# fender_vibrochamp_ab764.pdf



## keeperofthegood (Apr 30, 2008)

Does anyone have a copy that opens of this one? Or know of a place to get it in a non-pdf format? There are only 2 'free' places online and both of them their copies are corrupted 

Mostly I want to know what the differences are specifically between

Fender Vibrochamp aa764

and

Fender Vibrochamp ab764


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## jimihendrix (Jun 27, 2009)

schematics...???...i'm not sure why the ab764 shows up as a bronco...but there are schematics here...

http://thetubestore.com/latefenschm.html

http://www.thevintagesound.com/ffg/schem/champ_vibro_aa764_schem.gif










http://lib.store.yahoo.net/lib/thetubestore/broncoab764.gif


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## keeperofthegood (Apr 30, 2008)

jimihendrix said:


> schematics...???...i'm not sure why the ab764 shows up as a bronco...but there are schematics here...
> 
> http://thetubestore.com/latefenschm.html
> 
> ...



THANKS!! 

Google for ab764 had brought up lots of the aa764 schematics instead :| and no idea about naming, from the schematics I have looked at so far, the lines that had both a and b names, the b's (_I cant recall if it was on TDPRI or TGP that a person broke down the upgrades between an aa and ab schematic for one of the bigger amps_) seem to be minor "improvements" over an original releases so, not sure on their names either. I would have to hunt and read through some of the fan sites bloggings on the 'history of the Fender amps'.


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## Wild Bill (May 3, 2006)

keeperofthegood said:


> Does anyone have a copy that opens of this one? Or know of a place to get it in a non-pdf format? There are only 2 'free' places online and both of them their copies are corrupted
> 
> Mostly I want to know what the differences are specifically between
> 
> ...


Merry Christmas, Keeps!:smile:

The difference is with the power transformer. Notice that the second schematic shows a higher plate voltage on the 6V6. Also, the power transformer is an export model with a dual primary. This gives the option of wiring the primaries in series/phase to run on 240 volts.

That higher B+ will increase the headroom a bit, so it will sound different but not hugely so...

:food-smiley-004:


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## keeperofthegood (Apr 30, 2008)

Wild Bill said:


> Merry Christmas, Keeps!:smile:
> 
> The difference is with the power transformer. Notice that the second schematic shows a higher plate voltage on the 6V6. Also, the power transformer is an export model with a dual primary. This gives the option of wiring the primaries in series/phase to run on 240 volts.
> 
> ...



Merry Christmas Wild Bill :wave:

Thank you too. 

Interesting the company name changes as I just also noticed the AA is company headlined "Fender" Musical Instruments , and the AB is "CBS" Musical Instruments as well. Maybe there was a corporate shift overall in the marketing at the time, opening up new venues for sales. I see they went from 0.02uf caps to 0.022uf which is probably a supplier deal and 20% tolerances anyways makes no big difference. The plate voltage on the 6V6GTA is the same in both, beefier OT I gather? The plate/cathode voltages though on V2 are higher on the AB with the difference between being 30volts more. Would that mean more power on the vibrato or more range (less chance of clipping)?

Hmm, and I am wondering, what is the deal with that line side cap to ground? The 0.047/400v cap? It's listed as 'not used for export', but why was it used in domestic at all?


XD Fun stuff!!


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## Wild Bill (May 3, 2006)

keeperofthegood said:


> Merry Christmas Wild Bill :wave:
> 
> Thank you too.
> 
> ...


I didn't notice the greater voltage drop across the OT. You're right, there must be a transformer difference. The voltage drop comes from the resistance of the wire in the winding. A higher resistance implies more turns and/or smaller gauge wire. Not necessarily beefier, although the CBS engineers may have felt the need for a bigger transformer to be more "hifi". Or more rugged. When you run Class A the current through the OT is always at a maximum. There's no point in the cycle where the OT can cool down. That's why Class A rated OT's are always so much heavier.

I've never played both amps but there MUST be a tone difference from the OT's! Which is better would be personal taste, of course.

The higher voltage in the vibrato circuit wouldn't do much, I expect. There's no power generated, just a wave that adds to or fights the cathode bias voltage of the triode driving the 6V6. Might get a stronger intensity but not that you'd notice when there's an intensity pot anyway.

That .047 is one of those infamous "death caps". The amp came out in the days of 2-wire cords. The cap bypassed one side of the line to ground. The plug could be in only one of two ways. If there was a lot of line hum you could reverse the plug and the cap would then be bypassing the "hot" wire instead of the neutral, hopefully reducing the hum. In a more expensive amp you got a "ground reverse" switch so you didn't have to flip the plug around yourself.

As you probably know, it's called a death cap 'cuz if it shorted out that side of the line would be connected to the amp chassis. You had a 50/50 chance that the plug would be inserted so that it was the hot wire that was connected. This meant that if you were playing your guitar and happened to touch something grounded at the same time you got the full line voltage as a shock. That's normally not nearly enough to kill you, especially since the voltage would usually make you jerk away and break the connection but it made for a great plot device in old murder mystery shows, where a guitarist in some lame ass hollywood idea of a rock band would plug in to his amp and step on his wah pedal with no shoes or socks on. He would then light up like the Electro Monster from the Outer Limits and fry like someone in the chair on Death Row!

Shorted death caps are actually rather rare. Notice that the voltage rating of the cap is normally far higher than the line voltage. Still, it happened occasionally. Even though serious harm was extremely rare it wasn't the sort of thing that made it easy for marketing men to get repeat customers for their products. It gave them a vested interest in not using cheapo death caps of low ratings. Besides, there was almost certainly some electrical code forcing them to use an over-rated cap.

How's Christmas morning going? My daughters are now 13
and 19. They didn't get up until after 10:00! What a difference from when they were small...:smile:


:food-smiley-004:


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## keeperofthegood (Apr 30, 2008)

Wild Bill said:


> I didn't notice the greater voltage drop across the OT. You're right, there must be a transformer difference. The voltage drop comes from the resistance of the wire in the winding. A higher resistance implies more turns and/or smaller gauge wire. Not necessarily beefier, although the CBS engineers may have felt the need for a bigger transformer to be more "hifi". Or more rugged. When you run Class A the current through the OT is always at a maximum. There's no point in the cycle where the OT can cool down. That's why Class A rated OT's are always so much heavier.
> 
> I've never played both amps but there MUST be a tone difference from the OT's! Which is better would be personal taste, of course.
> 
> ...



Thanks Wild Bill

The day is great!! The kids were up at 7AM!! amazing as it sounds!! I mean, here we are half way through the school year and every day it is like pulling teeth to get them up by 8AM to be at school by 8:40 and today NO PRODDING or anything they are up and bouncing like jumping beans!! Hahaha mine are 8 and 11, but they do grow up fast though eh  and gosh, 13 is SO VERY grown up LOL isn't it and 18 is just plain OLD to my kids XD

At least they let me have my morning coffee before really getting into things. We just had our traditional "breakfast" now (mom and dad SO need to lay down for an hour or two LOL) of Eggs Benedict. Used cooked ham this year instead of bacon or peameal bacon., and I have to say it wasn't bad, even my 8 year old daughter ate most of hers up; she usually eats half and is full, this year, she ate 2!!

Right now they are enjoying Madagascar 2 for the first time, a new movie on a new dvd player (the last one died 3 months back). I have some nice 5 year old cheddar and some nice blue cheese to enjoy with tonights nibble tray dinner and a couple other new movies to watch then too. Maybe I will even be able to have one of the cornbreads LOL we were too full to have them last night as desert, cranberry, chili, and plain (the cranberry is stuffed with cranberry syrup under it):












The rain finally coming means the pressure in my head has also released some. The rains over here on the Burlington side are not nearly as heavy as I had expected, maybe tonight we shall see if they increase or not.


Ahh so, that 0.047 was a per-cursor to the modern line-filter modules that is interesting to know. LOL I forget now, I think it was Quincy that had a singer light up off her mic. I miss that show, when medical mysteries were simple mysteries.

I was reading up on what a 'fatal' current is, and the consensus seems to be 100ma is a sure death (down to 1 or 2 or 3ma, usually across the heart). I am not 100% on this, but this is what my maths tell me. I know from grabbing my meter that (with dry skin) I tend to be 100K to 150K ohms (closer to 10K with wet) , and ohms law E = IR... 0.1 * 100000 would imply 10,000 volts would be needed to have that current in me (1,000 if wet). Which is, needless to say, a LOT more than 120, so I am missing some information in that I am sure.

Haha thanks Wild Bill!! I should put up that photo on the other thread too!!


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