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Post subject: Re: Fender 1967 DUAL (in block letters) Showman
Posted: Wed Oct 09, 2019 7:17 am
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AFAIK, your Dual Showman has a circuit similar to the AB763 Twin Reverb. Negative voltage is fed off the bias supply diode to a portion of the 12AX7 driving the oscillation-timing part of the vibrato circuit. BUT, I believe the current draw on the bias supply is not as great as the output bias waggle of the Princeton's vibrato circuit. Which directly feeds the bias voltage to the output tubes.


TR AB763:
http://ampwares.com/schematics/twin_reverb_ab763.pdf

Princeton AA964:
http://ampwares.com/schematics/princeton_aa964.pdf


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Post subject: Re: Fender 1967 DUAL (in block letters) Showman
Posted: Wed Oct 09, 2019 11:09 am
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Retroverbial...

Yes I can READ a schematic but for many of us still learning, reading and understanding a schematic are two different knowledge level/skill level attributes.
If you are referring to your comment earlier and my question in it...
royb3988 wrote:
From a contributor to this thread... "The bias-supply current has no effect whatsoever on the tremolo circuit." Does it or does it not?"...I asked that question because I could plainly see on the schematic the foot switch has a wire running from it that hits that 2.2M ohm resistor that is then connected to the bias control pot...but I did not understand it.

"It's getting better all the time."

Royb3988


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Post subject: Re: Fender 1967 DUAL (in block letters) Showman
Posted: Wed Oct 09, 2019 1:45 pm
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The bias supply's only purpose is to provide the necessary -52VDC to the power tubes. That's all it does.

Do you see a *DIRECT* electrical connection between that -52VDC and the tremolo circuit?

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: Fender 1967 DUAL (in block letters) Showman
Posted: Wed Oct 09, 2019 7:17 pm
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[url]Image[/url]

Is that 100mF 100V capacitor in there backwards? In the photo the top leg goes to chassis ground, the bottom leg goes to the bias control pot. According to those arrows, shouldn't it be turned around?
RB3988


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Post subject: Re: Fender 1967 DUAL (in block letters) Showman
Posted: Wed Oct 09, 2019 10:55 pm
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royb3988 wrote:
According to those arrows, shouldn't it be turned around?
RB3988


Normally, yes. But the bias supply provides a negative voltage so for this application the cap is correctly installed.

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: Fender 1967 DUAL (in block letters) Showman
Posted: Wed Oct 09, 2019 11:28 pm
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http://ampwares.com/schematics/twin_reverb_ab763.pdf

There a line from the bias supply diode to the signal grid of one-half of 12AX7 triode via a 2.2meg ohm resistor. Not sure how this works. As it would be a negative voltage.

:?:


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Post subject: Re: Fender 1967 DUAL (in block letters) Showman
Posted: Thu Oct 10, 2019 9:12 am
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The negative voltage from that 2.2-Meg resistor is fed through the two 1-Meg resistors above it. This equalizes both sides of the .01 capacitor, preventing the oscillator from operating. This gives you a DC switchable circuit. The footswitch grounds the negative voltage to allow the cap to charge up and oscillate.


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Post subject: Re: Fender 1967 DUAL (in block letters) Showman
Posted: Thu Oct 10, 2019 4:25 pm
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I replaced the 1.6M ohm with a 2.2M ohm as called for in the schematic.

I left the amp off for 6 hours, turned it on and the Tremolo engages immediately after hitting the foot switch.
I'll leave it off for the next 18 hours, run another test...and report back tomorrow.

RB3988


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Post subject: Re: Fender 1967 DUAL (in block letters) Showman
Posted: Thu Oct 10, 2019 5:38 pm
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TimsAudio wrote:
The negative voltage from that 2.2-Meg resistor is fed through the two 1-Meg resistors above it. This equalizes both sides of the .01 capacitor, preventing the oscillator from operating. This gives you a DC switchable circuit. The footswitch grounds the negative voltage to allow the cap to charge up and oscillate.

Thanks Tim, for the explanation. :)


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Post subject: Re: Fender 1967 DUAL (in block letters) Showman
Posted: Fri Oct 11, 2019 10:24 am
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TimsAudio wrote:
The negative voltage from that 2.2-Meg resistor is fed through the two 1-Meg resistors above it. This equalizes both sides of the .01 capacitor, preventing the oscillator from operating. This gives you a DC switchable circuit. The footswitch grounds the negative voltage to allow the cap to charge up and oscillate.

It also means that the negative voltage is applied to both grids of the vibrato tube, effectively biasing the tube into "cutoff" when the footswitch is not grounded.

JMO,


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Post subject: Re: Fender 1967 DUAL (in block letters) Showman
Posted: Sat Oct 12, 2019 1:23 pm
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Hello All.....

A couple of resistors and capacitors later and the amp is fixed! I have left it off for 24 hours twice and the Tremolo engages immediately after hitting the foot switch...it sounds great. The Tremolo seems to have a greater intensity available and I can dial in a slower pulse than previously was available...no more motor boating either!

This has been quite a learning experience for me and to one and all I say thank you!

RB3988


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Post subject: Re: Fender 1967 DUAL (in block letters) Showman
Posted: Sun Oct 13, 2019 1:09 pm
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Anything further to report?

Arjay

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Post subject: Re: Fender 1967 DUAL (in block letters) Showman
Posted: Sun Oct 13, 2019 8:53 pm
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Hello Retro...

Nothing else to report for now...I am using the amp and it operates properly and sounds great.
Thanks.

RB3988


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Post subject: Re: Fender 1967 DUAL (in block letters) Showman
Posted: Sun Oct 13, 2019 8:56 pm
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Outstanding.

Rawk on!

Arjay

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