Fender Circuit Changes, AB763?
Len Moskowitz
I'm repairing a late-60's or early-70's silverface Fender Twin Reverb.
I note that the circuit differs from the AB763 schematic by having 150
Ohm resistors between the 6L6GC cathodes and ground while the schematic
has the cathodes connected directly to ground. There's also three
5uF/25VDC capacitors that interconnect the cathodes, and two resistors
near the pilot light that go from the 6.3V heater leads to ground.
Does anyone know what the purposes of these circuit changes were?
Should I delete them to return this to a blackface configuration?
Thanks to all!
--
Len Moskowitz
Core Sound
mosk...@panix.com
Brant Montgomery Vogel
: I'm repairing a late-60's or early-70's silverface Fender Twin Reverb.
: Thanks to all!
What you have there might be the AC568 Twin. Do you have the tubing
chart inside the cabinet? I got a bunch of schematics from Fender for my
twin, which is a 69 AB763. The differences you describe are on the AC568
schematic. It looks to me that this is an older design, perhaps closer
to blackface (ask someone who knows the history better than I). The amp
is a 70-80 watt configuration. Thus it might be a good thing, if you are
living in an apartment building. If you call Fender at their consumer
number in Scottsdale AZ, they will sned you all the possible scematics,
in nth generation photocopy, for $5.00 american. They might even have a
wiring chart for it as well, as they did for the AB763. In any event, as
well as I can dope it out, the AC568 is from 1968 or before. The 763 is
c.1969. I'd send you a copy from mine, but it might be too hard to
read--I wouldn't want to give you something that misinformed you,
considering the voltage. Good luck.
--
Brant Vogel I represent neither my department
ILA in exile nor the University. I find
[somewhere in Brooklyn] representation problematic.
Charlie Metcalf
Two years ago when I had it serviced I had it conformed to the blackface
specifications. Tone did improve. IMO it's money well spent.
Charlie
Len Moskowitz
Brant Vogel <bvo...@unix.cc.emory.edu> wrote:
>What you have there might be the AC568 Twin. Do you have the tubing
>chart inside the cabinet?
The tube chart says it's an AB763 but it doesn't match the AB763
schematic I have per my previous post.
>I got a bunch of schematics from Fender for my
>twin, which is a 69 AB763. The differences you describe are on the AC568
>schematic. It looks to me that this is an older design, perhaps closer
>to blackface (ask someone who knows the history better than I).
>...In any event, as
>well as I can dope it out, the AC568 is from 1968 or before. The 763 is
>c.1969.
Might anyone know for sure when the AC568 circuit was produced and if
this is what I have?
>...Good luck.
Thanks very much for your help!
Len Moskowitz
Thanks for the benefit of your experience. Any idea what they did?
Steve Cowell
Brant Montgomery Vogel <bvo...@unix.cc.emory.edu> wrote:
>Len Moskowitz (mosk...@panix.com) wrote:
>
>: I'm repairing a late-60's or early-70's silverface Fender Twin Reverb.
>: I note that the circuit differs from the AB763 schematic by having 150
>: Ohm resistors between the 6L6GC cathodes and ground while the schematic
>: has the cathodes connected directly to ground. There's also three
>: 5uF/25VDC capacitors that interconnect the cathodes, and two resistors
>: near the pilot light that go from the 6.3V heater leads to ground.
>
>: Does anyone know what the purposes of these circuit changes were?
What you have is a transition era Twin. Those circuit changes were
put there to add stability (cathode bias adds negative feedback) as
well as to protect the transformer from shorted tubes (the cathode
resistors limit current in case of a plate-cathode short).
>: Should I delete them to return this to a blackface configuration?
Absolutely. Rip out the terminal strip with the caps on it and ground
the cathodes to the chassis. You'll need to up the bias (more negative
volts) as the amp depended on the cathode resistors for some of the
bias. Might as well add the bias adjust circuit.... it's got the
balance ciruit now. You know about the phase inverter stuff.
I have one of these.... it has the blue jewel, doesn't it? Mine has
been hacked beyond recognition... in original configuration, it was
the coldest sounding Twin ever.
--
Steve........scowell@aoc.nrao.edu................
Robert Fries
The dreaded 'CBS mods'. Tube Amp book Vol 3 shows this circuit,
'AC 568'. The stuff on the power tubes is what Weber refers to
as the combination of cathode- and fixed-bias, which he says
sounds horrible.
The resistors on the heater circuit, hmmm..., I don't know
why they're there.
I'd remove this stuff, and convert it to Blackface specs. If you
haven't already got them, get the Tube Amp Book, and the Weber
book.
Robert
Len Moskowitz
>>: Does anyone know what the purposes of these circuit changes were?
>
>What you have is a transition era Twin. Those circuit changes were
>put there to add stability (cathode bias adds negative feedback) as
>well as to protect the transformer from shorted tubes (the cathode
>resistors limit current in case of a plate-cathode short).
I suppose they succeeded but in the process ruined the sound.
Oddly enough, the tube placement diagram says that the amp is an AB763
despite the revised circuit in the chassis.
>>: Should I delete them to return this to a blackface configuration?
>
>Absolutely. Rip out the terminal strip with the caps on it and ground
>the cathodes to the chassis. You'll need to up the bias (more negative
>volts) as the amp depended on the cathode resistors for some of the
>bias. Might as well add the bias adjust circuit.... it's got the
>balance ciruit now.
Ah, so that's what that 10k pot with three resistors coming off it is! (I
don't have the AC568 schematics.) I should be able to convert it to the
bias adjust circuit pretty easily.
> ... You know about the phase inverter stuff.
Do you mean the change in components around the 12AT7 that another
poster mentioned?
Do the resistors to ground in the heater circuit matter?
>I have one of these.... it has the blue jewel, doesn't it?
I think that mine has a red one.
>Mine has
>been hacked beyond recognition... in original configuration, it was
>the coldest sounding Twin ever.
Mine sounds awful. Of course one 12AT7 was gassy, the other had poor
transconductance, and the baffle board was rattling. Let's see if I can
get it in shape.
Dr.Distortion
: wiring chart for it as well, as they did for the AB763. In any event, as
: c.1969. I'd send you a copy from mine, but it might be too hard to
That is incorrect. The AB763 came BEFORE the various silverface mutations.
The AB763 models were produced from about '64 through '67; and except
for some transition models, were equipped with the "blackface" control panels.
--Dr.Distortion
Dr.Distortion
: The resistors on the heater circuit, hmmm..., I don't know
: why they're there.
: I'd remove this stuff, and convert it to Blackface specs. If you
: haven't already got them, get the Tube Amp Book, and the Weber
: book.
It's a pair of 100-ohm resistors strung from the filament supply
(at the pilot light) to ground, right? Don't remove them. This
arrangement provides the necessary grounded-centertap for the
heater supply, and is more accurate than using the transformer
centertap. If you remove these resistors, chances are the amp
will hum like a son of a bitch.
--Dr.Distortion
Robert Fries
Yep, I see that now. Anyway, I meant to write 'remove the stuff besides
the resistors on the filament supply', as I'm sure changes made on the
filament supply were changes for the better, and not subject to the
vagueness of 'does this improve the sound?'
Robert
Brant Montgomery Vogel
Your not the only one to correct me on this. Thus I see the best way to
get information is to say something dumb on newsgroup. My amp has a
penciled mark inside that dates it to 1969. The face is consitent with
68-69. The aluminum strip on the grill is consistent with 68. And the
tubing chart claims the amp to be an AB763. I should double check my
wiring against the schematics. Anyhow, I guess I must have one of those
transitional amps. Lucky me. But one question: why do some of the other
70-80 watt plans look older? Duh! maybe the Fender guys updated their
paper work on older plans. So, a general question: does anybody know the
history of those other mid-late 60s designs? Is the Black Silverface an
oddity? Thanks in advance for info.
Kobe's local login
I have a Super Reverb that is silverfaced, has the alum. around the grille cloth
and an AB-763 designation on the tube chart. The guts also check with the AB-763
schematic I got from Gerald Weber's book. Incidentally I tried some of the
Weber mods and they did make a difference especially in the bottom end. Check
the book for details. All are reversible so it shouldn't de detrimental to try
them.
Kobe
--
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Karl Oberlander ko...@vitruvius.ar.utexas.edu
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