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#1
12-02-2009, 10:24 AM
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Which Fender Twin circuit is this???
I'm hoping some of the technical amp guru's here can help me out. I bought this '69 Fender Twin and was trying to figure out which circuit it was. I think it's just past the AB763 circuit as far as the timeline goes, but perhaps it's still the AB763, or a slight variation. Can anyone tell me?? I'm not very schooled in what to look for. Here are the gut shots. I tried to take a close up of the right and left side and one of the whole thing. Thanks in advance!!
  
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#2
12-02-2009, 11:33 AM
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AB568...AC568....one of the '68 designs. The "partial cathode bias" scheme is the dead giveaway
I have drawers full of those 150 ohm resistors..... http://schematicheaven.com/fenderamp...c568_schem.pdf
__________________
The bus came by and I got on, that's when it all began... ++++ As per TGP Rule #8: I wind pickups for profit.
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#3
12-02-2009, 12:09 PM
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The only thing I know about "bias" has to do with tubes. Is the "partial cathode bias" something you see on the tube sockets? Sorry if this is a dumb question, just trying to learn what is what here.
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#4
12-02-2009, 12:25 PM
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See those tan rectangles on the power tubes? Cathode resistors.
This amp uses a (IMO flawed) method of "combination fixed & cathode biasing".
__________________
The bus came by and I got on, that's when it all began... ++++ As per TGP Rule #8: I wind pickups for profit.
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#5
12-02-2009, 12:29 PM
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...the "partial cathode bias" refers in part to the four, square, brown ceramic (cathode) resistors shown in your first picture...they're part of the infamous CBS-era "quasi-cathode bias" improvements made in 1968 (ie: AB568, AC568, etc.)...which nobody liked.
...one component of those '68 circuit changes, the "bias balance" adjustment, was actually a very GOOD idea...it provided a "rocking" bias adjustment which allowed you to lower bias to one tube or the other from a common "null" bias voltage level...essentially, lowering a "hot" tube down idle-wise to the level of the lower tube.
__________________
GTRS - 1968 Fender Telecaster || 2002 Gibson ES-135LE || 1986 Ovation Balladeer (1117-1) || 2004 Ovation Viper (EA68-5). AMPS - 1969 Fender Super Reverb-Amp with JBL/D110F's || 2004 Line-6 Spider-I || DIY VibroClone with JBL/D130F.
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#8
12-02-2009, 01:59 PM
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To my ears it sounds "choked". I prefer the earlier designs...AB763, etc.
If you like the 568 design, who am I to argue?
__________________
The bus came by and I got on, that's when it all began... ++++ As per TGP Rule #8: I wind pickups for profit.
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#9
12-02-2009, 02:12 PM
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Thanks for the info guys! I now understand what you mean. I'm not sure I'm going to make any changes, I do like the way it sounds now - HOWEVER - since I have not yet heard a true AB763 circuit to compare to, how do I know which I like better?
 What other changes would need to be made to get closer to the AB763 circuit? And, what would be used in place of the partial cathode biasing? Again, probably not a simple question, but it's worth a shot asking. Thanks!
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#10
12-02-2009, 02:13 PM
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Quote:
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