Re: [Harp-L] Champ Feed Back Loop mod (RICK DAVIS)

[MARK BURNESS <markwjburness@xxxxxxxxxxxxxx>]

You don'tsay which Champ you have?

On any champ the NFB loop is tapped from the second stage cathode of the 12A#7 
tube. Find pin 8 by counting clockwise from the gap in the tube pins whilst 
looking at the top of the tube socket, inside the chassis. Follow the wire from 
pin 8 and it will run to a 1500 ohm resistor (brn/green/red). 


On a 5F1 champ, the end of this resistor that connects to the 12AX7 will also 
connect to a 22K resistor (red/red/orange - NFB dropper) that runs to the hot 
speaker jack terminal. Remove the connection from this resistor to the speaker 
jack, insulating the disconnected wire, but do not disturb the 2 wires (usually 
yellow & black) that run from the output transformer to the speaker jack, if you 
do you will kill the power tube & probably the OT in a few seconds!

On a BF/SF Champ procedure is much the same except that you will find the NFB 
wire comes from the end of the 1500ohm resistor at 12AX7 pin 8 that is 
FARTHEST from the tube socket. The 1500 ohm resistor will also be bypassed by a 
25uf cap (you might want to experiment with having this cap in/out of the 
circuit?) which is in parallel and there will be an additional 47ohm resistor 
(yel/violet/blk) between 1500 and ground, from where the NFB loop is tapped. The 
NFB dropper resistor in a tolex champ is 2.7Kohms (red/violet/red). Again, 
remove the connection from this resistor to the speaker jack hot connection, 
without disturbing the OT wires, & insulate.

Should you accidentally disconnect the 1500ohm resistor from the 12AX7 socket, 
or from ground at the other end,
 the amp will cease to make sound, don't panic just check & reconnect if 
necessary, no harm will be done.

Personally I prefer the loop connected. In a tolex champ I prefer the tweed 
style loop with the 25uf bypass cap at 12AX7 pin 8 removed. Without the NFB loop 
connected, you may also want to experiment with removing the 25uf bypass cap (up 
the voltage rating of this cap to 50v minimum and dress it away from the hot 
cathode resistor) at pin 8 of the 6V6 (a common arrangement in amps where no NFB 
loop is installed - again do not disturb the connections of the 470ohm 6V6 
cathode resistor, or it will turn off the tube). Talking of the 470ohm 6V6 
cathode resistors, tolex champs are biased very hot, you could also try 680ohms 
or 800ohms (at 5W rating) and see which you prefer?

One of the mods I think is most useful on a tolex champ is a reduction in the 
value of the 15K middle resistor (brn/grn/orange) that is grounded from bass pot 
LH tab (adding resistors in parallel will reduce the value - R1xR2 then divide 
by R1+R2 for the new value)...most Fenders with a fixed middle value use 6.8K 
here, you might want to try 10K, or even bypass the 15K with bare wire for no 
middle at all?

All these mods are easily reversible and the "right" ones are simply the ones 
you like the sound of best. Do them one at a time so you hear what each one does 
& determine which you like. Make sure the amp is fully serviced and functioning 
properly before modding.

As ever, when working on tube amps, drain the flter caps (I use a jumper wire 
from 12AX7 pin 1 or 6 to chassis ground, drains voltage in 30 seconds via power 
supply resistors, keep in place during any soldering/modding & remove before 
powering up) & never work on an amp that is connected to wall AC even if the 
amp is turned off, as you will still get a jolt from the AC cord & fuse 
assembly.



________________________________
From: joe hagins <joe_hagins@xxxxxxxxx>
To: Harp L <harp-l@xxxxxxxxxx>
Sent: Saturday, 3 September, 2011 0:04:28
Subject: [Harp-L] Champ Feed Back Loop mod (RICK DAVIS)

So I got my Champ today and it sounds pretty good. The speaker wasnt original so 
I put in my Weber 8A125 and it sound pretty. I want to cut the Feed back loop 
wire and looking at the picture on Rick's site I'm not sure which wire he speaks 
of nor where the best spot to cut it would be. SO Rick if you are out there can 
you let me know 


Cheers,
Joey Anchors

http://joeyanchors.wordpress.com/