Ampeg SB 12 information please?

[Neil H]

The holes in the chassis for the 7868 tube sockets seem to be 1-1/8th inch dimeter, with the mounting holes for the rivets 1.5" apart. I haven't used the calipers on them, but eyeballing against a tape, I think I'm pretty close to actual value.

[sluckey]

Here are nine schematics for the SB-12... https://el34world.com/charts/Schematics ... matics.htm

Most of them are for fixed bias 6L6s. I would not hesitate to convert to 6L6.

[Neil H]

The really fun thing for reading schematics ... they don't show actually anything in "real" locations!

So I have to try to read the data off the item, hoping I can move it so I can actually see all the relevant data, and then look up on the schematic to see if there's something mostly like that listed.

Much, much handier would be a simple diagram of everything laid out on that long board, as it is.

I think the long big brown cap along the back side, near where the power cord on my pic comes in, is the 30MFD/600V cap in the schematic. Numbers on the cap are:
20-42417
235-6442A

The bigger of the two brown caps mounted to the board itself:
EMB 00559
2X 20MFD 450V
1081 6527

The smaller is:
EMB 02650
20MFD 450V
1081 6506

All I can see of the two blue ones is a 50uF value.

[lonote]

The holes in the chassis for the 7868 tube sockets seem to be 1-1/8th inch dimeter, with the mounting holes for the rivets 1.5" apart. I haven't used the calipers on them, but eyeballing against a tape, I think I'm pretty close to actual value.

So, it seems you may be all set for a switch to octal sockets if the need arises.

I saw the gut shot this AM & noticed the discrepancy myself, that being the size of the the power tube sockets compared to the 12AX7 sockets. Then wondered if maybe all the chassis were punched one size in the power tube position & they had appropriate tube sockets with integral reducers so either tube type could fit.

Anyway, I have a chassis mount Belton octal right on my desk, dial indicator shows 1.171" (spec'd at 30mm). That 30mm hole size is to flush mount the mounting flange to the chassis, with either inside or outside mounting. Towards the solder tab end of the socket, the diameter of the body reduces down to .99".

So the current Belton socket could outside mount in a hole as small as 1", with just a .070" shim under the mounting ears. That is so close to flush that I would think that the tubes would still have clearance from the cage that goes over them.

Mounting holes are approx 1.5" center to center, with elongated holes, so a bit of wiggle room there.


Hope that makes sense, socket data sheet attached.


https://s3.amazonaws.com/tubedepot-com- ... 1383248496

[Neil H]

And I'm curious what the brown multi-pin thing is mounted to the treble/bass pots is? Looks to have something like 6 pins ... and that gray tubular thing?

I'm trying to sort out cap types, too ... the yellow ones are ... what? The blue ones ... what? The brown paper wrapped ... what?

[didit]

Hello --

And I'm curious what the brown multi-pin thing is mounted to the treble/bass pots is? Looks to have something like 6 pins ... and that gray tubular thing?

It's a compact module with resistors and capacitors that make up the filter elements for their unique tone stack. The gray "thing" is the first of the power string resistors.

I'm trying to sort out cap types, too ... the yellow ones are ... what? The blue ones ... what? The brown paper wrapped ... what?

The paper wrapped are the large high voltage electrolytic power filters. The blues are smaller lower voltage electrolytic. The yellows are CDE film/foil "signal" capacitors.

..

[Neil H]

Thank you, you are being most helpful!

The gray thing, so it's the first of the power resistors. Great ... um ... so, what value is it, and how would one know what to replace it with?

I take it a first order of business is to replace the large brown paper caps, and above it was recommended to do the blue caps also.

Past that, if I replaced the sockets, as has been suggested is probably possible ... which tubes would then be needed, and what other changes might be needed to accomodate them?

[Neil H]

So this would be the modern replacement for the big brown 30MFD 600v cap?

https://www.studioelectronics.biz/sunsh ... ail&p=1586


Neil

[Neil H]

And Brendan from TubeWorld (in Wisconsin) says yes, he has a matched set of RCA 7868 tubes in stock. Maybe I'll order those and consider a socket swap at a later date ...

Neil

[Neil H]

Ok ... it would be $130 plus shipping for the pair of RCA 7868's from TubeWorld.

Add in a pair of AX7s, and the 5AR4, that's going to be well north of $175 to get a replacement set.

A pair of JJ 7591's on the other hand is around $65 plus shipping. Would be probably around $100 for the complete replacement set. Maybe some more.

I see sockets are like about $5 or something like that? And as the octals would fit in the hole I've got, how much change to the wiring/parts of the rest of the amp would be involved in swapping over to the 7591 in the octal base?

[didit]

Hello --

The gray thing, so it's the first of the power resistors. Great ... um ... so, what value is it, and how would one know what to replace it with?

That resistor almost certainly will not need replacing. Suggest your next step is to print out the schematic (Ampeg original for '65) and trace through the circuit to get a closer understanding of all the components. Maybe mark up the photo with the assigned part numbers to lock knowledge and for future reference. A few hours well spent. At least do this around the power supply string and bias circuit.

I take it a first order of business is to replace the large brown paper caps, and above it was recommended to do the blue caps also.

Yep. And you could sensibly stop once that's done, assuming all tests out in fully working order.

Back at top post you said ..

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.

.. and advice is leave the rest alone, sounding nice as is. Buying a fresh pair of 7868s to have ready for when the working set finally does wear out makes sense. Have a few new 12AX7s on the shelf too maybe?

I've an original 1964 Deluxe Reverb that still has, with exception of one 12AT7, the entire complement of tubes it had when I found it in a pawn shop back in 1996. All electrolytics were replaced back then, and perhaps overdue again. Some long while back I bought a pair of quality NOS Sylvania 6V6 that are identical to the Fender branded tubes in the amp. They sit unused. It's my main amp to play through so it gets used regularly but gently, perhaps a few hundred hours a year. I've a B15N from the same early '60s era purchased around the same time that came with Ampeg branded 6L6s. It gets less use, but still chugging along. If they were gigged hard or pushed aggressively wear & tear would doubtless have been enough to require more servicing.

Closing advice :: Save your money, at least for the moment. Get the key stuff taken care of. Spend your time playing rather than tinker when it ain't broke.

Best ..

[Stevem]

That flag tan pack on the back of those pots is the whole tone circuit as in what you see here in this Ampeg V2 preamp with part number 6470000

[Neil H]

Very helpful information and it seems, rather pracitcal advice.

Thank you kind sirs!

[didit]

So this would be the modern replacement for the big brown 30MFD 600v cap?

https://www.studioelectronics.biz/sunsh ... ail&p=1586


Neil

Yes. You might assess the rewiring and decide to use a radial variant. I source those MIEC e-lytics from:

https://www.justradios.com/axialelectrolytics.html

.. but mostly to minimize cross border paperwork and charges. You should see all the values you’ll need, or at least adequate substitutes.

Best ..

[Neil H]

Wow, that's a great link! Thanks for posting ... I'll be ordering from them I think.

I've never seen a 2x capacitor ... two leads on one end, on on the other, before ... that's an interesting thing.