Fender Vibro Channel out of phase?

Karl Oberlander · Post by **Karl Oberlander** » 26 Oct 2003 7:02 am

I was told recently that on all Fender amps with a Vibro channel, the Vibro channel is out of phase. Meaning that if you put a signal through the Normal channel it will push the speaker cones out and if you do the same thing on the Vibro channel it will pull the speaker cones inward. i. e. the opposite of the Normal channel. Is this truw and what does that do to the tone of the Vibro channel? Also, can it be reversed?

I'm using a SF Super Reverb and I want to use the Reverb on the Vibro channel.

Thanks in advance for any info.

Kobe

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Gibson D-8 Console Grande - Stringmaster T-8 - Alkire EHarp D-10
Fender Super Reverb
kobe@austin.rr.com
http://home.austin.rr.com/kobeco

Jon Light (deceased) · Post by **Jon Light (deceased)** » 26 Oct 2003 7:27 am

Yes. If you run the same signal into both channels of a trem. or reverb Fender simultaneously they will be out of phase. This manisfests as a thinner, weaker signal as you turn them up. No harm done except to the tone. Yes, it can be reversed. Not by a layman but it's not a major job for a technician.

Note that the issue is feeding the same signal to both sides. Two different instruments on either side presents no such problem.

Ken Fox · Post by **Ken Fox** » 26 Oct 2003 8:27 am

It requires de-soldering one wire and moving it to a new location. You will then have reverb/tremolo on both amd the added gain stage for channel one will now have them in phase.
Just remove the wire that goes from the .047 cap off of tube #1 (gain stage #2) to the 200K summing resistor. Place that wire at the output side of the .02 cap, that is on the output of tube #2, gain stage #2. This can be done at the juction of the 3.3M and the 10 pf cap, along the top side of the board, closer to the volume control panel.
I have done three of these mods in the last two weeks. Sounds and works great. The amps were a 1969 Super Reverb, 1969 Deluxe Reverb and a 1970 Dual Showman Reverb.
If you use the amp for steel, mod channel #2 slope resistor to 56K. You now have a guitar and steel channel, both with reverb!
If the steel is really hot and overdriving the input, use input jack #2 on channel #2 (padded down) or you can mod it like the 90's Vibrasonic by removing the cathode bypass cap on channel #2 preamp tube #2, stage 1. I do not prefer that mod, tried it on the bench and preferred just using the #2, padded input.

Karl Oberlander · Post by **Karl Oberlander** » 26 Oct 2003 9:50 am

Thanks guys,

I do not "ever" plug anything into the Normal channel since I have two of these '67 AB763 Super Reverbs.

One I have moded to the steel spec on the Vibro channel already with the 56k slope resistor and the silver mica 250pF cap replacing the old ceramic treble cap. I switched out the bass amd mid EQ pots for a 1M and a 25k as well. Also, on this one I moded it to take a pair of 6550's and it really sounds great.

I did the same mods on the other one except I switched the bypass cap on the Vibro channel to 250Mf x 6v. This one I left with the standard 6L^'s and it realy sounds good with a Tele or Strat.

I also seperated the cathodes so both channels are independent. Both amps are incredibly clearer and way louder than they used to be.

I was just wondering how that out of phase thing worked but it looks like I'm OK as is now.

Thanks again for the info.

Kobe

------------------
Gibson D-8 Console Grande - Stringmaster T-8 - Alkire EHarp D-10
Fender Super Reverb
kobe@austin.rr.com
http://home.austin.rr.com/kobeco

Earnest Bovine · Post by **Earnest Bovine** » 26 Oct 2003 11:51 am

Why would you care?

Karl Oberlander · Post by **Karl Oberlander** » 26 Oct 2003 1:00 pm

Curiosity is all. I hadn't heard of the out of phase thing and wondered how it worked.

Kobe

------------------
Gibson D-8 Console Grande - Stringmaster T-8 - Alkire EHarp D-10
Fender Super Reverb
kobe@austin.rr.com
http://home.austin.rr.com/kobeco

C Dixon · Post by **C Dixon** » 27 Oct 2003 5:23 am

The above is true.

However inovative persons which I believe most musicians are, have found ways to turn "out of phase" problems actually into some desired results.

By playing around with the volume and tone controls and listening carefully a "spacial" effect can be achieved. If one plays Rock with a distortion box on, this is particularly true.

"What hath man wrought"?

carl

Hiro Keitora · Post by **Hiro Keitora** » 28 Oct 2003 4:38 pm

Hey guys, can you back up a little, and tell me which resistor I should change to 56K for steel? <Hiro>

Gino Iorfida · Post by **Gino Iorfida** » 28 Oct 2003 5:30 pm

Ken> thanx for saving me having to type the full page reply

Ken Fox · Post by **Ken Fox** » 28 Oct 2003 6:19 pm

Gino, it was a pressure, I mean pleasure! Always glad to jump in and confuse the issue where I can!!!

The 56K would replace a 100K in the tone stack of channel 2. It is at the junction of the 100K plate load resistor on the 2nd half of tube #2. You can download a schematic and a board layout for the amp from the Fender Amp Field Guide:
http://www.ampwares.com/ffg/index.html

If you can not find it there, then try http://www.schematicheaven.com

Hiro Keitora · Post by **Hiro Keitora** » 29 Oct 2003 3:23 am

Yeah, but which 100K thou? The one between 250pf and .1 ?

Jay Ganz · Post by **Jay Ganz** » 29 Oct 2003 6:47 am

Yeah, that's the one. It's called the slope
resistor.