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Post subject: Help Identifying Mods Done to 1968 Bassman
Posted: Fri Jun 14, 2019 11:50 am
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Hi all,

I have a 68 dripedge blackline Bassman with the small cab. Sounds great. I know it is an AB and I know it has been modified a bit. It appears to me the Bias Balance has been modified. I was hoping someone can tell me exactly what has been modified on the amp. I do not wish to change it further as it sounds great but I would like to know more about it.

https://drive.google.com/open?id=1FSytL ... DzVc3r4Si_

Also, how do you test and bias this in the event it has been changed to adjustable bias? I have only ever biased an amp with a labeled test point like a HRD.

Thank you!

Phil


Last edited by RShakleford88 on Sat Jun 22, 2019 2:05 pm, edited 1 time in total.

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Post subject: Re: Help Identifying Mods Done to 1969 Bassman
Posted: Sat Jun 22, 2019 1:23 pm
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Without pictures, your post is useless. Change your pictures to "public."


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Post subject: Re: Help Identifying Mods Done to 1969 Bassman
Posted: Sat Jun 22, 2019 1:32 pm
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vinyl wrote:
Without pictures, your post is useless. Change your pictures to "public."


I made them public. Thank you for pointing that out!


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Post subject: Re: Help Identifying Mods Done to 1968 Bassman
Posted: Sun Jun 23, 2019 2:32 pm
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AB165 circuit. Bias balance has been modified to bias adjust. Electrolytic caps have been replaced. No other mods apparent. Phase inverter and all the weird negative feedback appears stock. Half of V2 not used.

Unless you know what you're doing I'd leave the bias adjustment to a pro.


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Post subject: Re: Help Identifying Mods Done to 1968 Bassman
Posted: Sun Jun 23, 2019 5:00 pm
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I appreciate it. That’s what I was hoping to learn.

I thought the AB165 used both halves of V2 as a second gain stage for each channel, is that not the case?


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Post subject: Re: Help Identifying Mods Done to 1968 Bassman
Posted: Sun Jun 23, 2019 5:53 pm
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vinyl wrote:
Half of V2 not used.


Wrong.

It's V3 that goes half unused.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Help Identifying Mods Done to 1968 Bassman
Posted: Sun Jun 23, 2019 7:28 pm
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Retroverbial wrote:
vinyl wrote:
Half of V2 not used.


Wrong.

It's V3 that goes half unused.

Arjay



If you want to call it V3, that's up to you. I'll continue to call it V2, and we can agree to disagree.
Image

I still have the utmost respect for you, sir.

Cheers


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Post subject: Re: Help Identifying Mods Done to 1968 Bassman
Posted: Sun Jun 23, 2019 7:46 pm
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You may in fact be correct, Vinyl. I just checked every official Fender schematic from revisions AA864 to AA371 and none of the "V" positions are specifically labeled. And that pictorial illustration you posted plainly shows the inputs from the "NORMAL" channel connected to the third 7025 in the physical line-up. Thus, the second bottle is the de facto V2, with half of it unused.

Keep us honest, kid.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Help Identifying Mods Done to 1968 Bassman
Posted: Mon Jun 24, 2019 2:21 am
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vinyl wrote:
Electrolytic caps have been replaced.


You talk about bias circuit filter cap and all bypass caps.

We don't know about filter caps in the "dog house"


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Post subject: Re: Help Identifying Mods Done to 1968 Bassman
Posted: Mon Jun 24, 2019 4:54 am
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Tone Lizard's Bassman Mods site may help. :)

http://tone-lizard.com/bassman-mods/


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