Adding Negative Feedback to a Vox AC4C1?

Bartimaeus

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Well that isn't very punk, is it?
I have a blue ac4c1 reissue that I've done a ton of mods to. Bright cap switch, tonestack lift switch, C5 bass cut switch, vintage-ac4-style tone cut control, G10 Gold speaker, the list goes on...

Stock schematic for the AC4C1 amp can be found here:

I'm not really concerned about keeping the original Vox top boost sound on all settings, so i thought it'd be fun to add a 3-position switch with a couple negative feedback values and a "stock" setting. But I've never added it to an amp before, so I'm looking for some second opinions.

The easy part is grabbing the output from the speaker jack, where it connects to the output transformer. but I can't decide where to insert the feedback.

Based on the Fender Champ and Deluxe designs, seems like it's usually added at the last preamp stage. On this amp, I think that'd be around C15. But seems like the power amp is inverting, so perhaps I need to put the negative feedback earlier? Or can I wire it to the cathode instead of the grid?

The last preamp stage is also serving as a make-up for the tonestack, so I'm not sure if adding negative feedback would interfere with that? Though that doesn't seem to be an issue for the Fender Princeton: https://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-Princeton-AA964-Schematic.pdf
 

Bartimaeus

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Well that isn't very punk, is it?
What do you want to achieve with the mod?
Partly, I just want to experiment and hear new things in the context of a circuit I'm familiar with. But I also like the idea of more consistent cleans at low volume, which could be useful when recording with certain pedals like reverbs.

Easy enough on the schematic. Lift R17 and C12 from ground and use a 49R resistor to ground like in Fender Champs. Then use a 2.7k resistor or other values according to taste.

Oh cool, thanks very much for the reply, I'll give that a go!

Be sure to do an A/B comparison so you can see how the feedback kills the magic.
:lol:
 

Bartimaeus

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Well that isn't very punk, is it?
So I give printer2's idea a shot, mimicking the Fender Champ, and I was met with some painful oscillation! Positive feedback... Turns out the output transformer on the Vox is set up to reverse polarity. Reversed the connections on the secondary and now I'm getting the sounds I was looking for. Not sure if I'll use them, but it's interesting to see the dynamics disappear when I add the negative feedback haha! Has some extra punch instead.
 




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