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Post subject: 72 twin phase inverter circuit
Posted: Tue May 29, 2018 6:37 am
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maybe somebody here can help.

on the output of the phase inverter lies a 220k + .01 cap to ground via 100ohm. but its only on one side. anybody know what its for? its in the 100w MV circuit but not in the AB763, AA769 or AA270. i want to remove it.

my 72 twin has a MV but no push/pull.

/ron


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Post subject: Re: 72 twin phase inverter circuit
Posted: Mon Jun 18, 2018 4:22 pm
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Look it is a mod to your amp , show a picture

Twin or Twin reverb ?


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Post subject: Re: 72 twin phase inverter circuit
Posted: Mon Jun 18, 2018 8:24 pm
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It is part of the global feedback network in those Master Volume amps. You can't remove it without adversely affecting the amp's tone and maybe stability.

Pretty sure there are sites, on the Net, on how-to-do complete and correct reversion of these circuits back to the AB763 one. If that is what you are after.


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Post subject: Re: 72 twin phase inverter circuit
Posted: Tue Jun 19, 2018 1:26 am
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BMW2002Ti wrote:
It is part of the global feedback network in those Master Volume amps. You can't remove it without adversely affecting the amp's tone and maybe stability.

Pretty sure there are sites, on the Net, on how-to-do complete and correct reversion of these circuits back to the AB763 one. If that is what you are after.



Feedback is not at Phase Inverter output as rborja3 write


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Post subject: Re: 72 twin phase inverter circuit
Posted: Tue Jun 19, 2018 3:58 am
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See your other post here: https://forums.fender.com/viewtopic.php?f=12&t=109964

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Post subject: Re: 72 twin phase inverter circuit
Posted: Tue Jun 19, 2018 5:19 am
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It is the 135 watts amps and as I wrote before this negative feedback is at Phase Inverter Input not output.

Did you see my picture ?

Image[url=https://flic.kr/p/28hrvA9]Capture d’écran 2018-06-19 à 08.14.58

Schematic here ;
http://el34world.com/charts/Schematics/ ... _schem.pdf


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Post subject: Re: 72 twin phase inverter circuit
Posted: Tue Jun 19, 2018 6:30 am
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I see your picture, Louis.

rborja3 wrote:
maybe somebody here can help.

on the output of the phase inverter lies a 220k + .01 cap to ground via 100ohm. but its only on one side. anybody know what its for? its in the 100w MV circuit but not in the AB763, AA769 or AA270. i want to remove it.

my 72 twin has a MV but no push/pull.

/ron


Complete schematic here http://schematicheaven.net/fenderamps/fender_quadreverb_100w_nomstrvol.pdf

Image

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Post subject: Re: 72 twin phase inverter circuit
Posted: Tue Jun 19, 2018 6:34 am
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vinyl wrote:
I see your picture, Louis.

rborja3 wrote:
maybe somebody here can help.

on the output of the phase inverter lies a 220k + .01 cap to ground via 100ohm. but its only on one side. anybody know what its for? its in the 100w MV circuit but not in the AB763, AA769 or AA270. i want to remove it.

my 72 twin has a MV but no push/pull.

/ron


Complete schematic here http://schematicheaven.net/fenderamps/fender_quadreverb_100w_nomstrvol.pdf

Image


+1000


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Post subject: Re: 72 twin phase inverter circuit
Posted: Wed Jun 20, 2018 6:10 am
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its a 72 twin reverb with master volume, but no push pull. see the pic below of the RC combo im talking about (circled). is this for oscillation purposes?

Image


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Post subject: Re: 72 twin phase inverter circuit
Posted: Wed Jun 20, 2018 6:34 am
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Image

Are you talking about the cap and resistor network in the red box? It appears to be a local feedback tapped from PI cathode and GNFB circuit, ahead to the 6L6GC stage. Prolly intended to maintain linearity.

Since the rest of the PI stage is setup to utilize this feedback control. It is fed the global feedback & therefore seems part of that circuit. You can't just remove it without changing the tone. Maybe for the worse.

I'd say, if you want more of the "blackface" AB763 tone... you need to fully restore the PI stage back to AB763 parameters. Including feedback schemes, plate resistors (and voltages), and cathode setup.

Here is schemo of the AB763.

http://www.electronicstudio.net/schemat ... _schem.pdf


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Post subject: Re: 72 twin phase inverter circuit
Posted: Wed Jun 20, 2018 3:01 pm
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Even if you are trying to return this amp to AB763, they're notoriously clean amps. That's why they are sought after by pedal steel and keyboard players. If you want dirt, plug in a pedal. I'd leave it as is.

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Post subject: Re: 72 twin phase inverter circuit
Posted: Wed Jun 20, 2018 4:34 pm
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vinyl wrote:
Even if you are trying to return this amp to AB763, they're notoriously clean amps. That's why they are sought after by pedal steel and keyboard players. If you want dirt, plug in a pedal. I'd leave it as is.


+1

A speaker upgrade would make more sense.

Arjay

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