down in resistance to find a happy medium. I might put in the resonance control as described by robrobinette next.

Contributing Member


Life makes a man tired.
Feb 6th, 2018 07:43 PM   Edit   Profile  

You can pot a pot in there too. I’ve done that before. Put a 820 in series with it so that becomes the low value in the circuit.



Bobby Roadrunner
Feb 6th, 2018 09:10 PM   Edit   Profile  

Hi Leftee: Yes I have done that with the master volumes. I was thinking of installing it on the back of the chassis. thanks

Contributing Member

Whitehorse Canada

I don't get out much
Feb 6th, 2018 10:56 PM   Edit   Profile  

"What I would like to do is to put a capacitor in line with the 820 feedback resistor, which will aid in the bass harmonics."
What do you mean "which will aid in the bass harmonics"?

Are you wanting less low frequency information in the negative feedback circuit?
If so...do you know how to calculate the corner frequency of the cap to achieve your goal?
I don't.
I asked a similar question months ago and learned that it's not just a simple matter of changing a cap value.
One must take into account many other things before determining the cap value that gives you the corner frequency that is needed to achieve your goal.

Just adding this as food for thought and as a buffer before the big dogs step in.



Feb 7th, 2018 03:06 AM   Edit   Profile  

Yes, the cap value needs to be appropriate for the circuit resistances, in order to achieve suitable corner frequencies.
As the feedback resistors in a TR are way lower values than those in a 5150, the cap value would need scaling up to compensate. The 4n7F of a 5150 is way to small for a resonance cap in a TR feedback circuit.
Whatever, it seems a bad idea to me, a recipe for flub; TRs tend to have way too much bass available anyway.
But as the idea is in your head, you may need to prove that to yourself.



Bobby Roadrunner
Feb 7th, 2018 08:46 PM   Edit   Profile  

Hi Pete: Thank you for your thoughtful input when I ask questions. By Luck in the on line issue February issue of Premier Guitar Magazine, Jeff Bober brings up this issue with some recommendations for the resistor and capacitor combination. Its on Page 174.Right now I'm really liking this Twin reverb with the 22K negative feedback resistor. Its quite amazing sounding as to the 820 ohm resistor. What I am finding, is with the volume just under 3, treble at just over 4, the mids at 8/9 ish and the bass at 4/5 ish the sound is very rich and full without being loud. I think my next step is to measure the DB level to see where I am at. The next step is to try the different cap/resistor values. I think this is real interesting.



Feb 8th, 2018 04:41 AM   Edit   Profile  

If the 820 ohm resistor has been replaced by 22k, and the 100 ohm resistor it relates to is unchanged, then I don't think there will be much negative feedback.
Have you tried comparing open loop to closed loop, ie lifting one leg of the 22k, or putting a switch in series with it?



Bobby Roadrunner
Feb 11th, 2018 10:58 AM   Edit   Profile  

Hi Pete: Yes, I did. I temporarily used the ground switch that was not being used to switch between the 820 and the 22K and then no feedback.

Basically with the 820 compared to the 22K, the 820 seemed sterile and had to play the amp louder to get the responsiveness I was looking for.

Also, it really took away a lot of the harmonic structure I was looking for as well as reduced the lower bass register for the A and E strings. I mostly play rhythm guitar. Also, the amp was not real responsive at a volume below 4.

I thought the no feedback resistor was a little too aggressive and it seemed the amp was too much on the edge. I see in the layout where the 100 ohm resistor is you are referring to, I am not sure what it does in series with the 820 ohm, I do see it comes off the 100k/25/25 from pin 8 of the 12AX7resistor/capacitor for the tremolo.

What I can tell you, with this setup, I can have the Twin on a little over 2 to 2 1/2 which is a nice volume and get the touch response, the frequency response dialed in.

I have been using the AB763 layout as my base, parts/wiring scheme and some mod suggestions from the Rob Robinette site.

I will post next what I have pulled together exactly what I ended up with. Thanks Again, Bob

Steve Dallman

Merrill, Wisconsin

Age is just a number...mine is big
Mar 13th, 2018 06:45 AM   Edit   Profile  

I've installed a "resonance" control in quite a few Fenders.

A .0047uF cap in parallel with a 1 meg ohm audio taper pot, then put in series with the nfb wire.

It's worked fine in all.

FDP Forum / Amp Mods, Repairs, and Projects / Negative Feed Back Mod

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