Scary SF 73 Twin in for service

[Madison]

Man, where to start?
Might be fun for a discussion anyways.

I haven't had enough of these in here to know but, that cannot be the original wiring!!!???(God help us if it is)
Obvious to me, some caps (tremolo) are updated.
How about those Mallories? Oldies?
Board warped like crazy.
Doghouse caps leaking.
Tubes old RCAs.
Sounds like crap and weaker than a 40 watt amp.
Still pretty quiet as far as hum goes.

Maybe I am being to harsh on this beast.

Looking for an aged, seasoned, eye to have a peek and say what is original or not....at a glance.
Oh Crikey! Fender put wax on components?? 

I used to have a SF Pro Reverb that sounded great after a simple re-tube.
Me thinks this beast is going to need a lot more love than that amp.

Thanks and peace all

[Madison]

#2

[Madison]

#3

[Geezer]

Ha ha!  

Looks like a couple of caps might have been changed, but yep....that's stock wiring for that era of Fender amps.

How do I know? I have TWO of them (a 100w and a 135w UL) right here!
A friend gave them to me (closing down his music store after 33 years in business) and I can't decide what to do with them....part them out(?), build a Dumble clone or "inspired" amp(?)

Anyway,  THAT is what CBS did to Fender after the takeover.

G

[Zipslack]

Yep, that looks just like the late-70's Pro Reverb I've got opened right now for various issues....pretty nasty/sloppy in there.

[Ed_Chambley]

I bought one a few weeks ago that looks just like this.  I already had a AB763 board with some mallorys and orange drops and metal film resistors.  Just replaced the board and and updated the bias section to fixed/adjustable.  I did remove the pots and cleaned the inside of the chassis.  This one did have fairly new F&T filters.  Great sounding amps, but I do love a BF twin.  Nothin twangs like a twin IMO.

They are not bringing much money round here.

[Willabe]

but, that cannot be the original wiring!!!???

Yep, like the other guys have already said, it sure is.


             Brad    

[echuta13]

Had one like that.      Their exceptional wiring is one of the reasons why they had to add several caps to their builds in order to prevent them from oscillating. 

[Madison]

Ok, well.....this should be an adventure.
Peace all

[LooseChange]

Hey Madison,
The amp looks almost factory. The trem caps are changed out.
Does it hum? Whats the B+?
Do a complete cap job. All electrolytics. Don't worry about blackfacing it or changing that bias circuit until after you do the cap job.
All may be just right.

[HotBluePlates]

Looking for an aged, seasoned, eye to have a peek and say what is original or not....at a glance.

It looks 100% original to me, for an early 70's Twin. Except for the trem caps, as LC pointed out.

And yes, in the past I've gutted all that stuff, but that might not be necessary, the parts/wiring layout is bad in those.

[Madison]

Hey guys, thanks for chiming in.

The power caps are definitely leaking like mad just looking at them.

No obvious hum issues.
PT is about spec.
Everything seems to work as expected.

I am thinking like you LC and HBP.
I went ahead and wrote up an estimate for the guy.
Replace-
power tubes, power caps, bypass caps, and see what happens.
If it goes beyond that I suggested possibly replacing the board..........which may exceed his budget, I worry.

I am not a total SF hater.......besides the wiring.

Thanks guys.
I'll try to report back.

[gldtp99]

Here's a '74 Twin Reverb that I recently re-did-------I got a different cab off ebay--- the orig one had the baffle cut out and a 1x15 baffle crudely installed--- the cab was ugly and ready to fall apart--- so I got the cab in the vid and installed new WGS speakers----- I installed new filter and bias caps---new cathode bypass caps on circuit board-----converted to non pull MV because orig pull pot was falling apart---- new output tubes.
Yes, I'd prefer to work on older BF Fender amps but this SF Twin is good sounding and powerful----it's making its 100 watts.
Vid: Fender Twin Reverb 1974

[Ed_Chambley]

It is not necessary to convert it to an AB763 if it is not in the budget, but I could not get a proper bias with the balance pot.  It was cold and sounded like crap.  I have tons of 6l6 and even the hottest ones I have would not get the amp to open up.  I was not fixing it for a customer tho.  I was fixing it to sell.  Got it for 350 dollars and it is for sale for $1100 now.

When installing the new board, I did not have to do anything to the lead dress other than move and bend the wires where there were parallel or crossing at 90.  I did however shorten a couple and added the 1 wire reverb to the normal channel.

Players prefer Blackface Fenders for a reason, but people are paying big bucks for silvertone guitars too.  IMHO, silverface amps sound good if in good shape, but do not compare to the AB763.

[gldtp99]

Another vid of the '74 Twin I re-did------ this one has my friend Harry playing a Gibson 335----the other vid is my friend Bobby playing an Ibanez 7 string semi hollow body.
It is possible to change the bias range on these amps by changing the value of the bias ground resistor, located on the bias balance pot--- Instead of rebuilding the whole bias circuit to BF specs----- I think these SF 100 watt non UL Twins sound great just the way they are---no need for radical circuit changes--- the ratty looking wiring doesn't seem to effect the tone.

Vid: 1974 Twin Reverb

[gldtp99]

My friend Harry again--- same SF Twin------ with a Tele:
1974 Twin Reverb w/Telecaster

[birt]

Hey guys, thanks for chiming in.

The power caps are definitely leaking like mad just looking at them.

No obvious hum issues.
PT is about spec.
Everything seems to work as expected.

I am thinking like you LC and HBP.
I went ahead and wrote up an estimate for the guy.
Replace-
power tubes maybe not needed at first. make it work with the old ones and go from there, power caps, bypass caps yes and yes, and see what happens.after replacing those caps check if any of the coupling caps are leaking, if there is hum check heater wiring orientation, for hiss/crackles the plate resistors and/or dirty tube sockets.
If it goes beyond that I suggested possibly replacing the board..........which may exceed his budget, I worry.

I am not a total SF hater.......besides the wiring.

Thanks guys.
I'll try to report back.

after all standard checks i suggested i think this amp will sound just fine. if there is still hum or crackle search further. when you get it working well you could replace the power tubes to make it sound better. i've never seen a twin where this was really needed for it to work well. and i don't care too much about a board looking bad. (i did a lot of work in maintenance of vintage twins for a rental company. they were on the road ALL the time.

[LooseChange]

I can only think of one reason to install a new board... If it was conductive.
You can do anything in these amps on the boards that are installed.
You want to keep the repair bill low. Do the caps, check the tubes and replace as needed. If bias is a problem, with the bias balace knob, you can either change it to a bias level or just swap out the bias level resistor.
I have worked on many of these. Lead dress is usually not an issue and rarely need to adjust... Except around the tremolo tube.

[Madison]

I can only think of one reason to install a new board... If it was conductive.
You can do anything in these amps on the boards that are installed.
You want to keep the repair bill low. Do the caps, check the tubes and replace as needed. If bias is a problem, with the bias balace knob, you can either change it to a bias level or just swap out the bias level resistor.
I have worked on many of these. Lead dress is usually not an issue and rarely need to adjust... Except around the tremolo tube.

I went a head and gave a "budget option" basically as you described.
Seems the most logical way to get it tonally cleaned up at a decent price.
Thanks!

[gtrplayr1976]

I bought one a few weeks ago that looks just like this.  I already had a AB763 board with some mallorys and orange drops and metal film resistors.  Just replaced the board and and updated the bias section to fixed/adjustable.  I did remove the pots and cleaned the inside of the chassis.  This one did have fairly new F&T filters.  Great sounding amps, but I do love a BF twin.  Nothin twangs like a twin IMO.

They are not bringing much money round here.

Hi, I'm getting ready to replace the board in my friends  73 SF Twin Reverb with MVS.  Do you have any gut shots of the project ? I just purchased a Twin board from Doug ,and since it's a different layout than the fiberboard I could really use some help.

Thank you

[jim]

Madison--You should warn your customer about the speakers.  When you get this up to full power those ancient brittle paper cones
might not be up to twangin'.   Price out some new ones or recones. From the looks of the chassis this amp has not been operated at full power for a while.  Jim










 

[Madison]

Madison--You should warn your customer about the speakers.  When you get this up to full power those ancient brittle paper cones
might not be up to twangin'.   Price out some new ones or recones. From the looks of the chassis this amp has not been operated at full power for a while.  Jim

Yupe, already warned him that something else might go once this thing starts to function properly again.
Waiting for his decision on direction.
Peace








 

[Ed_Chambley]

I bought one a few weeks ago that looks just like this.  I already had a AB763 board with some mallorys and orange drops and metal film resistors.  Just replaced the board and and updated the bias section to fixed/adjustable.  I did remove the pots and cleaned the inside of the chassis.  This one did have fairly new F&T filters.  Great sounding amps, but I do love a BF twin.  Nothin twangs like a twin IMO.

They are not bringing much money round here.

Hi, I'm getting ready to replace the board in my friends  73 SF Twin Reverb with MVS.  Do you have any gut shots of the project ? I just purchased a Twin board from Doug ,and since it's a different layout than the fiberboard I could really use some help.

Thank you

The amp is sold and I usually do not take photos.  I guess I should start, but I usually only take photos of scratch builds and I have only used Hoffman layouts in 2 amps.  It is the same as many others have built.  I made an eyelet board just like the original fiberboard.  A lot of people put the filter caps inside the chassis rather than using a doghouse when using a Hoffman Board.  Look in the photo section of people's builds and I would assume you will see an AB763 Hoffman Board.  I find if I am just replacing a board, the fender layout works best because once built all you have to do is lift the wires from the board and after installing the new one the wires are right there.  Not much thinking involved other than predating the bias section.

[gtrplayr1976]

I bought one a few weeks ago that looks just like this.  I already had a AB763 board with some mallorys and orange drops and metal film resistors.  Just replaced the board and and updated the bias section to fixed/adjustable.  I did remove the pots and cleaned the inside of the chassis.  This one did have fairly new F&T filters.  Great sounding amps, but I do love a BF twin.  Nothin twangs like a twin IMO.

They are not bringing much money round here.

Hi, I'm getting ready to replace the board in my friends  73 SF Twin Reverb with MVS.  Do you have any gut shots of the project ? I just purchased a Twin board from Doug ,and since it's a different layout than the fiberboard I could really use some help.

Thank you

The amp is sold and I usually do not take photos.  I guess I should start, but I usually only take photos of scratch builds and I have only used Hoffman layouts in 2 amps.  It is the same as many others have built.  I made an eyelet board just like the original fiberboard.  A lot of people put the filter caps inside the chassis rather than using a doghouse when using a Hoffman Board.  Look in the photo section of people's builds and I would assume you will see an AB763 Hoffman Board.  I find if I am just replacing a board, the fender layout works best because once built all you have to do is lift the wires from the board and after installing the new one the wires are right there.  Not much thinking involved other than predating the bias section.

How do I find the photo section ? I don't see a place for people's builds ? 

Thanks

[sluckey]

How do I find the photo section ? I don't see a place for people's builds ?

Click on the "Media" link at the top of this page.

[gtrplayr1976]

How do I find the photo section ? I don't see a place for people's builds ?

Click on the "Media" link at the top of this page.

Well geez...I feel like an idiot... Thanks a heap Sluckey !

[Ed_Chambley]

I bought one a few weeks ago that looks just like this.  I already had a AB763 board with some mallorys and orange drops and metal film resistors.  Just replaced the board and and updated the bias section to fixed/adjustable.  I did remove the pots and cleaned the inside of the chassis.  This one did have fairly new F&T filters.  Great sounding amps, but I do love a BF twin.  Nothin twangs like a twin IMO.

They are not bringing much money round here.

Hi, I'm getting ready to replace the board in my friends  73 SF Twin Reverb with MVS.  Do you have any gut shots of the project ? I just purchased a Twin board from Doug ,and since it's a different layout than the fiberboard I could really use some help.

Thank you

The amp is sold and I usually do not take photos.  I guess I should start, but I usually only take photos of scratch builds and I have only used Hoffman layouts in 2 amps.  It is the same as many others have built.  I made an eyelet board just like the original fiberboard.  A lot of people put the filter caps inside the chassis rather than using a doghouse when using a Hoffman Board.  Look in the photo section of people's builds and I would assume you will see an AB763 Hoffman Board.  I find if I am just replacing a board, the fender layout works best because once built all you have to do is lift the wires from the board and after installing the new one the wires are right there.  Not much thinking involved other than predating the bias section.

How do I find the photo section ? I don't see a place for people's builds ? 

Thanks

This looks like a Hoffman Layout in a Super Reverb.  Very similar.

[gtrplayr1976]

Thanks Ed ,but that one looks a lot different than mine. I may just build the board like the original to get it done for my friend.

[sluckey]

There is a lot of info right here on Hoffman's site to help you build an AB763 TR. Look in the library.

[Madison]

Did power caps, tubes, cath b/p caps.
Decent increase in power.....not ear splitting, but good.
Speakers seem in good working order.
Sounds pretty "ratty" when it gets up above 8 on the volumes, especially with the MV pulled out.(maybe the norm for these MV models?)
Some things hardly make a difference; ie bright switches. Lacking "chime" for sure.
I'll see what the customer wants to do.
I suppose a full cap job would bring life back into it.

[Willabe]

Which model is it exactly? Does it have the dreaded parasitic oscillation caps on the grids of the OP tubes to ground?


              Brad     

[Madison]

Not sure.
How can it be ID-ed for which model?
Is it in the serial # somewhere or....?

No signs of those caps.

Thanks a lot.

[HotBluePlates]

How can it be ID-ed for which model?

Model number is on the tube chart, and the circuit should be verified against the various Twin Reverb schematics (AB763, AB568, AA769, AA270).

[Madison]

That's what I thought.I thought it is usually just stated what model.
Maybe I am missing something here?
Exports didn't get a model number?