ampage
Tube Amps / Music Electronics
For current discussions, please visit Music Electronics Forum. New: view Recent Searches.
New: visit Schematic Hell!
The sunn still shines online!

 
Listen to great tunes streaming live right now!

ampage archive

Vintage threads from the first ten years

Search for:  Mode:  
previous: Joe Movich Anyone know what the concept is beh... -- 11/12/1998 12:47 PM View Thread

Re: Fender "Fat" Switch

11/13/1998 12:27 AM
Steve A.
Website
email
Re: Fender "Fat" Switch
Joe:  
 
    I don't have much to add to Doc's reply, but just wanted to add that it is actually more of a "Skinny" switch than a "Fat" switch, since the switch normally removes the 22-25uF cathode resistor bypass cap (aka Ck) that is generally present on the input stages of most Fender guitar amps. On other amps it is labelled as a "Boost" switch, but likewise it would technically be an "Unboost" switch.  
 
    In other words, if your amp doesn't have a "Fat" or "Boost" switch (as explained in Doc's post), it probably already has a cathode resistor bypass cap in which case adding the switch will offer an unboosted option (rather than a "superboosted" option). It's best to check the schematic for your amp to see if a Ck cap is being used and what the value is. If you don't have a schematic, just follow the leads from the tube socket- look for the stage that the signal from the input jack goes to first (very often pin #2 of V1, in which case the cathode is pin #3- just follow its path to ground through the cathode resistor and see if there is a capacitor hooked up in parallel).  
 
    Using a switch to select between values for Ck (or none at all) is a simple mod that improves practically any amp. A large cap between 10uF and 25uF gives a louder, fuller sound. A small cap between 0.47uF and 1.0uF gives a louder, brighter sound with the lower frequencies rolled off for more midrange (Marshalls often use a 0.68uF Ck cap). No Ck at all has lower gain, but with increased clean headroom and a flatter frequency response. It may take awhile to get used to the unboosted option (since its not as loud) but it works great with clean sounds and is a worthwhile addition.  
 
    Subsequent preamp stages may or may not use a Ck cap; if they do the value is usually much smaller- like maybe a 1.0uF cap for the second stage. By the time you get to the 3rd stage (if there is one) there probably isn't one present and if you add one, the additional gain and bass response may push the circuit into oscillation or ugly distortion. But you can generally play around with the first stage without making your amp unstable, and optionally REMOVE a Ck cap used on later stages for different tone and gain characteristics from your amp.  
 
Good luck!  
 
Steve Ahola

 
Replies:
Steve, again     If you are ... -- 11/13/1998 12:41 AM