Louie7
Joined: Jan 30, 2007
Posts: 448
50,000 Light-Years Beyond Planet Claire
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Posted on Apr 08 2007 01:13 PM
Here's a quick account of my effort to 'understand' this 1971 SF Bassman
a. Tube chart says AB165 - I pull the chassis to bias the tubes and discover amp has a different bias circuit than what is shown for AB165. Use of left-over earlier tube chart seems consistant with practises of Fender.
b. The 'hum balance pot' is discovered to be part of the AA371 circuit which seems consistant with 1971 time period.
I successfully add a true bias adjust pot alongside the hum pot and all is well.
c. I change filter caps (with Sprague Atoms), consistant with AA371 schematic, and all is well.
d. I decide to replace electrolytics (5 white Mallory 25/25) on the eyelet board; and then decide to replace the so-called 'brown-turd' ceramics at the same time - and I order the same parts that actually exist in the amp.
e. Parts arrive and I begin working across the eyelet board, only to discover a number of caps and resistors of values not found (and in one case, in positions not found) on the AA371 layout sheet! I take this as an assembly line anomoly, and post this thread to see what gives....
f. It is suggested that I look at the AB165 paperwork and IN FACT, the eyelet board in my amp does almost exactly match that of the AB165.
CONCLUSION: So it appears that I have, basically, an AB165 amp, with an AA371 era Bias system (that I modified, of course). Fine.... but it doesn't seem to fit the model. My understanding was that the AB165 was the BF Bassman. 1971 seems awfully late for that circuit to pop up. Never the less, it matches perfectly (sans bias componants).
NOTE: As regards circuit AB763... I can find no direct relation of any AB763 to a Bassman amp. Pittmans Tube Amp book and Webers Vintage Amp book both do not show any Bassman amp with an AB763 circuit. The book Fender Amps by Teagle and Sprung for some reason has very limited information on SF Bassmans (even though other amps are thoroughly assessed in detail). For some reason the authors get to the SF Bassman and then just skip on ahead to later 70's and 80's versions.
THEREFORE: Today, I installed the remaining componants I ordered, according to the AB165 circuit, since they match identically and presume that it's the way it should be. The picture below shows the final result.
LASTLY: I'm still vexed about two things.....
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If I have essentially an AB165 circuit (which should be superior to an AA371) then why does the amp have such a wimpy Bass channel?
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Why, in the wide-world-of-sports does a BF Bassman circuit pop up in a 1971 SF Bassman well into the evil CBS era?
Fun stuff, Maynard!
the everquesting, slightly conservative, but still tremendously open to new ideas LOUIE SEVEN
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JakeDobner
Joined: Feb 26, 2006
Posts: 12159
Seattle
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Posted on Apr 08 2007 01:40 PM
Replaced tube chart as the circuit may have been modded by a previous owner.
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diceophonic
Joined: May 20, 2006
Posts: 2174
PacNW (Vancouver, Wa U.S.A.)
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Posted on Apr 08 2007 01:51 PM
L7 AB165 is not the greatest ciruit for guitar, AB165 is a CBS creation and one of the 1st circuits CBS frankensteined and changed, you should re-build it to pre-CBS AA864 you'll hear better and sweeter tones for bass and guitar, loose the orange drops and go with Mallory caps, Mallory caps are as close as you'll get to the original fender blue molded caps 
As Jake mentioned it had to be modded as some point to AB165
so go the extra mile AA864 it 
This bassman I have is a stock AA864 64' Bassman
— -Kyle
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Louie7
Joined: Jan 30, 2007
Posts: 448
50,000 Light-Years Beyond Planet Claire
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Posted on Apr 08 2007 03:14 PM
Appreciate the comments.... Here's the deal.
It's not JUST the values of some of the caps and R's that are like the AB165..it's the way the eyelet board is wired. The components are in different configurations. The bias section resembles the AA371 and the preamp sections resemble the AB165.
But I see no real evidence that anyone went in wholesale and turned everything around.
I've showed the chassis to guys who should know about these things and they tell me that (before I did any work) the amp looked complelely stock and showed no signs of mods or repairs.
Also, I'm told by these same peoples that the Orange Drops are going to be warmed and rounder than the Mallory 150's..... beyond that, I purchase the TORRES Engineering Orange Drops which they claim are superior to the garden variety Orange Drops typically available at other vendors.
I'm not sweating it.... I can always change them out if I don't like the results, but I think it'll be OK.
wool sea, won't eye ?
heh
L7
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WR
Joined: Feb 27, 2006
Posts: 3832
netherlands
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Posted on Apr 08 2007 03:57 PM
Louie7
a. Tube chart says AB165 - I pull the chassis to bias the tubes and discover amp has a different bias circuit than what is shown for AB165. Use of left-over earlier tube chart seems consistant with practises of Fender.
b. The 'hum balance pot' is discovered to be part of the AA371 circuit which seems consistant with 1971 time period.
I successfully add a true bias adjust pot alongside the hum pot and all is well.
both ab15 and aa371 have a hum balance bias, though with different value components. not sue whetehr I understand your posting correctly. I think aa165 still had adjustable bias.
— Rules to live by #314:
"When in Italy, if the menu says something's grilled, don't assume it is."
https://www.facebook.com/The-Malbehavers-286429584796173/
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Louie7
Joined: Jan 30, 2007
Posts: 448
50,000 Light-Years Beyond Planet Claire
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Posted on Apr 08 2007 04:31 PM
heh I'm not sure I understand your post either....... 
I don't recall mentioning anything called an AB15.
Anyway... remember that my original entry tries to explain the series of confusions that I had when first dealing with the amp.
I went into the amp to bias the tubes and discovered the hum-balance pot which disallowed me from accomplishing that - and also tipped me off that the tube chart was inaccurate.
I then (incorrectly) surmised that my amp was an AA371 circuit since all the bias section components matched the AB165 layout diagram.
It wasn't until later, when I elected to change out the eleytrolytics and ceramic caps on the eyelet board that I discovered that the amp was actually wired up with AB165 values in an AB165 arrangement (excepting, of course, the bias section, which is throroughly AA371.
Again, everyone who has actually looked at the chassis agrees that it doesn't appear to have been modified. THEREFORE, I pretty much must conclude that this is an AB165 with an AA371 bias section.
But I still don't understand why AB165 values and arrangements were showing up in 1971. I mean, I know CBS/Fender was playing fast and loose with these things....but it jsut doesn't jive with all the information I've been about to locate regarding these SF Bassmans.
I'm not upset about it, really... it's just mysterious.
The amp is currently plugged into the VARIAC. I'm bringing those Sprague 25uf/25V electrolytics up slowly. I'm told you that 'newer' caps don't require this step. But I'm doing it anyway, as a safety measure, just in case I made a boo-boo (or a yogi) when buttoning things up.
comments ?
Louie Lou-EYE
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WR
Joined: Feb 27, 2006
Posts: 3832
netherlands
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Posted on Apr 08 2007 04:58 PM
typo...
I meant ab165. I thought you maybe saw a hum balance pot and figured it was aa371, but anyway, I rembered only half right, I looked at the schematics now, ab165 has the three-lug hum balance pot, aa731 the four lug. entirely different indeed.
interesting though. so how did you date it to '71? by tranny code and pot-codes?
— Rules to live by #314:
"When in Italy, if the menu says something's grilled, don't assume it is."
https://www.facebook.com/The-Malbehavers-286429584796173/
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Louie7
Joined: Jan 30, 2007
Posts: 448
50,000 Light-Years Beyond Planet Claire
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Posted on Apr 08 2007 05:29 PM
I dated it by observing the mfg date stamped inside the chassis.
Have a look ! That's how it looked before I added the bias level adjustment trim-pot mod.
This is after I added the trim pot. It works in conjunction with the balance pot.
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PhatTele
Joined: Jun 05, 2006
Posts: 445
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Posted on Apr 09 2007 11:37 AM
Louie7 - I wouldn't worry about identifying the circuit too much. I've never found two Bassman amps that were wired the same after the last BF AB165s were made. Starting in 1968 with the drip edge SFs, I've found all sorts of combinations of circuits and biasing and component values. I finally gave up and said to myself "self, if it's after 1967, then it needs to be brought back to AB864 specs anyway, so why worry about it." I've slept easier ever since.
http://blueguitar.org/new/schem/fender/modnotes/mga-aa864_vs_aa165.pdf
BTW - The Mallory 150s are supposed to be mellower like the blue tube Mallorys of the BF era. The Orange drops will be a bit brighter, especially if you use them as tone caps.
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Louie7
Joined: Jan 30, 2007
Posts: 448
50,000 Light-Years Beyond Planet Claire
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Posted on Apr 09 2007 12:23 PM
Phattele ~
well, those are some consoling words. That's pretty much it I guess. I'm calling it an AB165/AA371 circuit just for identifrying porposes, now. Moving on. hehe
As regards the Orange Drops, I think that's exactly what the fellows at TORRES told me when I called them to get their take on the differences between the Mallories and the Spragues. OD's will be brighter......
...and that's a good thing for me. Because that's the sound I like. And ya know what? I have the amp up and running and it sounds quite Black-face like! Believe it or not. It's got a good bit of sparkle now and much much quieter too. I think I dun good. Of course new tubes helped too, but I was still getting some splattery treble after the new tubes but before the new tone caps. That seems to be history now.
I'm taking this amp to the next show. May as well put it thru its paces.
Louie Seven
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diceophonic
Joined: May 20, 2006
Posts: 2174
PacNW (Vancouver, Wa U.S.A.)
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Posted on Apr 09 2007 05:52 PM
PhatTele
Louie7 - I wouldn't worry about identifying the circuit too much. I've never found two Bassman amps that were wired the same after the last BF AB165s were made. Starting in 1968 with the drip edge SFs, I've found all sorts of combinations of circuits and biasing and component values. I finally gave up and said to myself "self, if it's after 1967, then it needs to be brought back to AB864 specs anyway, so why worry about it." I've slept easier ever since.
http://blueguitar.org/new/schem/fender/modnotes/mga-aa864_vs_aa165.pdf
BTW - The Mallory 150s are supposed to be mellower like the blue tube Mallorys of the BF era. The Orange drops will be a bit brighter, especially if you use them as tone caps.
Amen to it all PhaTele AA864 you said what I was going to post in reply
— -Kyle
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Louie7
Joined: Jan 30, 2007
Posts: 448
50,000 Light-Years Beyond Planet Claire
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Posted on Apr 09 2007 06:56 PM
I think I'm reading "black-face that amp, now" between the lines, here.
heh....guys.... this is a Sliver Faced amp....I've read awl about SF-BF conversions and it's a fun topic, but I really don't need to do that.
This Bassman has no future competing with my 73 Pro Reveb (which HAS been black-faced and sounds like it, too)...or my 65 Showman. That's why I have those amps - so I don't have to BF my Bassman.
Anyway, I've read about the motorboating and oscillation freakouts that often result in BF'ing amps like that. I'm not that good of a tech; and as I say...no need, really.
but thanks for the concern and stay tuned...next on the operating table....
my 68 SF DR...... it's getting new electrolytics!
Louie SEVEN
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diceophonic
Joined: May 20, 2006
Posts: 2174
PacNW (Vancouver, Wa U.S.A.)
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Posted on Apr 09 2007 07:20 PM
Louie7
I think I'm reading "black-face that amp, now" between the lines, here.
heh....guys.... this is a Sliver Faced amp....I've read awl about SF-BF conversions and it's a fun topic, but I really don't need to do that.
This Bassman has no future competing with my 73 Pro Reveb (which HAS been black-faced and sounds like it, too)...or my 65 Showman. That's why I have those amps - so I don't have to BF my Bassman.
Anyway, I've read about the motorboating and oscillation freakouts that often result in BF'ing amps like that. I'm not that good of a tech; and as I say...no need, really.
but thanks for the concern and stay tuned...next on the operating table....
my 68 SF DR...... it's getting new electrolytics!
Louie SEVEN
L7 the beauty about your showman and the pro reverb thier ciruits are BF AB763 and the Bassman stands alone in it's circuit bieng AA864/AB165  "nice big beefy tone" very very different tones from the AB763 more flat but killer tone
— -Kyle
Beyond The Surf YouTube channel
Beyond The Surf Instagram
The Verbtones @ Instagram
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zak
Joined: Sep 24, 2007
Posts: 2728
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Posted on Sep 13 2008 01:12 AM
This post has been removed by the author.
Last edited: Sep 23, 2009 19:17:27
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Louie7
Joined: Jan 30, 2007
Posts: 448
50,000 Light-Years Beyond Planet Claire
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Posted on Sep 15 2008 08:48 PM
right......
well, actually, mine really rips!
heh
I mean... it's too savage to even use at a normal gig.
(no headroom to speak of, is what I mean.... 
L7
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