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Post subject: Phase inverter circuit ('72 twin reverb)
Posted: Wed May 30, 2018 11:02 am
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Hobbyist
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Joined: Tue May 29, 2018 6:24 am
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looking at the 100w SFTR MV schematic, there lies a 220k res and .01uF cap going from one of the outputs of V6 to ground via a 100ohm resistor.

anybody know what this is for? is it safe to remove? its not in the early SF circuits. thanks.


ron


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Post subject: Re: Phase inverter circuit ('72 twin reverb)
Posted: Thu Jun 07, 2018 9:08 am
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Welcome to the forum, rborja3, sometimes the delay on 1st posts can cause it to become "buried".

The change you seem to be referring to introduces some negative feedback back into the phase inverter, probably to prevent the amp going into oscillation.

I'll let the other more knowledgeable users chime in here as they have more experience in terms of what happens if you try to remove it without any other changes.

I see you have a 72 Twin from your other posts and enjoy vinyl and vintage stereo gear.

Cheers!


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Post subject: Re: Phase inverter circuit ('72 twin reverb)
Posted: Thu Jun 07, 2018 9:35 am
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vinyl wrote:
The change you seem to be referring to introduces some negative feedback back into the phase inverter, probably to prevent the amp going into oscillation.


+1

A secondary purpose is to shunt some of the even-order harmonics to ground, with the intent being to enhance the more "musical" aspects of the signal particularly when compressed and saturated. IIRC these additional components were added to the PI sub-circuit when the MV control was introduced. Unless the MV feature was being disabled I'd leave the PI as is.

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: Phase inverter circuit ('72 twin reverb)
Posted: Tue Jun 19, 2018 5:23 am
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More answer here;

https://forums.fender.com/viewtopic.php ... 4#p1228644


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Post subject: Re: Phase inverter circuit ('72 twin reverb)
Posted: Tue Jun 19, 2018 5:54 am
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Stratele, this is a 100-watt Master Volume Twin Reverb. Not 135-watt model. Here is the portion of the circuit, in question. I did not see his other post.

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Post subject: Re: Phase inverter circuit ('72 twin reverb)
Posted: Tue Jun 19, 2018 6:12 am
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BMW2002Ti wrote:
Stratele, this is a 100-watt Master Volume Twin Reverb. Not 135-watt model. Here is the portion of the circuit, in question. I did not see his other post.



I send answer there and find shematic just before you .

https://forums.fender.com/viewtopic.php ... 4#p1228644


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Post subject: Re: Phase inverter circuit ('72 twin reverb)
Posted: Tue Jun 19, 2018 6:20 am
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Stratele, you are referencing the 135-watt MV circuit, in your replies. It has a different GNFB scheme than the 100-watt MV model.


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