Grateful Dead Music Forum

Great amp, great price. Nice work.

Ebearius wrote:

Searing75 wrote:tone stack Change to .022's.

How can I visually confirm that this has not already been done? I'm wondering if this is the "tweak" that Jeff Bakos had performed.

You'll need to open her up to see the tone stack.

Searing75 wrote:

Ebearius wrote:

Searing75 wrote:tone stack Change to .022's.

How can I visually confirm that this has not already been done? I'm wondering if this is the "tweak" that Jeff Bakos had performed.

You'll need to open her up to see the tone stack.

Do I need to unplug it to do that?

Seriously, I looked up "Twin Tone Stack" online and saw a bunch of schematics. Is there an easily identifiable part that I can look for to confirm that this mod has been done? If it has not, how much benefit does it provide? I appreciate your help. Thanks.

During those years, they put the ab763 tube chart sticker on many silverface amps that were not ab763 because that's all they had and the tubes would be the same anyway.

The easiest way to tell if it's ab763 is to compare your bias circuit with that of the ab763 layout diagram.

hippieguy1954 wrote:During those years, they put the ab763 tube chart sticker on many silverface amps that were not ab763 because that's all they had and the tubes would be the same anyway.

The easiest way to tell if it's ab763 is to compare your bias circuit with that of the ab763 layout diagram.

Yes. Agreed. I was going by the serial number, not the sticker. However, I was going to open it up for a visual confirmation this weekend. Assuming everything else checks out, how do I identify a .22 tone stack?

Ebearius wrote:

hippieguy1954 wrote:During those years, they put the ab763 tube chart sticker on many silverface amps that were not ab763 because that's all they had and the tubes would be the same anyway.

The easiest way to tell if it's ab763 is to compare your bias circuit with that of the ab763 layout diagram.

Yes. Agreed. I was going by the serial number, not the sticker. However, I was going to open it up for a visual confirmation this weekend. Assuming everything else checks out, how do I identify a .22 tone stack?

I'm not totally sure what he's referring to as far as "tone stack change to .022".

This article is very good and explains a lot. Good photos towards the bottom too: https://seriouslygoofy.wordpress.com/20 ... onversion/

hippieguy1954 wrote:I'm not totally sure what he's referring to as far as "tone stack change to .022".

This article is very good and explains a lot. Good photos towards the bottom too: https://seriouslygoofy.wordpress.com/20 ... onversion/

He's talking about swapping out the .1uF and .047uF capacitors that govern tone for ones of .022uF value. Sometimes only applied to reverb channel. Good photos, but should be flipped so pots are at the top.

It should be noted that 1968 is a turbulent year for Twin Reverbs and other high powered Fenders...The complaints were rolling in that the amps were "too loud"...or "too clean"...or "one dimensional". So the new floor engineers brought in by CBS decided to shake things up a bit. 1968 bears the brunt of these changes...

Early 68's remain virtually unscathed. January to March are virtually the best of the "blackface" design with no circuit variations. In April you start to see weird and unique transitional designs like Power Transformers with low power secondaries. Shortly after that the hybrid cathode bias design rears its ugly little head. By July of 1968 Twin Reverbs are distinctly a pale comparison of what they were just a few months earlier. In late summer of 1968 the spaghetti wiring starts to show up(which 50 years later is proving to be a big mistake as these amps arc over once the plastic coating breaks down)...

I have worked on hundreds of Twins, maybe a thousand at this point... I find 68' Twins to be an unusual animal with its own set of burps and farts. I used a hybrid cathode biased 68' Twin here in the shop for many moons as a bench amp for guitar/bass repair. One of my customers eventually bought it and thought it sounded absolutely fantastic! Its all totally subjective, right?

I can guarantee that Garcia at some point in 1968 had to use an off the shelf Twin Reverb that had yet to be "Alembicized"....or "Owsley-ized", etc... and I bet he made it sing. That's the thing about a Twin Reverb in general...it does a good job of reproducing what the rest of your signal chain is doing without any, or limited coloring of the tone. Including what your fingers can do!

I have a '74 Twin. Master volume. Definitely not desirable. But it kills in a band setting. K120 in it and a jensen. NOS preamp tubes.
It has casters so not too bad. I'm used to heavy amps anyway.
It does the Jerry thing very well, IMO.....

I would love to come across a '68 with D120fs. I have been looking for one. I like the drip edge amps. I have a '68 Deluxe Reverb.

Brandon Bowlds wrote:I have a '74 Twin. Master volume. Definitely not desirable. But it kills in a band setting. K120 in it and a jensen. NOS preamp tubes.
It has casters so not too bad. I'm used to heavy amps anyway.
It does the Jerry thing very well, IMO.....

I would love to come across a '68 with D120fs. I have been looking for one. I like the drip edge amps. I have a '68 Deluxe Reverb.

I have a '74 as well as the '68. The '68 has a bit more "grit" at lower volume, as expected. I also notice a fairly dramatic advantage in the '68, in that the high strings seem better balanced relative to the mid/low strings at moderate volume. The '74 has a tendency to sound weak on the trebles unless you crank it a bit more than comfortable home levels. I think this may be what is referred to as "tone bleed"? Correct? Or, it could just be a characteristic of the JBL's.