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Post subject: '66 Bassman AA165 or AB165
Posted: Thu Dec 04, 2014 8:30 pm
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I purchased a '66 Bassman head and cab a couple years ago. The tube diagram says it's the AA165 circuit but some scratched out the AA and wrote AB with a pencil. I opened up the amp to see if I could determine which one it really is. Well, someone's been in there before me as all the caps have been replaced and there are some new looking resistors. In comparing to layout and schematic diagrams, I see unique elements of both circuits - ie the 10k resistors connected to bias pot of the AB circuit (look original) and the RC network around the 12AT7 from the AA circuit (looks like some new resistors added or replaced). Is that a common mod or a mistake?


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Post subject: Re: '66 Bassman AA165 or AB165
Posted: Fri Dec 05, 2014 4:31 am
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Sometimes the tubes chart with amp model is the wrong one . I see that often


I just have a look for 1 minute to both circuits and I see;

Both models do not use same bias circuits , more than one resistor.
And Phase Inverter circuits 12AT7 are not same ,

Some coupling caps values are not same too


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Post subject: Re: '66 Bassman AA165 or AB165
Posted: Fri Dec 05, 2014 11:37 am
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dscholz,
It sounds like someone tried to mod an AA165 to a AB165.
Try looking up the chassis serial number at the site below.
That may answer your question.

Bob

http://www.hendrixguitars.com/FendAmpDate.htm


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Post subject: Re: '66 Bassman AA165 or AB165
Posted: Sat Jan 03, 2015 12:00 pm
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Fender tube charts are notoriously un-reliable. My un-molested '66 AB165 has an AA864 tube chart with one of the "A"s scratched out and a hand-written "B" in it's place. I bought the amp new in 1966 so I know it was done at the factory. Also, there never was an AB864 circuit. And I've seen several AB165's with AA165 tube charts. Fender used what they had and didn't waste anything-even tube charts.


The best way to tell is by the date code stamped on the chart, the serial#, and transformer and pot codes. http://www.superiormusic.com/page195.htm
http://www.ggjaguar.com/biblio.htm


Last edited by t-luxe on Sat Jan 03, 2015 3:08 pm, edited 1 time in total.

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Post subject: Re: '66 Bassman AA165 or AB165
Posted: Sat Jan 03, 2015 12:33 pm
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t-luxe wrote:
[

Bob

Fender tube charts are notoriously un-reliable.


]


+1

I see often error


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Post subject: Re: '66 Bassman AA165 or AB165
Posted: Sat Jan 03, 2015 3:55 pm
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If it helps any, I have a 66 Bassman and it is an AB165...Tube chart on mine is actually correct, but mine was made in last week of Aug 66. Depending on when yours was made, this may help you determine for sure what you got, regardless of what the tube chart says (as stated prior, they can be quite unreliable). So at the very least, late August of 66 the switch to the AB165 was on.

HTH

T2

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Post subject: Re: '66 Bassman AA165 or AB165
Posted: Sat Jan 03, 2015 7:18 pm
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dscholz wrote:
I purchased a '66 Bassman head and cab a couple years ago. The tube diagram says it's the AA165 circuit but some scratched out the AA and wrote AB with a pencil. I opened up the amp to see if I could determine which one it really is. Well, someone's been in there before me as all the caps have been replaced and there are some new looking resistors. In comparing to layout and schematic diagrams, I see unique elements of both circuits - ie the 10k resistors connected to bias pot of the AB circuit (look original) and the RC network around the 12AT7 from the AA circuit (looks like some new resistors added or replaced). Is that a common mod or a mistake?



The AA165 has a BIAS pot. No 10k-ohm resistors hang off the pot, itself. The AA165 has traditional blackface style global feedback. A 820-ohm/100 ohm divider.

http://bmamps.com/Schematics/fender/bas ... _schem.pdf



The AB165 has a BALANCE pot. Two 10k-ohm resistors hanging off the same pot (usually onto a terminal strip). The AB165 has a resistor/capacitor (47k-ohm/0.1mfd) feedback circuit onto the OPPOSITE plate of the 12AT7 phase inverter. This is why you need to reverse the output tranny leads, when you mod the AB165 back to the more desired AA165.

http://bmamps.com/Schematics/fender/bas ... _schem.pdf

The 7025 triode-half, second gain stage has 470k-ohm local feedback --- from the anode to the signal grid. All of this to "try" and make the amp more linear, for bass use. But basically, help to neuter the tone.


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Post subject: Re: '66 Bassman AA165 or AB165
Posted: Sun Jan 04, 2015 9:46 am
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BMW2002Tii wrote:
dscholz wrote:
I purchased a '66 Bassman head and cab a couple years ago. The tube diagram says it's the AA165 circuit but some scratched out the AA and wrote AB with a pencil. I opened up the amp to see if I could determine which one it really is. Well, someone's been in there before me as all the caps have been replaced and there are some new looking resistors. In comparing to layout and schematic diagrams, I see unique elements of both circuits - ie the 10k resistors connected to bias pot of the AB circuit (look original) and the RC network around the 12AT7 from the AA circuit (looks like some new resistors added or replaced). Is that a common mod or a mistake?



The AA165 has a BIAS pot. No 10k-ohm resistors hang off the pot, itself. The AA165 has traditional blackface style global feedback. A 820-ohm/100 ohm divider.

http://bmamps.com/Schematics/fender/bas ... _schem.pdf



The AB165 has a BALANCE pot. Two 10k-ohm resistors hanging off the same pot (usually onto a terminal strip). The AB165 has a resistor/capacitor (47k-ohm/0.1mfd) feedback circuit onto the OPPOSITE plate of the 12AT7 phase inverter. This is why you need to reverse the output tranny leads, when you mod the AB165 back to the more desired AA165.

http://bmamps.com/Schematics/fender/bas ... _schem.pdf

The 7025 triode-half, second gain stage has 470k-ohm local feedback --- from the anode to the signal grid. All of this to "try" and make the amp more linear, for bass use. But basically, help to neuter the tone.


Beemer, don't you have some gut shots of the two different ones? Thought I saw somewhere you posted a AA165 and a AB165, might help to post em and show the differences. Just a thought.

T2

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Post subject: Re: '66 Bassman AA165 or AB165
Posted: Sun Jan 04, 2015 9:58 am
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T2Stratman wrote:

Beemer, don't you have some gut shots of the two different ones? Thought I saw somewhere you posted a AA165 and a AB165, might help to post em and show the differences. Just a thought.

T2


Just look at the layouts , its is close to black and white photos and better to see difference than a real color photos.


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Post subject: Re: '66 Bassman AA165 or AB165
Posted: Sun Jan 04, 2015 11:10 am
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Here you go.


Stock AB165 with BALANCE pot. Note how the AB165 also has two 220k-ohm resistors acting as local feedback on the 6L6GC plates (from the PI section)? See them hanging off the board, on the lower left part of board to terminal strip by 6L6GC sockets? These are seen on the earlier AA165.

Image



AB165 modded back to AA165 BIAS pot. 220k-ohm feedback resistors removed.

Image


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Post subject: Re: '66 Bassman AA165 or AB165
Posted: Sun Jan 04, 2015 1:09 pm
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AA165

Image




AB165

Image


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