Fender 68 Custom Twin Reverb

[nevetslab]

I have a client's Fender 68 Custom Twin Reverb here in the shop, and have always loved the sound of the two channels of this amp. It was here a month ago, but I didn't have to pull it apart to deal with a problem then. This time, I pulled the chassis out, to take advantage of seeing what I could find in the differences they made in the voicing of the two channels' EQ/tone circuits, as they sound VERY different from the Standard Twin Reverb.

One of the differences in the overall wiring is both the Reverb AND the Tremolo circuits are active from both channels. That is achieved by tying the plates of V1B Pin 6 to that of V2B Pin 6 (at the bottom of the plate resistors R11 & R22). The other, which yields Tremolo on Ch 1 is removal of R34 220k & C18 47nF from the board. So, both channels are coupled together and join the Reverb return, feeding the LTPI circuit thru C19 100nF & amp; R35 220k from V4B Pin 6. There's a lot more gain on both channels than on a standard Twin Reverb.

I expected to find different cap values there, but all were the same. I did find a couple extra 220pF caps to ground off of the frames of the Treble pots, but not tied into the EQ circuit. They did replace the jumper W2 that comes from the Wiper/Top of Midrange Pot to the bottom of the Bass Control with an 18k resistor, but only done on the Normal Ch, which on this Custom TR is called CUSTOM while the Vibrato Ch is called VINTAGE. Even the Bright switches are more aggressive, but all the cap values & resistor values are the same. Maybe I've missed something.

The speakers are also different....Celestion G12 8 ohm 12's.

Does anyone actually have the Schematic of the 68 Custom Twin Reverb?

65_Twin_Reverb_SvcMan.pdf

[nevetslab]

Photo images to go along with this:





Also different with the front apperance, besides the silver Escutcheon (front panel), is there's some thin frame around the baffle.

[mikeydee77]

I think this might be the one...

[mikeydee77]

I am not sure I can vouch for this schematic but it is in my library and the blue link wire certainly appears to match the red link on the schematic. I presume it came from this site or maybe electrotanya... ?

[g1]

I think this might be the one...

That's a 65 re-issue schem. with some of the changes added in.

Attached is the 68 Custom service manual.

[g1]

Rob Robinette did a rundown of the differences from the standard blackface to the '68 Custom. It's for the Deluxe, but should be basically the same: https://robrobinette.com/AB763_Modif...Reverb_(68_CDR)

[nevetslab]

Rob Robinette did a rundown of the differences from the standard blackface to the '68 Custom. It's for the Deluxe, but should be basically the same: https://robrobinette.com/AB763_Modif...Reverb_(68_CDR)

I figured there had to be some very cool documentation out there for all the Fender amps! I've never seen this, nor had I seen the Service Manual for the 68 Custom Twin Reverb until now. YUP....those are the changes I see, and seemingly subtle, the sonic difference between that amp and the 65 reissue are dramatic! Thank you so much for adding both documents! Rob's collection of documents will take a lot of time to digest.

I haven't seen that SS Tremolo circuit before, and doesn't appear to be in place in this amp my client has.

[loudthud]

G1 linked the correct schematic. The channels are called CUSTOM and VINTAGE. The Custom channel has a 0.022 cap where a normal blackface would have a 0.047 cap.

I just noticed a Pro Reverb AA1069 has the same tone stack as the Custom channel on both channels ... WTF?

[Pedro Vecino]

And the Super Reverbs too. More low mids and deeper bass.
The 18K resistor is there to limit the minimum adjustment threshold on the bass pot.
In the reverb circuit, the 1M resistor in series with the potentiometer makes lighter adjustments easier but makes the reverb very dark. Better 390K, which added to the 82K resistor make 470K as in the classics.

[Mick Bailey]

I haven't seen that SS Tremolo circuit before, and doesn't appear to be in place in this amp my client has.

Maybe its only fitted to export models. Here in the UK we get the '68 DRRI with a SS trem, but in the USA you get a regular LDR/neon roach. Maybe due to ROHS constraints on the LDR in Europe etc.

[g1]

Maybe its only fitted to export models. Here in the UK we get the '68 DRRI with a SS trem, but in the USA you get a regular LDR/neon roach. Maybe due to ROHS constraints on the LDR in Europe etc.

Yes, they mention this on pg.5, in the 'circuit description' section. All non-European models have the standard LDR/neon.

[nevetslab]

Rob Robinette did a rundown of the differences from the standard blackface to the '68 Custom. It's for the Deluxe, but should be basically the same: https://robrobinette.com/AB763_Modif...Reverb_(68_CDR)

WHAT a GOLD MINE of information and documentation on Fender amp circuits!! And a great Reference book list at the end. Nothing like having all this info and a large collection of Fender Combo Anps to play with to add more variety to what's in our inventory!! I'll have to get permission to do some of these mod's, of course, but this will be an interesting summer with all this info.

Many Thanks!!

[Tom Phillips]

There is a lot of interesting information in this thread.
I was not aware of the circuit configuration that Fender used in the ’68 Custom series of amps. They seem to have come full circle in that they have adopted some of the mods that various techs have been doing since the 1990’s. I first saw the mod to connect the V1b and V2b anodes in Kevin O’Connor’s The Ultimate Tone Vol I (TUT) book published in 1995. He called it “Equal-Weighting of Fender Preamp circuits.” I wonder if there was earlier documentation of this mod and how many others may have independently arrived at the same circuit topology. It’s just an anode mixer circuit. At least Mr. Smith didn’t try to patent it. Anyway, it turned out to be a much simpler mod that the one I was using up until I bought the TUT book.

Rob Robinette sure has added a lot of nice content to his web site since the last time I checked it out.

Cheers,
Tom

[pdf64]

Bruce Zinky designed a bunch of Fender amps in the 90s, the Custom Vibrolux for one included anode mixing of the channels, its schematic is dated Oct 1994.
https://el34world.com/charts/Schemat...lux_manual.pdf
I’m surprised that method wasn’t used more in vintage amp designs, as it saves on parts
Watkins used it for the normal channel input stages on the Dominator etc https://el34world.com/charts/Schemat..._dominator.pdf
Fender used it for the 5C8 Twin but dropped it for the 5D8.
https://el34world.com/charts/Schemat..._5c8_schem.pdf
https://el34world.com/charts/Schemat..._5d8_schem.pdf

I think that the reason the 68 custom amp version sounds brighter and louder may be due to the lower degree of nfb on the power amp (820 & 47, compared to 820 & 100).

[Leo_Gnardo]

Bruce Zinky designed a bunch of Fender amps in the 90s, the Custom Vibrolux for one included anode mixing of the channels, its schematic is dated Oct 1994.

I'm surprised that method wasn't used more in vintage amp designs, as it saves on parts

Could it be . . . that it makes sound quality SUCK?

I ran across some modded amps mid 80's where this cheap & dirty technique was used - owners complained they paid exorbitant $$$ to a local amp gooroo. *

Addition of mix resistors, say 220K, fixed the problem quick & cheap.

Then Fender started in on making amps with this "feature" built-in. How clever! They must have saved millions in profits by leaving out a pair of cheap resistors. Guaranteed income for me, but honestly I'd rather they built their amps correctly.

"SO what's your beef LG???" Simple enough - without mix resistors, the plate of one tube, besides driving the grid of the following stage, is trying to drive the plate of the other tube to which it is attached. Not enough drive current available to accomplish successfully. NOT a good design. Cruddy sounding distortion instead of the kleen-teen Fender sound we know and love. Those who don't notice the distortion, bless you for not complaining & bringing your amps in, I have plenty to do without them. Live it and live with it, that's what I always say.

* Said gooroo left a trail of tears, doing plenty other half assed mods for which he charged typically $1000 - 2000. Said gooroo actually told me face to face "I have no idea what I'm doing. If it doesn't smoke or cause a shock then it must be OK." His parts supply was made of resistors & caps yanked out of old TV's he found at the dump. What a character...