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Ampeg SB12 authentication

T

thegrapeguy

Jan 2, 2020
9
5
#1
Hi - New here. So 1st post. I've been a bass player for about 10 years and currently record in my studio with the Reddi DI but when I heard this amp I wanted to start micing it up. I was wondering if anyone can tell me specially how to authenticate the transformers, tubes and circuit for this amp. Found one that sounds incredible and I want to buy it but I'm trying to see if it's original. The speaker in the portaflex cabinet isn't. Someone put a 15" speaker where there once was a 12". But it still sounds incredible on guitar especially. The speaker rattles on the lowest notes of the bass and I predict I would put a better speaker in it. At a super low volume it sounds good on bass but turn it up and it starts to rattle. I think it's coming from the speaker not the cabinet.

I also researched how to date with the serial number but I couldn't find any info about authenticating the transformer, tubes and the circuit. When I typed it into google I did find a picture of the circuit and a schematic but I'm not too savvy on the schematics but can locate caps, resistors and such visually from a picture. I read through that portaflex forum and couldn't quite find what I was looking for so I figured I'd start here, THX!
 
#2
thegrapeguy said:
Hi - New here. So 1st post. I've been a bass player for about 10 years and currently record in my studio with the Reddi DI but when I heard this amp I wanted to start micing it up. I was wondering if anyone can tell me specially how to authenticate the transformers, tubes and circuit for this amp. Found one that sounds incredible and I want to buy it but I'm trying to see if it's original. The speaker in the portaflex cabinet isn't. Someone put a 15" speaker where there once was a 12". But it still sounds incredible on guitar especially. The speaker rattles on the lowest notes of the bass and I predict I would put a better speaker in it. At a super low volume it sounds good on bass but turn it up and it starts to rattle. I think it's coming from the speaker not the cabinet.

I also researched how to date with the serial number but I couldn't find any info about authenticating the transformer, tubes and the circuit. When I typed it into google I did find a picture of the circuit and a schematic but I'm not too savvy on the schematics but can locate caps, resistors and such visually from a picture. I read through that portaflex forum and couldn't quite find what I was looking for so I figured I'd start here, THX!
Click to expand...

Do you have any photos?
 
#3
A 15 in that cab is going to be a really tight fit. I'd be curious to see how they mounted the replacement. Also, are you able to approximate the year or does the seller know?

Depending on the condition of the cab, I wouldn't be shy about wanting to purchase just the head by itself and reaching out to fliptops for a replacement.
 
#4
Yes so the seller says it is a 1965. They want $700. I took some photos. I'm going to try and see from the codes from the transformers what year it truly is. I looked everywhere and could not find a serial number. I played it a bunch today and it is almost useless on bass because whatever 15" they installed just woofs and rattles when the lowest notes are played. Even at a low volume... You can tell it's prowess on bass when you play down the neck to higher notes or ignore the rattles and hear the clarity of the lower notes. But again useless at any volume. The guy said it was "meant for a stand up bass" and I did read that when I was researching it today. However this speaker/cabinet could not handle the low notes of a stand up bass.

Now this thing absolutely rocks with guitar. I played a Strat and a Les Paul through it and the cleans are full (because of big speaker) yet still very detailed, very warm because of tubes. Both guitars are absolutely incredible for crunch/blues/70's when the volume is dimed and the treble almost dimed and bass down to 3 or less.

I do wonder if it is worth the $700 given it is the wrong speaker and presumably the cabinet is altered. And I don't know how much the circuit/tubes have been messed with. Casters have been put on and there are old drill holes for maybe another set of casters at some point. Not sure if the grill screen is original. The internal mounting of the speaker relates to how the screen is stapled and it might have been altered. The tolex if original is in very good shape for it's age. It's a cool little setup and I'm stoked about the guitar tone.

The advantage with this deal is (assuming it's mostly original) I heard it, it rocks for guitar. I do wonder what I could do to get it to play my Fender Jazz Bass without any rattles (even at low volume). I don't know, new speaker, new flip top just for bass? Thanks for responding.
 

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#5
It's actually in decent shape. The Transformers look to be in decent shape. It looks like they just cutout a bigger hole in the baffle to accommodate the 15" driver or just mounted the 15" in the 12" cut out. If they opend up the original hole, it would have to have the original 12" baffle duplicated and refit.
 
Last edited:
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#6
Good condition, good price.

Some changes were made. Not a problem. Buy an amp for the sound.

There are a lot of possibilities for the rattles.

Check out the TB Ampeg Portaflex Wiki for dealing with buzzes and rattles:

Technical - Speaker Cabinet | TalkBass.com

The portaflex wiki:
Ampeg Portaflex Wiki | TalkBass.com

There's a detailed SB-12 cabinet restoration in the wiki. Worth checking out.

A good replacement 15" bass speaker such as the Eminence Alpha 15A or Beta 15A is available for around $100US. If you want to install a 12" speaker, try and Eminence Beta 12A. All 8 ohm.
 
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#7
Thanks for replies. Can anybody tell me what's not original in the circuit? Any comments about whether transformers are original? Tubes? I almost subscribe to the idea that "if it sounds good, get it". But it would be nice to know what this really is in case I go belly up and have to sell...
 
#8
I looked closely at transformers and their numbers indicate 682 651 and 682 639. Based on an SVT document I read this means the Electrical Windings Company (682) and the 51st and 39th week of production in the year 1966. SO would it be a safe bet to assume this is a 1967 amp?
 
#9
The power transformer part number (PT114) is correct per the schematic, same for the output transformer (OT215). I'm assuming the wire nuts are on a replacement 3 prong plug? You'll want to correct that.

looks like a bunch of original electrolytic capacitors, those will likely need to be replaced at some point in the future but if the amp isn't excessively noisy I wouldn't worry right away. Check the circuit to be sure the polarity switch cap was disconnected or replaced with a safety cap.

does it have the tube cage?

[Invalid or Expired Link Removed]
 
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#10
Thx coreyfyfe. If the tube cage is the big metal mesh basket that sits on top of the amp and screws in then yes (and protects me from hot tubes:)). It has the plexiglass decorative A Ampeg logo as well. And yes to the 3 prong modification. What's the reason to correct that? Hard wire it in or get an original 2-prong chord? Can you tell me where the polarity switch cap would be? Get an amp tech to look at schematic and tell me? Thx again.
 
#12
So, there you have it. When the guys in the know tell you what's up with a SB-12 Ampeg, then you can decide what to do. I owned a SB-12 and if it were mine, after I made sure the power cord issues were adressed, I would look

at restoring the speaker and baffle board to it's original 12" driver with one oblong port. The SB-12's speaker placement in the cabnet was always offset to the side a little, in a more rectangular frame than the B-15N's . Putting a 15"

in there changes the configuration so that the speaker is even more to off center to the port. That has to impact what it sounds like, since the air space inside the cabinet is blocked by the larger 15" speaker. Just my take of course,

but I'm not fond of someone putting something in a speaker cabinet, especially and Ampeg speaker cabnet, other than what the designer intended.
 
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#13
Totally agree Ric. Thx. I'll get into it and see if the baffle can accept an originally sized speaker. Any other ideas besides the Eminence Beta 12A? I guess in a perfect world I would want a replacement speaker to handle the low notes of my bass (not at a loud volume but reasonable) and still crunch up my guitars.
 
#15
thegrapeguy said:
Yes so the seller says it is a 1965. They want $700. I took some photos. I'm going to try and see from the codes from the transformers what year it truly is. I looked everywhere and could not find a serial number. I played it a bunch today and it is almost useless on bass because whatever 15" they installed just woofs and rattles when the lowest notes are played. Even at a low volume... You can tell it's prowess on bass when you play down the neck to higher notes or ignore the rattles and hear the clarity of the lower notes. But again useless at any volume. The guy said it was "meant for a stand up bass" and I did read that when I was researching it today. However this speaker/cabinet could not handle the low notes of a stand up bass.

Now this thing absolutely rocks with guitar. I played a Strat and a Les Paul through it and the cleans are full (because of big speaker) yet still very detailed, very warm because of tubes. Both guitars are absolutely incredible for crunch/blues/70's when the volume is dimed and the treble almost dimed and bass down to 3 or less.

I do wonder if it is worth the $700 given it is the wrong speaker and presumably the cabinet is altered. And I don't know how much the circuit/tubes have been messed with. Casters have been put on and there are old drill holes for maybe another set of casters at some point. Not sure if the grill screen is original. The internal mounting of the speaker relates to how the screen is stapled and it might have been altered. The tolex if original is in very good shape for it's age. It's a cool little setup and I'm stoked about the guitar tone.

The advantage with this deal is (assuming it's mostly original) I heard it, it rocks for guitar. I do wonder what I could do to get it to play my Fender Jazz Bass without any rattles (even at low volume). I don't know, new speaker, new flip top just for bass? Thanks for responding.
Click to expand...

Well, shut my mouth. I never would have thought a 15 would be suitable in that cabinet. The speaker in my B15s looks massive compared to whats in my SB-12. I'll leave it to the professionals (@beans-on-toast and @coreyfyfe ) to give direction on these vintage Ampegs.
 
#17
Raw N Low said:
Well, shut my mouth. I never would have thought a 15 would be suitable in that cabinet. The speaker in my B15s looks massive compared to whats in my SB-12. I'll leave it to the professionals (@beans-on-toast and @coreyfyfe ) to give direction on these vintage Ampegs.
Click to expand...

The SB-12 really is a tiny budget cabinet. Too small for a 15 when used for bass at higher volume levels. More suited for guitar.
 
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#18
beans-on-toast said:
The SB-12 really is a tiny budget cabinet. Too small for a 15 when used for bass at higher volume levels. More suited for guitar.
Click to expand...

Not to derail the thread but, would you happen to have a how-to guide on how to tear down the SB-12? I have a lightbulb out and would like to change it. I have no clue on how to disassemble the head to make the repair.
 

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