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Post subject: 1968 Bassman
Posted: Fri Feb 07, 2014 5:07 pm
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Received the Bassman head today. Some photos. Quick look seems that this amp has the AB165 circuit layout. Prolly, modified from a AC568 (I can see the added cathode ground strap on the 6L6GC). Will try to revert to AA864.

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Last edited by BMW2002Ti on Fri Feb 07, 2014 5:11 pm, edited 1 time in total.

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Post subject: Re: 1968 Bassman
Posted: Fri Feb 07, 2014 5:08 pm
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Post subject: Re: 1968 Bassman
Posted: Fri Feb 07, 2014 5:09 pm
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Guess the last tech didn't have clear silicone sealer. :lol:

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Post subject: Re: 1968 Bassman
Posted: Fri Feb 07, 2014 5:10 pm
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AB165 circuit schemo:

http://www.webphix.com/schematic%20heav ... _schem.pdf

AA864:

http://www.webphix.com/schematic%20heav ... _schem.pdf


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Post subject: Re: 1968 Bassman
Posted: Fri Feb 07, 2014 5:19 pm
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Looking good, how does it sound at the moment? :D


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Post subject: Re: 1968 Bassman
Posted: Fri Feb 07, 2014 5:54 pm
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By the ink-stamp on the chassis it's an AB165 revision. And you can plainly see that one half of V2 isn't wired up to anything -- for an AA864 circuit you'll be using that idle half to provide an additional gain stage for the bass channel.

Arjay

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Post subject: Re: 1968 Bassman
Posted: Fri Feb 07, 2014 7:03 pm
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Nice... Congrats !

There sure is a lot of room on that Board !

Isn't the Bias circuit a little funny? It looks as if it's a balanced circuit rather than adjustable.

cheers!

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Post subject: Re: 1968 Bassman
Posted: Fri Feb 07, 2014 7:32 pm
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Can you guys tell me why the newer tone caps always seem smaller, in fact much smaller than the old Blue Tone Caps? Is this because the newer caps are more efficient or just a manufacturing technique?

Beemer sorry for the short Hijacking, but you live for this stuff so thought it was a good time to answer my stupid question of the day.

BTW...nice amp head.

T2

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Post subject: Re: 1968 Bassman
Posted: Fri Feb 07, 2014 7:57 pm
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Lightnin MN wrote:
Isn't the Bias circuit a little funny? It looks as if it's a balanced circuit rather than adjustable.


You are correct, sir. The AB165 Bassman was designed with a bias-balance adjustment (the first Fender amp to use this system).

Arjay

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Post subject: Re: 1968 Bassman
Posted: Fri Feb 07, 2014 8:00 pm
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T2Stratman wrote:
Can you guys tell me why the newer tone caps always seem smaller, in fact much smaller than the old Blue Tone Caps? Is this because the newer caps are more efficient or just a manufacturing technique?


Correct. As well, you'll find that modern metal-film resistors are approximately 10% smaller than their vintage carbon-comp equivalents.

Arjay

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Post subject: Re: 1968 Bassman
Posted: Fri Feb 07, 2014 9:02 pm
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Retroverbial wrote:
By the ink-stamp on the chassis it's an AB165 revision. And you can plainly see that one half of V2 isn't wired up to anything -- for an AA864 circuit you'll be using that idle half to provide an additional gain stage for the bass channel.

Arjay





Wonder how using the second half of V2, as additional gain stage, will sound with a guitar and not a bass?

Retroverbial wrote:
Lightnin MN wrote:
Isn't the Bias circuit a little funny? It looks as if it's a balanced circuit rather than adjustable.



You are correct, sir. The AB165 Bassman was designed with a bias-balance adjustment (the first Fender amp to use this system).

Arjay




Yeah, when Fender went to the balance/hum adjustment pot (from the original bias pot) they just added a terminal strip to place the neccesary resistors. I guess this allowed the use of the original AA864 tagboards.


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Post subject: Re: 1968 Bassman
Posted: Fri Feb 07, 2014 10:46 pm
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BMW2002Ti wrote:
Wonder how using the second half of V2, as additional gain stage, will sound with a guitar and not a bass?


A pleasing, overabundance of "fattitude".

8)

Arjay

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Post subject: Re: 1968 Bassman
Posted: Sat Feb 08, 2014 11:16 am
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That's a conversion, that I pulled off some years ago. Mine is a 2X10 combo, now. Good Luck!!! Art

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Post subject: Re: 1968 Bassman
Posted: Sat Feb 08, 2014 10:37 pm
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Rectifier board and bias wiring, before (stock AB165)

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Post subject: Re: 1968 Bassman
Posted: Sun Feb 09, 2014 4:34 am
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Rectifier board redone. 6 X Fairchild UF5408 in main PSU. One Fairchild UF4007 in bias supply.




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Main caps. Ohmite wire-wound 1K-ohm and Dale ww 4.7k-ohm resistors. 27k-ohm, 8mfd/450VDC stage of AB165 --- replaced with 22mfd/500VDC cap. First stage 70mfd/350VDC replaced with 100mfd/450VDC F&T's.




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