Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: 22uf on February 15, 2022, 04:34:55 pm

Title: Does anyone have an AC568 Twin schematic/layout? That is actually legible? :)
Post by: 22uf on February 15, 2022, 04:34:55 pm

Just picked up another knackered old 73 SFTR and I'm totally bored of AB763. This time I'm trying to nail that Sticky Fingers tone instead.

I figured I'd go for a SG as above in to an AC568 circuit.

I even still have those eminence legend 1258 speakers in a box ... !  :laugh:

Does anyone have a schematic/layout that is legible? I only found these (attached) and my old eyes can't read the squiggles like they used too.

Title: Re: Does anyone have an AC568 Twin schematic/layout? That is actually legible? :)
Post by: sluckey on February 15, 2022, 05:12:18 pm

Here's another. Your look better.

     https://el34world.com/charts/Schematics/files/Fender/Fender_showman_ac568_schem.pdf

Title: Re: Does anyone have an AC568 Twin schematic/layout? That is actually legible? :)
Post by: 66Strat on February 15, 2022, 06:24:35 pm

I thought that the Stones used Ampegs for the Stickey Fingers album.

Title: Re: Does anyone have an AC568 Twin schematic/layout? That is actually legible? :)
Post by: PRR on February 15, 2022, 07:07:57 pm

> old eyes can't read

I bet it is 99% the same as other Fenders of the period. The six main rectifier diodes nails the period within a few years. What part do you think is different?

Title: Re: Does anyone have an AC568 Twin schematic/layout? That is actually legible? :)
Post by: 22uf on February 16, 2022, 02:06:48 am

Quote from: sluckey on February 15, 2022, 05:12:18 pm

Here's another. Your look better.

     https://el34world.com/charts/Schematics/files/Fender/Fender_showman_ac568_schem.pdf

Maybe 20 years ago. Now that's just a blur.  :icon_biggrin:

Title: Re: Does anyone have an AC568 Twin schematic/layout? That is actually legible? :)
Post by: 22uf on February 16, 2022, 02:17:34 am

Quote from: PRR on February 15, 2022, 07:07:57 pm

> old eyes can't read

I bet it is 99% the same as other Fenders of the period. The six main rectifier diodes nails the period within a few years. What part do you think is different?

I don't know exactly, that's why I want a legible schematic.  :icon_biggrin:

I figure there'll be the snubber caps on the power tubes and an extra bias filter cap, different dropping resistor values and the usual lower SF PI values..

I also read they briefly used some kind of fixed bias arrangement but I have no interest in that naturally (other than to know what they biased it at) hehe.

Title: Re: Does anyone have an AC568 Twin schematic/layout? That is actually legible? :)
Post by: 22uf on February 16, 2022, 02:51:24 am

Here we go ..

That's quite different to AA270. Looks like the tag board is some kind of fixed/cathode mix!?

Also those huge 7w resistors .. :dontknow:

Title: Re: Does anyone have an AC568 Twin schematic/layout? That is actually legible? :)
Post by: Latole on February 16, 2022, 03:22:29 am

Look at this one ;

https://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-Twin-Reverb-AC568-Schematic.pdf


 

Title: Re: Does anyone have an AC568 Twin schematic/layout? That is actually legible? :)
Post by: 22uf on February 16, 2022, 03:29:19 am

 :l2:

Title: Re: Does anyone have an AC568 Twin schematic/layout? That is actually legible? :)
Post by: Bieworm on February 16, 2022, 05:29:45 am

Quote from: 22uf on February 16, 2022, 03:29:19 am

:l2:

this reminds me of the first digital camera images.  :laugh:

Title: Re: Does anyone have an AC568 Twin schematic/layout? That is actually legible? :)
Post by: sluckey on February 16, 2022, 06:47:30 am

Here's the best I have. I can read everything on this copy. If you still can't see it you may need some more powerful reading peepers...  :icon_biggrin:

Title: Re: Does anyone have an AC568 Twin schematic/layout? That is actually legible? :)
Post by: pdf64 on February 16, 2022, 07:30:52 am

Wow, that’s better!
How to get that to replace the one in the schematic library?

Note the non polarised ecaps on the cathodes.

It may be worth bearing in mind that ‘mixed bias’ era was pretty short lived, new fixed bias designs came along in ‘69.

Title: Re: Does anyone have an AC568 Twin schematic/layout? That is actually legible? :)
Post by: sluckey on February 16, 2022, 08:08:50 am

It's already in the library...

     https://el34world.com/charts/Schematics/files/Fender/Fender_twin_reverb_ab568.pdf

Note... This is an AB568 rather than AC568. Probably not a nickel difference.

Title: Re: Does anyone have an AC568 Twin schematic/layout? That is actually legible? :)
Post by: Latole on February 16, 2022, 09:17:56 am

Quote from: sluckey on February 16, 2022, 06:47:30 am

Here's the best I have. I can read everything on this copy. If you still can't see it you may need some more powerful reading peepers...  :icon_biggrin:

That is the best one !

Title: Re: Does anyone have an AC568 Twin schematic/layout? That is actually legible? :)
Post by: SILVERGUN on February 16, 2022, 11:39:48 am

I remember a very respected member saying that the '72 Twin was one of his favorites.
He posted some here and more on other threads: (search : 2deaf   - Twin) he has had a lot to say about these amps
'70's Fender Twin Reverb Trilogy of Schematics (el34world.com) (https://el34world.com/Forum/index.php?topic=24638.msg265672)

Title: Re: Does anyone have an AC568 Twin schematic/layout? That is actually legible? :)
Post by: 22uf on February 16, 2022, 12:29:17 pm

Cheers sluckey! o7o7o7o

Quote from: SILVERGUN on February 16, 2022, 11:39:48 am

I remember a very respected member saying that the '72 Twin was one of his favorites.
He posted some here and more on other threads: (search : 2deaf   - Twin) he has had a lot to say about these amps
'70's Fender Twin Reverb Trilogy of Schematics (el34world.com) (https://el34world.com/Forum/index.php?topic=24638.msg265672)

Yeah I dig them all tbh. Sadly the best one I ever had was the "Ultralinear" 135w. It was absolutely SUBLIME tone.

This is that very amp:

I can't speak highly enough of those Eminence Legend 1258's. They are absolutely tight as nuns. Every single frequency gets projected and the bass hits you in the chest like a teenage DUI.

I sold it for $700  :sad2: :sad2: :sad2: :sad2: :sad2:

I asked the guy recently if I could buy it back and he said no. I even offered him $1400. He said NOT A CHANCE is he ever selling that amp.

RAGAFRAGSHAHAHAGRRR.  :BangHead:

Title: Re: Does anyone have an AC568 Twin schematic/layout? That is actually legible? :)
Post by: tubeswell on February 16, 2022, 02:09:14 pm

Quote from: 22uf on February 16, 2022, 02:51:24 am

That's quite different to AA270. Looks like the tag board is some kind of fixed/cathode mix!?

Also those huge 7w resistors .. :dontknow:

Yep hybrid bias, and the 7W rating is overkill - probably had a bunch of 150R 7W lying around.