Franken...Twin?

[G-Yo]

My main question is - did Fender make a 70s Twin Reverb with 8ohms output?
If not, then what the heck is my amp?

Long story, but here's the summary. Bought it years ago (for cheap) about 15 years ago from a craigslist ad. It was banged up, but it worked and I just wanted a silverface twin. Sounded good. Played it gently, never gigged with it, etc. Tremelo and reverb did not work, but I used pedals and didn't worry about that. But, why did I need such a loud and heavy amp? That of course is a GASsy rabbit hole question for another thread...

I had figured out that the cabinet, speakers, reverb tank were not original. Maybe 90s reissue or something, but the chassis looked legit 70s, and the transformer codes dated it to '75 (plus a '73 choke). Back plate says 100W power, which I think aligns with a 73-77 (or there about) date. It was having some crackly, buzzy issues last few years. So I took it to a tech #1 - let's fix the buzz, and while we're at it maybe fix the tremolo and reverb. Tech #1 fixed the buzz (caps, new tubes, cleaned out pots, etc), but he gave up on the vibrato/reverb channel. Took it to amp tech #2. He opened it up and said someone has really hacked this thing up - mods upon mods. I believe him, and no it wasn't me who did the hacking, and I don't think it was tech #1. Tech #2 fixed the vibrato channel, reverb (replaced reverb transformer), and cleaned up some things. So, it works now - yay! The big thing he noted was - hey, this thing wants to see 8ohms, and the pair of blue label fender speakers are each 8ohms running parallel (so 4 ohms).

Oops! How did I not notice that? And how did tech #1 not notice that? So, on tech #2's advice, I disconnected one speaker so that the amp sees the 8ohm load it wants. I know that if I want to use 2 speakers, I should probably get a pair of 16 ohm speakers and wire them parallel. Speaker replacement discussion tabled for now, though...

BUT - now I'm confused. I've googled a bit and can't seem to find where fender ever made a twin reverb in the mid 70s that was 8ohm output, with 100W power. So, back to my question to the collective TDPRI expertise: is this thing really a Twin Reverb? What else would it be? Did somebody take another 70s fender amp, monkey with it, and slap on a silverface twin face plate?

Any thoughts are appreciated. And be kind - I do have some knowledge, but I'm clearly not an amp historian or tech.

 

[Dacious]

8 ohm output? Vibrosonic output transformer. Single 15" speaker in the VS, Otherwise it's a Twin Reverb wholus bolus. Your amp may be a Vibro with the TR faceplate.

 

[Ten Over]

What are the numbers on the output transformer?

 

[schmee]

Did the tech determine "this things needs to see 8 ohms" by tracking the OT model number, or just looking at the rear panel?.... how does he know?
One wonders if the factory just misapplied a Vibrosonic rear panel on assembly...

BUT ....If it was heavily messed with and non original cab and speakers, then yeah, the rear panel label could be any sort of thing. Those rear labels from BF into SF were all nearly the same.

How did you figure the cab was not original? Have you investigated the OT part number for what it actually is?

 

[G-Yo]

What are the numbers on the output transformer?

OT xformer numbers:
022897
606-3-31

There is some sort of black marking over the top number, but I think that's it.

 

[G-Yo]

Did the tech determine "this things needs to see 8 ohms" by tracking the OT model number, or just looking at the rear panel?.... how does he know?
One wonders if the factory just misapplied a Vibrosonic rear panel on assembly...

BUT ....If it was heavily messed with and non original cab and speakers, then yeah, the rear panel label could be any sort of thing. Those rear labels from BF into SF were all nearly the same.

How did you figure the cab was not original? Have you investigated the OT part number for what it actually is?

Tech #2 determined 8ohms by taking a measurement while running the amp. I'm not totally sure what he was measuring (bias at different loads? voltage while running?). He didn't necessarily just trust the label on the panel.

Regarding the cabinet, I have no guarantee, but:
The are some old mounting hardware holes on the top of the amp that don't match the installed mounting hardware.
Cab constructuion feels like MDF or fiberboard baffle (no grain inside like plywood would have)
It has that velcro strap inside the cabinet to hold a footswitch. I don't think that's a 70s feature.
The handle looks too new (but could be a replacement itself)
The speaker codes suggest 1990s speakers (sure, they could be replacements)
Tolex just look too new to be 70s

Right, I've not investigated the OT number, except here on this post.

Thanks!

 

[glenlivet]

8 ohm output? Vibrosonic output transformer. Single 15" speaker in the VS, Otherwise it's a Twin Reverb wholus bolus. Your amp may be a Vibro with the TR faceplate.

Did the vibro's have a master volume? (never had one, so I truly don't know?) I always thought the master volume was just a twin thing ?

edit: a quick google search says "yes, yes they do" Looks to me like the vibrosonic with a twin faceplate is probably the correct answer.
and could explaine why the techs were thinking that it was a twin that had been severly modded?

 

[schmee]

Tech #2 determined 8ohms by taking a measurement while running the amp. I'm not totally sure what he was measuring (bias at different loads? voltage while running?). He didn't necessarily just trust the label on the panel.

Regarding the cabinet, I have no guarantee, but:
The are some old mounting hardware holes on the top of the amp that don't match the installed mounting hardware.
Cab constructuion feels like MDF or fiberboard baffle (no grain inside like plywood would have)
It has that velcro strap inside the cabinet to hold a footswitch. I don't think that's a 70s feature.
The handle looks too new (but could be a replacement itself)
The speaker codes suggest 1990s speakers (sure, they could be replacements)
Tolex just look too new to be 70s

Right, I've not investigated the OT number, except here on this post.

Thanks!

Baffles were particle board stock in BF and most SF amps. A SF cab would be pine but not fingerjointed, there may be some later ply ones.
Yeah the speaker labels look like EMi's from what I can see. Probably decent speakers.
Here's what SF cabs look like at the joint: (After ~ 1968, and why rabbeting isn't as good as fingerjoints!)

And a finger joint:

 

[Jowes_84]

According to the 100W Silverface Schematic 022897 is correct for a Vibrosonic Reverb
Link here
https://www.thetubestore.com/lib/th...ender/Fender-Twin-Reverb-SF-100-Schematic.pdf
Pic attached
I have one in its original cab
The face plate is very worn so I would consider slamming a Twin Reverb Faceplate on like someone has done with yours

Great amp

 

[Jowes_84]

Did the vibro's have a master volume? (never had one, so I truly don't know?) I always thought the master volume was just a twin thing ?

edit: a quick google search says "yes, yes they do" Looks to me like the vibrosonic with a twin faceplate is probably the correct answer.

No.
Vibrosonic 100W came with MV
Yes.
It is.

 

[schmee]

022897 is indeed a Vibrosonic Reverb or Single Showman 100 watt 8 ohm transformer. That tranny may be worth something to someone who has a rare-ish single speaker Showman. ? I guess not if used on the VS.

I guess it must be a VS one because of the extra dash in the mfg code was mostly a 70's thing. So... 1973

 

[G-Yo]

Baffles were particle board stock in BF and most SF amps. A SF cab would be pine but not fingerjointed, there may be some later ply ones.
Yeah the speaker labels look like EMi's from what I can see. Probably decent speakers.
Here's what SF cabs look like at the joint: (After ~ 1968, and why rabbeting isn't as good as fingerjoints!)
View attachment 1022124
And a finger joint:
View attachment 1022125

Nice pics of different joint conditions! Yeah, those rabbet joints just rely on the fasteners too much don't they...

I did just google the OT number, and it supports Dacious's suggestion that I may have a Vibrosonic Reverb disguised as a Twin.

 

[G-Yo]

022897 is indeed a Vibrosonic Reverb or Single Showman 100 watt 8 ohm transformer. That tranny may be worth something to someone who has a rare-ish single speaker Showman. ? I guess not if used on the VS.

Looks like it was used on the VS, too.

 

[Jowes_84]

022897 is indeed a Vibrosonic Reverb or Single Showman 100 watt 8 ohm transformer. That tranny may be worth something to someone who has a rare-ish single speaker Showman. ? I guess not if used on the VS.

Check the link. I think so. Interesting though.

 

[Jowes_84]

Sorry too many posts crossing to quickly. I think we got it though
Have fun with the amp. Imho running the two speakers in parallel and the amp with a 4 Ohm load is not the most terrible idea in the world. I recently converted my Vibrosonic Reverb to a Head and occasionally use it with a 4 Ohm 2x15. No issues so far.

 

[schmee]

Nice pics of different joint conditions! Yeah, those rabbet joints just rely on the fasteners too much don't they...

I did just google the OT number, and it supports Dacious's suggestion that I may have a Vibrosonic Reverb disguised as a Twin.

Or the OT has just been replaced at some point. But yeah, if the cab doesn't seem correct, may be the VS with a Twin faceplate.
Trying to think if there are any other features which may indicate the difference.... The NFB resistor may be a different value for 8 ohm vs 4 ohm output......

 

[G-Yo]

Y'all are awesome.

Output Transformer Chart for Fender Guitar Amps

Transformer part Numbers & Output Impedances

www.bustedgear.com

The Vibrosonic idea seems to line up! Very Twin-like, similar panel, same control knobs. But it's 8 ohms, and the OT matches the VS. The newer cab/speaker makes more sense now.

So maybe somebody had a VS and for some reason (maybe damaged cabinet/speaker?), they tried to make a twin out of it. They put it in a 90s 2/12 cabinet with Eminence's, drilled new mounting holes, and even found a SFTR faceplate to stick on. This also might explain why the faceplate is sort of un-sticking at one end.

That's my working theory until there's a better one.

Thank you all!!!

 

[Dacious]

Occam's razor says it's a Twin maybe with blown output trans and someone didn't realise or didn't care that the output was wrong or just billy bobbed it.

It's possible it belonged to someone who had two and exchanged chassis in cabinets. But the original speakers and cosmetics coupled with hack-butchery indicates half-assedness to me. Someone whose abilities didn't match their estimation.

 

[Paul G.]

Since it's not the original cab, another vote for a Vibrosonic rehoused in a Twin cabinet. Yes, it is possible to determine the OT's output impedance by inputting a known AC voltage at the primary, measuring the AC voltage at the secondary and calculating the turns ratio.

Although less than ideal, you can run that with both speakers in parallel.