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Post subject: Silverface reverb unit 4th tube???
Posted: Sat Feb 01, 2014 6:06 am
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A few days back I was looking at the back of one of those re-issue Fender Reverb Units and it seemed pretty empty in there. I've had a Fender silverface Reverb Unit since the '70's and sure enough, when I checked it out I saw that it has 4 tubes instead of the 3 in the re-issue and evidently in the originals. The 3 tube setup seems pretty logical - one to drive the springs, one to pick up the sound and one to amplify it. What's the story on that 4th tube? BTW, that '70's unit has the sweetest reverb I've ever heard - everyone who listens to it is blown away!


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Post subject: Re: Silverface reverb unit 4th tube???
Posted: Sat Feb 01, 2014 9:04 am
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Redfarfisa wrote:
A few days back I was looking at the back of one of those re-issue Fender Reverb Units and it seemed pretty empty in there. I've had a Fender silverface Reverb Unit since the '70's and sure enough, when I checked it out I saw that it has 4 tubes instead of the 3 in the re-issue and evidently in the originals. The 3 tube setup seems pretty logical - one to drive the springs, one to pick up the sound and one to amplify it. What's the story on that 4th tube? BTW, that '70's unit has the sweetest reverb I've ever heard - everyone who listens to it is blown away!



We need pics man, pics!!

T2

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Post subject: Re: Silverface reverb unit 4th tube???
Posted: Sat Feb 01, 2014 12:08 pm
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Here's the '70s silverface schematics:
Image

Someone who has a deeper understanding of electronics will have to decipher it and explain why it has an extra tube


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Post subject: Re: Silverface reverb unit 4th tube???
Posted: Sat Feb 01, 2014 4:24 pm
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The final 7025 (12AX7) appears to be a zero-gain buffer stage. Prolly added to maintain a constant output impedance for the Reverb Unit. As the original 6G15 had the tone and mixer controls tied directly to the output. Adjusting these pots would effect the final impedance.

In theory, this buffer would allow a more linear response of the Unit's control settings and the tone injecting back into the main amp.

Also note: The grounding strap between input and output in the SF model? Thus, grounding out the unit, until a guitar is plugged into the Unit. I suppose a way to keep the Unit's injected noise to a minimum when moving it and/or the amp around.


6G15:

http://www.webphix.com/schematic%20heav ... _schem.pdf

SF Reverb Unit:

http://www.webphix.com/schematic%20heav ... _schem.pdf


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Post subject: Re: Silverface reverb unit 4th tube???
Posted: Sat Feb 01, 2014 5:59 pm
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BMW2002Ti wrote:
The final 7025 (12AX7) appears to be a zero-gain buffer stage. Prolly added to maintain a constant output impedance for the Reverb Unit. As the original 6G15 had the tone and mixer controls tied directly to the output. Adjusting these pots would effect the final impedance.

In theory, this buffer would allow a more linear response of the Unit's control settings and the tone injecting back into the main amp.


+1

Buffering and isolation stage.

One of the few instances where CBS's "tampering" actually resulted in a tangible improvement to an existing piece of Fender gear.

Arjay

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