Ampeg SB 12 information please?

[Neil H]

I was given an Sb 12 ... just the amp ... a few years ago. I just built an open back cab for an Eminence 15" Big Ben speaker, an am using it with a Lyle jazz box ... a laminated hollow body, identical shape/size as the Gibson L5-CES, but laminated (three layers) rather than carved top/bottom. And ... with the shorter 24.75" neck like an ES-175. I've had the guitar neck completely reworked, shaved, new frets and all, plus put early T-Top pickups in it. Plays and sounds great.

The old SB 12 is a natural for the rich, deep, smooth sound I prefer for that guitar. But I have no information on that amp whatever. Date of manufacture, what not.

There is some information stamped on a transformer, two lines ...
0T - 215
682 531

And , the current tubes are twin Ampere brand AX-7's, a Sylvania 7868, a GE 7868, and a GE 5AR4 rectifier.

I don't know how long any of those have been there or how close to needing replacement. And with all my years of guitar stuff, this is the first tube amp I've ever used. I don't even know how to judge when something needs replacing.

So ... what should I check on this, if anything? It sounds very nice, just a small amount of hiss if the treble and volume are up a bunch. It's pretty clean, but I expect all the caps/pots what not are original.

Oh ... someone did put in a 1/4" jack for speaker out, and I put a 3-wire power input cord in a couple days ago. Other than that, most stuff is probably original. I'm pretty good with a soldering iron, I've built all sorts of things over the years. Just never worked with a tube amp before.

[lonote]

That is a pretty nice gift!

I am not that familiar with those, but they were part of the PortaFlex line of bass amps (Wikipedia says '65-'71 for the SB12) It was a smaller sibling of the iconic B-15.

Betting that the 7868 power tubes will help narrow the window in dating your amp, as they also had a 6L6 version.

Some info here: https://www.talkbass.com/wiki/ampeg-portaflex-wiki/



& a 7868 version of the schematic attached

Ampeg SB12 w-7868.gif

[seveneves]

There is some information stamped on a transformer, two lines ...
0T - 215
682 531

682 is the EIA code for Electrical Windings, Inc. (Chicago). 531 *probably* indicates 31st week of 1965, though it could be 1975 or even 1985 if it's a replacement. If the solder connections/joints from/to that transformer look untouched, the amp's likely from 1965.

This October '65 schematic shows 7868s in the output, so that also seems to line up.

[Neil H]

Looking at the amp both above the case and below ... I really doubt there have been any replacement or modifications. It all looks of similar aging.

There are two big black metal blocks on the top of the case, the one with the stamped numbers, and another similar but slightly larger one. I'm thinking that may be a power supply unit of some kind? They both have much of their paint, but some has flaked off on both.

I was also noting the two different brands of 7868's, thinking at least one is a replacement.

Should I have another set of tubes on hand if one or more go out? And other than weird sounds or no sound at all, how do you tell if a tube is going or gone?

[Stevem]

Ampeg used a lot Sylvania output tubes, but not always .

Those 12 pin outputs are getting very hard and or expensive to acquire since they have not been made for over 50 years now .

That tube was specifically made for excellent hi fi audio reproduction.
If you intend to keep the amp I would pick up a NOS set if you can afford it.

I saved one of these Ampeg amps and a Alamo amp a few years ago.

Both use this 7868 tube .
Both owners wanted to keep the amp up and running for ever so I changed the output tube sockets over to 8 pin ones and used new manufactured 7591 tubes supplied by Euro tubes.

These 7591s are reproductions of a RCA tube that just like the 7868 was made for high audio standards, in fact Ampeg used this tube in some of there amps.

[Suckseeder]

This amp was the template for the Jim Kelley 30/60 amplifiers.

[Stevem]

There are tons of Ampeg combo amps like this that just like it shows in the schematic uses a output transformer stamped OT 215.
Every original one I have ever seen ( and I have seen many!) where stamped like I posted above.

[Roe]

The output transformer should have a reflected primary impedane of 5k5 if memory serves

[Neil H]

I see a couple places online that still carry 7868s. Interesting though to convert by swapping out the plug to an 8 pin one to use "newer" manufactured tube types. Huh.

NOS ... the term is not one I'm familiar with.

But then I've also noticed this amp is far more affected by the cord between the guitar and amp than my solid state rigs.

[martin manning]

New Old Stock. It's hard to believe, but there are still a great many vacuum tubes that have been sitting around unused for 50 years. Most of them were made for radios and TV's. MI and audio amps are the primary markets today, so prices for those types have been driven up as the supply has been depleted. If you buy, be sure the seller is legit.

[lonote]

NOS = New Old Stock

Old production but unused, usually in the original box.

Sounds like there aren't a lot of options since that tube is no longer made.

[LOUDthud]

My RCA tube book says the 7868 is a 9 pin tube with a 19W plate. Plate Voltage rated at 550V.

[Neil H]

Fliptops has an SB12 set for $190:
matched pair of EH 7868
2 AX7 JJ preamp
1 5AR4 Rectifier

It's about $50 less if you can use 5881 Sovteks, $30 less for 6L6 JJ/Tesla set.

Amplified Parts has a list of several for each, though only one option for 7686.

6L6 and 5881 are always cheaper and wider selection it seems. I see the point about converting the socket.

What other changes might be needed, bias adjustment or what?

[Stevem]

Don’t bother with the EH7868 tubes there a poor imitation!

[Phil_S]

It's your amp. You can do what you want with it! It uses obsolete tubes. If it was mine, I'd modify it to take tubes from current production, keeping as reasonably close to the original as I could.