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Post subject: 1965 Bassman - Circut - help?
Posted: Thu Dec 27, 2012 1:11 pm
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Hey Folks, I just scooped up a Blackface Bassman head for $500. All braging aside I'm tying to figure out what circut it is - I don't care which, but I'm particular about knowing the details. So here's the scoop:

1: It says Fender Electic Instrument on the back panel.
2: It's got the AA864 tube chart
3: It's Date Stamped OB for Feb of '65 (yes I know that in 66 the forgot to advance the year stamp so OB could be Feb of 65 or 66, but point number 1 would suggest that it would be a 65.)

I know that at some point in early 65 Fender quit using the AA864 circut. They switched to the AA165 for about a month, and then went to the AB165 for the remainder of 1965-66. I'm no electrician, so assuming it's not modded (other than the 3 prong chord) it could be an AA864, an AA165 or the AB165 in theory. So what I'm askin is, is there an easy way to tell those models appart with limited electrical knowledge? I've read that these are all a little different, and I'm not picky enough to argue the tone difference between the AA864 and the AB165. I just know that I like the amp. On the other hand it'd be cool to have the 864 because Leo designed it, and it'd be cool to have the rare AA165 model... Ugh, too much to think about! At any rate, any help'd be appreciated!

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Post subject: Re: 1965 Bassman - Circut - help?
Posted: Thu Dec 27, 2012 1:46 pm
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The most significant (and visible) difference between these three revisions lies with the bias-supply circuit. Both the AA864 and AA165 have a true bias pot which adjusts the negative grid voltage for both power tubes simultaneously. The AB165 is configured for a bias balance pot, which is used to match the current flow of one power tube against the other by adjusting only one leg of the bias-supply output.

Arjay

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Post subject: Re: 1965 Bassman - Circut - help?
Posted: Thu Dec 27, 2012 2:12 pm
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I guess I'm going to have to try to remember those basic electricity courses that I took in high school and start remembering how to read schematics. Got any other words of wisdom for someone who just dove head first into their first tube amp?

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Post subject: Re: 1965 Bassman - Circut - help?
Posted: Thu Dec 27, 2012 3:04 pm
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Unplug it before you execute that triple gainer with a half-twist.

And wear clean underdrawers......just in case.

:lol:

Arjay

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Post subject: Re: 1965 Bassman - Circut - help?
Posted: Thu Dec 27, 2012 5:46 pm
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Ahem...we have rules here...pics or it didn't happen. We love amp porn here!

T2

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Post subject: Re: 1965 Bassman - Circut - help?
Posted: Thu Dec 27, 2012 6:41 pm
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T2Stratman wrote:
Ahem...we have rules here...pics or it didn't happen.


Would you prefer to see photos of him riding off in the ambulance or a shot of the "smoking hole" on his workbench?

:lol:

Arjay

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Post subject: Re: 1965 Bassman - Circut - help?
Posted: Thu Dec 27, 2012 7:03 pm
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Retroverbial wrote:
T2Stratman wrote:
Ahem...we have rules here...pics or it didn't happen.


Would you prefer to see photos of him riding off in the ambulance or a shot of the "smoking hole" on his workbench?

:lol:

Arjay


:lol: :lol: :lol: :lol:

Well to be honest, just pictures of his amp would do!

T2

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Post subject: Re: 1965 Bassman - Circut - help?
Posted: Thu Dec 27, 2012 7:04 pm
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This may help you. A close up of the wiring may help determine what circuit you have. After draining the caps, of course. :D

http://www.justinholton.com/hotrod/caps_p.html


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Post subject: Re: 1965 Bassman - Circut - help?
Posted: Fri Dec 28, 2012 8:43 am
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No smoking hole in the work bench yet. I popped 'er open last night and took a look around. Upon further investigation I'm thinkin it's a AB165. Unfortunately, with closer investigation it's looking like the caps weren't just replaced, but someone mod'd the bass channel a bit... So sifting through wiring of a questionable origin concerns me. So now I'm trying to decide if I should be sending it to a tech to get it done right, or if I should use this as a good excuse to get back into electrical work... Probably taking it to a tech to avoid blowing myself - and more importantly - the amp up.

I'm traveling this weekend for the holidays, but I'll try to get some pictures up when I get home.

If it turns out to be an AB165 I'm tempted to have the bass channel reverted to the AB864 circut. If it turns out that it's an original AA165 or AA864 I'll probably have it all reverted to stock since those are the more rare circuts. Thoughts?

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Post subject: Re: 1965 Bassman - Circut - help?
Posted: Fri Dec 28, 2012 9:18 am
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The differences in pre-amp circuitry for the bass channel between those three revisions (AA864, AA165, AB165) are relatively insignificant. In all instances, that channel utilizes three gain stages -- both halves of one 7025 plus half of another, leaving an unused half of a 7025. Where it becomes interesting is with the earlier 6G6 Bassman. That amp provided four gain stages for the bass channel by using all four halves of those two 7025's (which is part of the 6G6 Bassman appeal, especially to guitarists). If your bass channel was modded to this earlier configuration and you like its tone, I'd leave it in situ and enjoy it as is.

HTH

Arjay

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Post subject: Re: 1965 Bassman - Circut - help?
Posted: Fri Dec 28, 2012 4:28 pm
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An easy test would be to pull V2, if the normal channel still works it's an 864, if not it's a 165. Yeah, I reverted my 165 to the 864 circuit. IMO This allows the ability to change the bass channel's tubes for different responses, without affecting the normal channel. Art

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Post subject: Re: 1965 Bassman - Circut - help?
Posted: Fri Jan 04, 2013 12:12 pm
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Alright, first off, I'm workin on pics but my camera is nowhere to be found after a recent move... Sorry.
Secondly,
After lookin over the amp it clearly has a bias pot, but the bass channel is a little hacked up. The normal channel sounds awesome however. So, the questions start piling up...
Everyone seems to like the sound of the aa864 - I haven't had the chance to play through an unmollested one, but the internet seems to agree that they are good sounding amps. I want to revert this amp to a stock circuit because I play bass too and would like to have the option to record through the bass channel (probably to quiet for gigging, but I've got that covered). So, what I'm getting at is, should I revert this to an aa165 (which it probably is) or an aa864? Obviously there aren't alot of reliable sound clips on the internet so I'm just running on hearsay. Does it matter? Is the aa165 really rare enough that changing one up is a sin? Would an aa165 be worth much more than an ab or an 864?

Fear not, amp porn to come.

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Post subject: Re: 1965 Bassman - Circut - help?
Posted: Fri Jan 04, 2013 1:17 pm
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"Worth" is of little consideration at this point since the amp has already been modded to extents still not fully understood. If you like the tone as it stands right now, I'd leave it all alone and perform only that maintenence and servicing which prudence dictates is necessary for safe and reliable operation.

JMOOC, YMMV

Arjay

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Post subject: Re: 1965 Bassman - Circut - help?
Posted: Fri Jan 04, 2013 5:01 pm
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I certainly appreciate that you want to get the amp back to factory spec, but as Retro stated, if you like the tone, just do the needed maintenace and keep it as is. Now if you bought it with an eye for resale, then by all means make getting it back to factory spec a priority. JMHO.

T2

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Post subject: Re: 1965 Bassman - Circut - help?
Posted: Fri Jan 04, 2013 7:27 pm
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My choice for recording bass would be the 165 circuit. It has some local negative feedback in the pre amp and an extra filter cap in the power supply. This contributes to a cleaner sound. Being a guitar player, I prefer the 864. But I also made some changes in component values to suit my ear. Actually, my guy was quite a project. Total solid core rewire, '63 Merc X-formers, appropriate pots, etc....Oh Yeah, it's a 2X10 combo/ with 1X10 ext spkr. Art

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