Anybody have an Ampeg SB-12?

[cazksboy]

New guy here, first post. I'm a bass player, Fender into Ampeg amplification is my fave thing. A fellow bassist sold me an Ampeg portaflex SB-12 for an hundred bucks. Needs a driver (a blown "Pyle" is in there now) and probably the usual going through the circuitry, as well as some cosmetic surgery on the outside.
Anybody else play through one?

[cazksboy]

Bump. Anyone at all ever even heard of the Ampeg SB 12? It's one of the portaflex "fliptop" amps. Probably the least-known one of them all. Hate to say it, but I once had a real 1969 B-15N, like Jamerson used...and I sold it. Wish I had 'er back. But I digress.

One thing I'm wondering about my SB 12 is whether to use it for guitar or bass, since it was designed to be used for either. I think I'm kind of leaning toward guitar since the output power would make it nice for overcranking a guitar through.

[Enzo]

Nope, I have a B-18X, but that is really a guitar amp, oddly enough.

[cazksboy]

B-18X...hmm, seems like I was reading about that one recently somewhere. Must be a monster to lug around. How does it perform as a guitar amp? Just curious.

[RWood]

I've got an SB-12, have had it since 1974. What do you want to know?

I have used mine for both guitar and bass, but for the latter it is really only sufficient as a practice amp, or a drummer-less jam. Or for recording - it has a great tone but just doesn't cut through very well when it is cranked.

There were two versions of that amp, one with 6V6 power tubes, and those like mine that have 7868s. These are tubes that are novar socket equivalents to 7591s. They are not that cheap or easy to come by but they last a long time in these amps as they are very conservatively biased.

There is a Joe Piazza-drawn schematic available, but I have scanned the schem that is in mine. Let me know if you need it. It has voltages and current draws on it, which Piazza's does not.

RWood

[cazksboy]

I've got an SB-12, have had it since 1974. What do you want to know?

I have used mine for both guitar and bass, but for the latter it is really only sufficient as a practice amp, or a drummer-less jam. Or for recording - it has a great tone but just doesn't cut through very well when it is cranked.

There were two versions of that amp, one with 6V6 power tubes, and those like mine that have 7868s. These are tubes that are novar socket equivalents to 7591s. They are not that cheap or easy to come by but they last a long time in these amps as they are very conservatively biased.

There is a Joe Piazza-drawn schematic available, but I have scanned the schem that is in mine. Let me know if you need it. It has voltages and current draws on it, which Piazza's does not.

RWood

RWood, thanks much. It's not that I really need to find out anything in particular, just that I've never encountered this model before. Mine has a pair of 6L6GC's, and I do indeed have the Joe Piazza schematic. If you wouldn't mind I'd sure appreciate your version of the schematic. You can shoot it to kcboy9940 at gee-whizmail dot com.

Mine has a serial number of 089338, and is stamped "Linden NJ, U.S.A.". Can't remember what reference I used to look up the serial number, but my notes show that it dates to 1969.

Thanks much. RWood, yes I'd definitely be able to use a schematic with voltages & current figures.

[RWood]

Ah, okay.... I was mixing this up with the GS series of amps when I was thinking of 6V6s.

What you have with the 6L6s is a different beast, more power, and I would think that with a good speaker in there it could be heard in a jam with a drummer.

In looking at the two circuits, the major differences that jump out between the earlier (mine is a '66, with 7868 power tubes) and yours from '69 are:

• First triode is biased differently in the two versions
• phase inverter on yours is the Long Tailed Pair type
• More filtering on yours, beyond the reservoir cap

There may be some other differences I missed.

I will email you the '66 schematic; just be aware of the potential for differences.

RWood

[cazksboy]

RWood, schematic received...many thanks from the newbie kid on this board.