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Post subject: SFDR "Modification"
Posted: Thu Aug 13, 2015 2:23 pm
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I read the following from FenderGuru.com and tried it. I really like the way my SFDR sounds now. That being said, I'm wondering if this will harm the amp?

"If you are like most players and only use the Vibrato channel (reverb, tremolo, the bright cap and the extra gain stage), you should pull out the V1 tube. This is the preamp tube for the normal channel which you are not using when playing the Vibrato channel. Vice versa; If you’re using the Normal channel, you can pull out the V2 tube. All AB763-similar circuits (Deluxe Reverb, Super Reverb, Pro Reverb, Twin Reverb, Vibroverb, Vibrolux) are designed so that they share the cathode cap and resistor (25 uF/1500 ohm) and pulling one of the tubes will changes the effective value of the resistor they both share. If you pull one tube the other channel’s tube will be hotter biased and offers more gain. The amp will play louder than before given the same volume knob setting. The stronger signal will push the second gain stage (V4 tube) harder and give you increased sustain, compression and harmomics. This mod does not change the amp’s clean headroom but increases the preamp gain and preamp distortion.

This mod is one of Cesar Diaz’ tricks in the Fender Custom Shop Vibroverb 64 which he always did to Stevie’s amps. This mod is safe. It has stood the test of time and been been done by players in 50 years in blackface Fender amps."


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Post subject: Re: SFDR "Modification"
Posted: Thu Aug 13, 2015 3:21 pm
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No harm will occur. What this mod does is, it disables the "normal" channel which in turn frees up some heater current from the power supply to enable the remaining five pre-amp bottles to run a little hotter.

Arjay

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Post subject: Re: SFDR "Modification"
Posted: Thu Aug 13, 2015 4:56 pm
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Thanks Retro. I suspect it knocks a little off the tubes lifespan, but I can live with that.


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Post subject: Re: SFDR "Modification"
Posted: Thu Aug 13, 2015 6:36 pm
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djwhitjr wrote:
Thanks Retro. I suspect it knocks a little off the tubes lifespan, but I can live with that.


No. This should not shorten the life-span. The tubes are cathode biased, so they compensate (within a range) for changes to voltages. Think it this as running a dead tube in a channel you don't use. This should not shorten the life of the other good tubes.

What I believe the author of that article was saying... is if separate tube cathodes share resistors, pulling one tube will alter the idle bias of the one remaining in the circuit. But, no tube in the DR shares resistors across two tubes. And only the reverb driver shares one resistor across its two halves.

Hope that made sense... :lol:


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Post subject: Re: SFDR "Modification"
Posted: Fri Aug 14, 2015 3:30 pm
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Or each tube can be wired with it's own cathode bias also giving more gain! Also a tube sharing bias to both halves of a tube can also have it's own bias cathode Cap and Resistor. Giving it more gain.

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Post subject: Re: SFDR "Modification"
Posted: Fri Aug 14, 2015 5:59 pm
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The worst that can happen is that you give yourself a good scare. Months will go by and one day you'll decide to use Chanel 1 and nothing will come out.....your initial reaction will be "WTF", followed by "oh yeah, I pulled that dang tube", then a huge sigh of relief......not that it's ever happened to me......im just sayin. :oops:

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Post subject: Re: SFDR "Modification"
Posted: Fri Aug 14, 2015 6:40 pm
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Cap47 wrote:
Or each tube can be wired with it's own cathode bias also giving more gain! Also a tube sharing bias to both halves of a tube can also have it's own bias cathode Cap and Resistor. Giving it more gain.


+1, but possibly at the cost of a decreased S/N ratio for that stage.

Arjay

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