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Tweed Disease


 
10/25/2004 3:04 PM
s2
email
Tweed Disease
So, I have heard a lot about tweed disease and how Hoffman's board layout is supposed to cure it. But, I don't really know what it sounds like. Anyone have any insight?
 
10/26/2004 1:20 AM
Benboom
I have heard that the fiber boards in the old tweed Fenders can become microphonic. How, I don't know - I'm just passing on old wives' tales here. I can't attest to this; my Deluxe is 49 years old and still has the original fiber board and it hasn't done that. But if it is true, then yeah, a new board would certainly cure the problem. Kind of drastic, though.
 
10/26/2004 2:49 AM
s2
email

That is some interesting information. I don't think that is it in my case (buzzing oscillation problem), though. I have seen Doug Hoffman's revised layout for the Deluxe, and people claim it was done to cure tweed disease. It appears the main difference is in moving one of the filter caps to the other side of the board. Anyway, Doug is a pretty sharp guy, so his doing this begs the question, "What exactly is tweed disease?"
 
10/26/2004 3:51 AM
bnwitt
S2,  
Tweed disease is the condition an original Fender fiber board develops if exposed to moisture (like when an amp that is stored in a garage or barn) for a long time. Some people have reported reversal of the symptoms by heating the boards with a blow drier. Some have said that makes it worse.  
 
What happens, is the board becomes conductive and allows stray voltages to travel between components (from eyelet to eyelet) that are not electrically connected. This has all kinds of consequences in the circuit depending upon where the conductivity happens. You can ground one lead of a test meter and poke an original board near eyelets to check for the presence of stray voltages as a test for tweed disease.  
 
The Hoffman layout does not eliminate tweed disease, but rather the use of G10 FR4 Garolite fiberglass circuit board typically used to make a Hoffman board does. G10 FR4 could be hosed down with water and after drying suffer no adverse consequences electrically. Most of the new material available for eyelet cards today also is not prone to tweed disease due to excessive humidity because it is not made of the paper type material of old Fender amps.  
 
The benefit to the Hoffman layout is that the filter caps of the power supply are separated by power and preamp section as you mentioned "..one of the filter caps to the other side of the board." Keeping these sections apart helps quiet the amp regarding power supply noise. The other two differences in Doug's layout are  
1. The addition of 470 ohm 3 watt screen resistors for the two power tubes.  
and  
2. Separate cathode capacitor/resistors for each triode of the dual triode preamp tubes.  
If you take a close look you'll see that instead of the 820 ohm/25uf25v combination tied to both cathodes of the dual triode 12AY7 that there are separate 1.5 kohm/25uf50v for each cathode of that tube.
 
10/26/2004 12:32 PM
bnwitt
For clarification I should have said "allows stray currents to travel" not voltages
 
10/26/2004 12:40 PM
Benboom
"If you take a close look you'll see that instead of the 820 ohm/25uf25v combination tied to both cathodes of the dual triode 12AY7 that there are separate 1.5 kohm/25uf50v for each cathode of that tube."  
 
If you do this you lose one of the defining characteristics of a tweed Deluxe! Namely, the interaction of the volume controls. While that may be nice for people who don't understand it you also lose some of the tones it's possible to get from one of these amps. That's not a Deluxe. I built one of these for my first (working) amp project and I thought it would be a nice thng to do, but when I heard how it sounded I put it back to the stock configuration and was happy again. Comparing it to my real tweed Deluxe before and after the change showed a night and day difference. With the stock cathode setup the amp nails the Deluxe tones; with the split setup it's still good sounding, but it's not a Deluxe IMO.
 
10/26/2004 4:21 PM
bnwitt
Benboom,  
I beg to differ. A shared cathode cap/resistor or individual cap/resistor has nothing to do with the channel interactivity of the deluxe. I have built almost 30 of these amps with Hoffman layout and they all have the channel interactivity of which you speak. That function takes place downstream of the 12AY7 in the tone stack area before the first triode of the 12AX7.  
Barry
 
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