View Full Version : AA864 Normal vs AB763 Normal
11-25-2011, 07:29 PM
So I was checking out the schematics on the AA864, AB763 and the AA763. I noticed that the "Normal" inputs are fairly identical. The main difference that I see is the lack of a .047uf coupling cap before the 220k mixing resistor. Would not having this cap provide added gain to the stage before it hits the PI? I can see that DC is stopped at the 500pf PI input cap on the AB763 and AA864, .001uf on the AA763 so I know it's not for that. Is there a filter formed between the cap and the mixing resistor?
Anyone with any insight?
11-25-2011, 07:49 PM
While I can't say with 100% certainty, but the cap may add a smidge of gain, but probably not enough to notice. Removing it is most likely equally unremarkable. On the Concert AA763, each channel has a different value cap (.1uf and .047uf) before the mixing resistor, probably to voice the different channels. Good question. The fact that later versions dropped this cap suggests that it was superfluous, although the AA964 vibrolux has them...
I lived at 945 E. Oakmont St. (corner of Oakmont and Waverly) in Orange, CA during the 50's. The houses there were brand spankin' new then. What's it like now?
11-25-2011, 07:57 PM
That is right near Chapman University. While I haven't driven by there in a while, it is likely all students living around there. That is still a sweet area!
And thanks for the insight on that cap! It seemed a little odd to me since the next cap at the input of the PI is much smaller.
11-25-2011, 08:07 PM
It's function I believe is to block DC from the vibrato circuit which the AA864 doesn't have, and cannot be removed for that reason. There would be no change in gain as it doesn't attenuate at guitar freq.
On the AB763 these caps are in series w/ the PI input cap which is probably why Fender used higher value for the PI cap.
11-25-2011, 08:16 PM
That makes sense. Though, wouldn't the PI grid impedance be lower than the output of the vibrato? So the signal would want to go to the PI anyway? But separation from the vibrato circuit does seem to make sense.
11-25-2011, 08:30 PM
Speaking of DC only which is blocked to the PI grids by the .001uf cap.
If you removed the .047 cap, you would then have a path for DC from the plate of the 12AX7 through both 220K mixing resistors to the the 50K vibrato pot. -BAD-
11-25-2011, 08:37 PM
Well... My question is answered! Thank you gentlemen!!
11-25-2011, 09:48 PM
It's function I believe is to block DC from the vibrato circuit which the AA864 doesn't have, and cannot be removed for that reason.
Doh! I went through this with another member not too long ago. My brain is leakier than a cheap sieve! Thanks for the update Guiltless. I used to play little league baseball and football at Yorba Junior High, which was about a block away. Saw and got an autograph from Ted Kennedy in downtown Orange when he came through campaigning for John F. There were still orange groves around us then, too... Happy times...:)
11-25-2011, 10:14 PM
Funny... I remember flying into orange county as a young adult. They still used those roll up stair ways. I felt like a Kennedy... Having seen him exit airplane one so many times as a child. Odd connection and apologies for the digression ; )
11-25-2011, 10:18 PM
No worries! There was farm area and Orange groves when I moved here 20 years ago. Now the Irvine Co. has made everything look the same. The only unique areas are the areas of Orange and Fullerton along with the beach towns. I personally live in San Clemente and wouldn't want to be anywhere else.
11-25-2011, 10:19 PM
And the airport is still small compared to LAX, but really nice and modern.
11-25-2011, 10:26 PM
I also grew up in Orange county, Los Alamitos, Westminister, Cypress JC in the mid 70's
Still somewhat rural then, strawberry feilds, Small world.
11-26-2011, 10:02 AM
Los Alamitos: we'd go straight south to Tin Can Beach [before the 605 was in]. Straight east on Katella for Disneyland prom dates.
Anyone remember the Marina Palace on Hwy 1? Old quonset hut club would have Strawberry Alarm Clock, Seeds, Ike & Tina, Gram Parsons, all our local garage bands.
Check out [and keep alive] the M.P. FB page for more...
[edit; just remembered this is the Amp Technical Info section. Pardon the typical derailing..]
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