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Post subject: AB165 Bass channel 3rd stage gain bypass
Posted: Mon Jul 22, 2019 9:34 am
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https://www.google.com/url?sa=t&source=web&rct=j&url=http://ampwares.com/schematics/bassman_ab165.pdf&ved=2ahUKEwjfuPXwuMvjAhXEY98KHXpsBGMQFjAKegQIBhAB&usg=AOvVaw0t-0Iwv8VDfh1KOBtPqlL5
Hello all, I am on here to inquire about my AB165 Bassman which I have been performing some mods. So far I have removed the NFB loop, changed out the .022 coupling caps to .1, and converted the output to cathode bypass. I am nearly satisfied with the normal channel, but now onto the Bass channel. I am looking for a slightly more 6G6-B sound. I have already converted the tone stack to 6G6-B specs which has made a noticable difference for the better, but a bit too much gain. I am wanting to bypass the 3rd stage of gain on the bass side. I tried taking the 220k coming off the second stage and put it into the .1 coupling cap going to the Phase Inverter, but I got very little sound after doing that. All suggestions are welcome. Thank you!


Last edited by Chase.parten on Tue Jul 23, 2019 9:21 am, edited 1 time in total.

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Post subject: Re: AB165 Bass channel 3rd stage gain bypass
Posted: Tue Jul 23, 2019 12:56 am
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The link do not work :(


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Post subject: Re: AB165 Bass channel 3rd stage gain bypass
Posted: Tue Jul 23, 2019 6:39 am
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stratele52 wrote:
The link do not work :(


+1

Why not simply replace the 3rd stage 12AX7 (gain 100) with a lower gain tube, like a 5751 (gain 70) or 12AT7 (gain 60) ?


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Post subject: Re: AB165 Bass channel 3rd stage gain bypass
Posted: Tue Jul 23, 2019 6:50 am
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vinyl wrote:
Why not simply replace the 3rd stage 12AX7 (gain 100) with a lower gain tube, like a 5751 (gain 70) or 12AT7 (gain 60) ?


+1

A 12AY7 would also work. Try all three to find the best compromise.

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: AB165 Bass channel 3rd stage gain bypass
Posted: Tue Jul 23, 2019 9:12 am
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Retroverbial wrote:
vinyl wrote:
Why not simply replace the 3rd stage 12AX7 (gain 100) with a lower gain tube, like a 5751 (gain 70) or 12AT7 (gain 60) ?


+1

A 12AY7 would also work. Try all three to find the best compromise.

Arjay


+1

12AY7 is more a direct replacement for a 12AX7 , not 12AT7


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Post subject: Re: AB165 Bass channel 3rd stage gain bypass
Posted: Tue Jul 23, 2019 9:24 am
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Thank you all for your suggestions. I posted a new link to the schematic (I'm still trying to figure this forum out). Though swapping out the 12AX7 would reduce gain, my understanding is that the normal and bass channels share the 3rd gain stage which is why I am wanting to bypass it in the bass channel. Keep the 3 gain stages in the normal, reduce to 2 in the bass.


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Post subject: Re: AB165 Bass channel 3rd stage gain bypass
Posted: Tue Jul 23, 2019 10:23 am
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Chase.parten wrote:
Thank you all for your suggestions. I posted a new link to the schematic (I'm still trying to figure this forum out). Though swapping out the 12AX7 would reduce gain, my understanding is that the normal and bass channels share the 3rd gain stage which is why I am wanting to bypass it in the bass channel. Keep the 3 gain stages in the normal, reduce to 2 in the bass.


So you don't really want to change the gain of the third stage, you want to change the gain of the Bass channel.

I think I've lost my patience for this thread.

Good luck re-inventing the wheel.


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Post subject: Re: AB165 Bass channel 3rd stage gain bypass
Posted: Tue Jul 23, 2019 10:57 am
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OK, Let me have a crack at it.

When you moved the 220K to the phase inverter and got very little sound, that demonstrates that there is not enough gain to drive the amp without the third stage.
Bypassing the tube lowers the amps multiplication factor by 100. You are looking for a change of 10 to 20, maybe less.
You can do that by following the tube changes recommended by others here.
You can also easily do that by changing the 220K mixing resistors to get the gain you want without changing any other characteristics.

A couple other things to do that may change tone include:
You can change the 470K on the third stage NFB loop from plate to grid. Lowering the value will lower the gain.

You can change plate resistors. Higher values mean higher gain.

You could even use the other half of the tube and build your own third stage. Pull out the 220K mixing resistors and install them at the phase inverter.


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