Vibrosonic Resurrection project

Wayne Garrett · From: Alberta, Canada

Hi Friends,

I recently came into possession of a silverface Vibrosonic Reverb that had been "converted" from a 1x15 to a 2x12 (basically a master volume twin). I tracked down a used JBL d130f (15" 8ohm) and am excited to change things back over to restore this amp to it's original glory. Before I get too far into it though, I wanted to run a couple questions by y'all.

First of all, thanks to everyone on this forum for their great advice, and for generously sharing your experience and expertise. It's such a wonderful online community and I am super grateful for all of y'all.

Ok, back to the amp...

So somebody before the previous owner "converted" it to a 2x12, but the more I look into it it seems like all they did was swap speakers. It has 2 12" 8ohm fender speakers in it when I got it, they're wired in parallel (4ohm total impedance) and the resistance across the speaker plug (both speakers together)reads about 3ohm on my multimeter.

This got me wondering if the output transformer had been swapped. A stock vibrosonic uses an 8ohm output transformer, while a twin uses a 4ohm I believe. I'm also aware that running a speaker load that is at a lower impedance than the output transformer is a NO-NO and can dammage your amp.

However, upon further inspection, the amp still has the stock Vibrosonic Output transformer Fender # 022897. The amp sounds good through the 2x12 setup, so I'm not too worried that it has suffered any significant damage yet. Ive attached some photos for reference in the comments below.

My questions are as follows:

1) is there any way someone has somehow modified this to be a 4ohm amp without swapping the OT (all?of the power tubes are still in place)

2) I noticed some fresh shiny looking caps, and I know the previous owner has it serviced by a tech at some point. Does anything look out of the ordinary here? Or does this just look like standard cap replacement service?

3) anything else I should be thinking of while going through this swap? Im gonna make a new baffle to fit the 15" JBL and have referenced some photos of stock vibrosonic amps to get an idea of how to fit it without interfering with the the transformers.

Thanks a million for reading this far, I appreciate all y'all taking the time!

Wayne Garrett · From: Alberta, Canada

Ken Fox · From: Nashville GA USA

Does the amp have a removable baffle? If not it is a later Twin Reverb cabinet and you will not be successful changing the baffle. The original amp would have had the speaker offset to the right and positioned to miss the transformers

Here’s an old photo of a smaller cabinet showing the rotation they used to avoid the transformers

1) is there any way someone has somehow modified this to be a 4ohm amp without swapping the OT (all?of the power tubes are still in place) No

2) I noticed some fresh shiny looking caps, and I know the previous owner has it serviced by a tech at some point. Does anything look out of the ordinary here? Or does this just look like standard cap replacement service? Not that I see

3) anything else I should be thinking of while going through this swap? Im gonna make a new baffle to fit the 15" JBL and have referenced some photos of stock vibrosonic amps to get an idea of how to fit it without interfering with the the transformers. See above

Dave Grafe · From: Hudson River Valley NY

Looks straight and clean to me, without seeing the psu caps.

If it's the original OT it is designed for 8 ohms and someone took liberties with it to run the 2x12 load. Put your 8 ohm JBL in there and you are golden.

Wayne Garrett · From: Alberta, Canada

Thanks for the replies y'all, while I've got your attention... any thoughts on what material to build the new baffle from? Been doing some reading and am leaning towards 5/8 Baltic birch ply, but would be grateful for your thoughts.

Bill A. Moore · From: Silver City, New Mexico, USA

Leo used 1/2" baffle with a 1/4" "plant-on" on the perimeter to keep the grill from flapping and hitting the baffle. (I know, I wouldn't hear it either!) When converting my Twin Mojo cab to a 15, I used 1/2" ply, and it worked fine. I did clock the speaker to clear the trannies!

Abe Levy · From: California, USA

If I were you I’d have a whole new cabinet built. I did and it made my Vibrosonic lighter and sound better. Those particle board cabinets are horrible. You won’t regret a new cabinet.
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Mostly Pre-1970 guitars.

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