Fender vibrolux negitive feedback

[paisley_tele]

How do u add negitive feedback to the fender vibrolux custom amp?

[rdjones]

Insert a 47 ohm resistor between the junction of R37/C19 and ground of the PI "tail".
Connect a 1K ohm resistor between that same junction and the "hot" lead of the output transformer secondary at the output jack.
You may need to swap the two leads of the secondary.

http://support.fender.com/schematics/gu ... ematic.pdf

rd

[martin manning]

Do you mean Custon Vibrolux Reverb? If so, then circuit-wise you could do it just like the original Vibrolux Reverb AA964. You'd have to lift the junction of R37 and C19 from ground, and put a 100 ohm in there, then run an 820 ohm from that junction to the speaker. I don't know how easily that can be done on that pcb, though.

Simul-posting with rd, who likes a bit less NFB... Good point about possibly having to swap the secondary leads, but that'll flip the speaker phase, and who knows what could happen if you do that! Swaping the speaker leads as well would restore it, in that case.

[rdjones]

http://support.fender.com/schematics/gu ... ematic.pdf

<.pdf edited to display top side up>

[martin manning]

Turns out this pcb is already set up for NFB. To make it look like the original Vibrolux, make changes as shown. R37 is the PI tail, which should be changed to the original value, I believe.

[fuzzheimer]

Just one note- I think you may possibly need to reverse the PRIMARY o/p transformer leads if the amp squeals after adding NFB (not the SECONDARY leads).

Good luck!

[martin manning]

Just one note- I think you may possibly need to reverse the PRIMARY o/p transformer leads if the amp squeals after adding NFB (not the SECONDARY leads).

Either way will work.

[Wicked.quick.18]

What does I guy say 100ohm and the othe fella say 47k .... Tweak to test????

[Wicked.quick.18]

Also since in in the amp would it be a wise choice to upgrade the caps to orange drops?

[sixstringer]

Orange Drops are an upgrade? ''

[rdjones]

Just one note- I think you may possibly need to reverse the PRIMARY o/p transformer leads if the amp squeals after adding NFB (not the SECONDARY leads).

Either way will work.

According to the schematic there are no taps on the secondary, only a single impedance winding.
I saw that the color code used was black=hot, green=ground and since this is not AC supply wiring, my OCD logic dictated the more conventional speaker wiring of black=ground, green=hot.
Yes, in the case of a multiple impedance secondary the primaries might have to swapped.

Also since in in the amp would it be a wise choice to upgrade the caps to orange drops?

There are (at least) 4 different types of construction used in "orange drops" by at least 2 manufacturers.
See several recent threads about capacitor types and orange drops.

What does I guy say 100ohm and the othe fella say 47k .... Tweak to test????

Be careful, it's 47 ohms, not 47k
The correct AA964 values (for a Vibrolux Reverb) are 100/820.
The values I gave will be slightly less feedback.
The 47/820 ratio is used in quite a few Blackface era amps, including the non reverb AB763 Vibrolux.
Personally I'd start with the lesser amount of FB and tweak if it still wasn't clean enough.

reddog

[PaisleyTube]

the custom vibrolux is more like a modded brownface than a blackface amp so check out the neg.feedback resistors values from the brownface super or ('63) vibroverb imo (both 40 Watts into 4 ohms)