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Post subject: Twin Reverb 1968 - AB763 or not?
Posted: Sun Jan 11, 2015 5:35 am
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Hello all, this is my first post here and I'm hoping that your expertise here can help me understand whether or not my 1968 Twin Reverb carries the Blackface AB763 circuit or the later AC568 circuit.

I've done a lot of reading and trawling forums to try and understand but honestly, I think it's neither of the above. The reason I'm keen to find out is that I'm selling it and I'd like to be able to give any buyers a definitive answer.

The serial number for this amplifier is A11707 which would appear to be an early 1968 code and the chassis exterior is ink stamped with "F 10 46 68" but I can't find an ink stamp inside the chassis. The output transformer is -608-8-18 and choke -606-8-21. The face plate does not have the black lines.

I'm particularly interested in the block of what appear to be white Mallory capacitors in the far left-hand side of the chassis? They're part of the circuit but I don't know what their function is. This part of the circuit doesn't appear in other chassis photos I've seen of AB763 Twins but nor does it appear in the AC568 photos... As far as I'm aware the only modification that has been performed is the tremolo "tick" capacitor.

http://s1297.photobucket.com/user/Timeline88/library/Fender%20Twin%20Reverb%201968

I'd appreciate any help you can offer on this. Thanks in advance!


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Post subject: Re: Twin Reverb 1968 - AB763 or not?
Posted: Sun Jan 11, 2015 8:05 am
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Are you talking about the "mustard" coupling caps on the right hand side of the photo? Located on the circuit board?

-----

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The can cap next to the PT is a 50/50mfd at 450VDC dual cap can. It was added later, after production.


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Post subject: Re: Twin Reverb 1968 - AB763 or not?
Posted: Sun Jan 11, 2015 9:45 am
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Hi, thanks for your reply.

I'm referring to the two small white components in the far left of the chassis photograph. On the left-hand side of the PT in the photograph you posted. Any ideas what that little block of components does? I'm asking because it seems irregular compared to other chassis photographs I've seen of either AB763 or AC568 Twins.

Also, any thoughts on what kind of circuit it is?

Cheers
Joe


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Post subject: Re: Twin Reverb 1968 - AB763 or not?
Posted: Sun Jan 11, 2015 10:12 am
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timeline wrote:
Hello all, this is my first post here and I'm hoping that your expertise here can help me understand whether or not my 1968 Twin Reverb carries the Blackface AB763 circuit or the later AC568 circuit.

I've done a lot of reading and trawling forums to try and understand but honestly, I think it's neither of the above. The reason I'm keen to find out is that I'm selling it and I'd like to be able to give any buyers a definitive answer.

The serial number for this amplifier is A11707 which would appear to be an early 1968 code and the chassis exterior is ink stamped with "F 10 46 68" but I can't find an ink stamp inside the chassis. The output transformer is -608-8-18 and choke -606-8-21. The face plate does not have the black lines.

I'm particularly interested in the block of what appear to be white Mallory capacitors in the far left-hand side of the chassis? They're part of the circuit but I don't know what their function is. This part of the circuit doesn't appear in other chassis photos I've seen of AB763 Twins but nor does it appear in the AC568 photos... As far as I'm aware the only modification that has been performed is the tremolo "tick" capacitor.

http://s1297.photobucket.com/user/Timeline88/library/Fender%20Twin%20Reverb%201968

I'd appreciate any help you can offer on this. Thanks in advance!


Just a shot in the dark, but reasonable speculation nonetheless... I suspect your TR started life with AB763 circuitry.

Seeing as your SN# is A11XXX and the 1968 model year SNs run A10500 to A16500, it's reasonable to assume your Amp was produced in the 1st half of the year.

The AC568 circuit was designed in May/'68 (568), but there is a lag time from design to implementation as old stock component inventory is exhausted and new stock components arrive. The AC568 Amps weren't likely produced until July or August '68 at the earliest making it probable that yours is the AB763 circuitry.

That said, it has definitely been modified over the years, possibly even to the extent that it's now so far away from AB763 as to bear little resemblance to it. We'd have to do a detailed comparison of the current photos to the schematic to determine the extent of this.

Hope this helps.

cheers!

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Post subject: Re: Twin Reverb 1968 - AB763 or not?
Posted: Sun Jan 11, 2015 11:06 am
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Hi Lightnin,

Thank you for your thoughts. I remember the tech who performed the capacitor work for me being slightly puzzled by the circuitry in this, in so far as it didn't completely match up with the circuit diagram. However, we put this down to it having been made in the 'transitional' period.

I'd be interested to know what mods you think may have been performed here? There are no changes to the controls on either the front or back panels that would suggest an adjustable modification. The only thing of not is that the voltage selector on the rear panel appears to have been intentionally broken off.

Also, you mention "we'd have to do a circuit by circuit comparison" - is that an offer or a service provided here on the forum...?

Thanks again!
Joe


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Post subject: Re: Twin Reverb 1968 - AB763 or not?
Posted: Sun Jan 11, 2015 2:37 pm
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Looks like two 5mfd/25VDC bypass caps on top with a 100mfd/6VDC cap on the bottom. I believe all three caps paralleled four 150-ohm, 7 watt sandbox-style cathode-to-ground resistors --- one off each 6L6GC. The resistors have been removed and replaced by a direct line from cathode-to-ground.

Prolly, the amp started out life as an AB568. The infamous "cathode & fixed hybrid" biased Fender amp line. Then, the cathode bias part was bypassed by removing these sandbox resistors and tying the cathode direct to ground = fixed-only bias.

The balance pot of the AB568 was re-wired to a bias pot. All of these mods were done afterwards. By an independent tech.

BTW... this amp has a multi-tap, primary winding. For various wall outlet voltages and Hz. I didn't see any of these unique components, the first time I looked at the photo. :D


AB568 Twin Reverb schematic:

http://bmamps.com/Schematics/fender/twi ... _schem.pdf


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Post subject: Re: Twin Reverb 1968 - AB763 or not?
Posted: Sun Jan 11, 2015 2:54 pm
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Hey BMW,

That's great, thank you! If I've understood you correctly (sorry, I'm operating at the limits of my electrical understanding here), has this amplifier been returned to the AB763 style biasing and effectively "blackfaced" or is it still different? Also, the block of white capacitors are they functioning in the circuit or are they being bypassed?

Thanks for your insight, much appreciated.

Cheers
Joe


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Post subject: Re: Twin Reverb 1968 - AB763 or not?
Posted: Sun Jan 11, 2015 2:58 pm
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Oh, and the voltage selector appears to have been snapped off on purpose on the rear panel. Does this mean it was an export model or did this become standard on Fender amps?

Cheers


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Post subject: Re: Twin Reverb 1968 - AB763 or not?
Posted: Sun Jan 11, 2015 3:24 pm
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I think that the voltage selector knob was accidentally snapped off, sometime in the past. Better check the shaft position and voltage settings, to be sure it is set to the outlet voltage of your area.

Doesn't matter whether the caps are bypassed or not. They are effectively out of the circuit, as soon as the cathodes were grounded by straight wire to the chassis. Path of least resistance and all that jazz. :lol:

The "blackfacing" done, that I can see --- are the bias supply modded back to BIAS pot. And the grid-to-ground resistors on the 6L6GC and plate loading resistors of the 12AT7 phase inverter. See those resistors in the "vee" and "inverted-vee" positions, circled in yellow? Those have been changed from AB568 to AB763 specs.

One resistor, seen just to the right of the yellow circle, has been changed. I'm guessing that this is one of the "tail" resistors of the long-tailed phase inverter. Prolly a 470-ohm replacing the 270-ohm one seen in several SF PI's. Setting this part of the circuit back to BF AB763.


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Post subject: Re: Twin Reverb 1968 - AB763 or not?
Posted: Sun Jan 11, 2015 10:42 pm
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It looks to me like the two white caps and sandbox resistor (on the left) are still connected to the output tubes, making it the AC568. Possibly, early AC568 circuits had the two caps and resistor mounted to the terminals on the left and those components were later just mounted directly to the output tubes. :?:

Edit:
After zooming in on the pic, I can see what appears to be pin 8 of each output tube grounded with red wires. As pointed out, this means that the caps and resistor on the left have been bypassed. Weird. :?

Image

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Post subject: Re: Twin Reverb 1968 - AB763 or not?
Posted: Sun Jan 11, 2015 11:54 pm
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Shimmy, these amps have two 5/25 and one 100/6 bypass caps. Better seen in this Showman AC568 schematic. I believe the top caps are the 5/25. And the one below is a 100/6.

The sandbox 7-watt cathode-tied resistors have been removed from the chassis and replaced with straight wires.

That's my WAG, at least. :lol:



http://bmamps.com/Schematics/fender/sho ... _schem.pdf

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