Definitely looking like replacing the 7868 sockets would be the wisest option. When I get home tomorrow I'll take a look at that chassis.
From memory I'm thinking there is some disassembly to do. Curious about how many hours this would take.
Ampeg SB 12 information please?
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Stevem
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Re: Ampeg SB 12 information please?
I forget if Ampeg machine screws on there sockets or rivets them, but either way working at a leisurely pace it should only take a hour and 15 once all the parts are in hand.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Ampeg SB 12 information please?
Isn't the amp working OK at the moment?
To me, that conversion would seem like something to consider when the time comes that replacing the power tubes is an issue.
Not real major surgery, but I imagine there is some chassis modification involved, on top of the rewiring.
Just my 2¢, as they say.
To me, that conversion would seem like something to consider when the time comes that replacing the power tubes is an issue.
Not real major surgery, but I imagine there is some chassis modification involved, on top of the rewiring.
Just my 2¢, as they say.
Re: Ampeg SB 12 information please?
Hello --
If this is an amp to simply use personally I'd go along the earlier advice and purchase a tested NOS set from a reputable dealer. Anticipate good success finding quality tubes for sensible cost. After testing to confirm they are clean leave them in reserve, assuming the pair that came with it are working.
I may have missed some earlier message in this thread recommending you look into replacing power filter capacitors, but that too is good advice.
Best ..
The 7868 are a 9 pin noval B9A base and therefore the chassis holes are 3/4" and so likely too small to simply swap in the large octal sockets. Ampeg used hard steel. Rework will be messy frustrating work.
If this is an amp to simply use personally I'd go along the earlier advice and purchase a tested NOS set from a reputable dealer. Anticipate good success finding quality tubes for sensible cost. After testing to confirm they are clean leave them in reserve, assuming the pair that came with it are working.
I may have missed some earlier message in this thread recommending you look into replacing power filter capacitors, but that too is good advice.
Best ..
Re: Ampeg SB 12 information please?
The sockets are riveted in this amp. I've got an extensive shop and experience drilling out rivets, so removing these wouldn't daunting me.
But if the new ones took a larger hole, that would be more work. Not that I couldn't do it, but it would be more.
My biggest concern, with a conversion process, is two fold:
1) tracking what is currently wired to where, and then getting things "condensed" to the new sockets. Every pin of the current sockets have at least one wire to them. With fewer pins, that will require knowing exactly how they're regrouped.
2) With a different set of tubes, it might need a different bias adjustment or whatever that is. I've not done it, so I'd want specific steps down first.
And from what I recall, the AX7s and rectifier stay the same ... ?
I've no problem with a conversion, but if I can get good tubes without, well, that's even less to do. Unless cost for 7868s becomes excessive.
But if the new ones took a larger hole, that would be more work. Not that I couldn't do it, but it would be more.
My biggest concern, with a conversion process, is two fold:
1) tracking what is currently wired to where, and then getting things "condensed" to the new sockets. Every pin of the current sockets have at least one wire to them. With fewer pins, that will require knowing exactly how they're regrouped.
2) With a different set of tubes, it might need a different bias adjustment or whatever that is. I've not done it, so I'd want specific steps down first.
And from what I recall, the AX7s and rectifier stay the same ... ?
I've no problem with a conversion, but if I can get good tubes without, well, that's even less to do. Unless cost for 7868s becomes excessive.
Re: Ampeg SB 12 information please?
I think that this is probably the best idea, certainly the most painless (other than the $$)
When I mentioned earlier a tube conversion wasn't "major surgery", it was purely from the wiring perspective & didn't really take into consideration all the potential issues that might go along with it. A poor choice of words on my part.
And I was not thinking that those tubes had as small a socket as they do, which really complicates finding a solution.
In my opinion, on an amp of that vintage & that caliber, the re-drilling the chassis alone would constitute major surgery.
You should explore further to decide for yourself, but I think you will find that with "vintage" amps (currently pre-1985-ish & ever-shifting), that the bottom-line criteria regarding modifications is: "could it be put back to stock by a skilled tech?"
I myself have a "no new holes" rules, which enlarging tube socket holes would fly in the face of.
Also, electrically-speaking, there may also be some changes in value to supporting components (resistors, capacitors) with a tube swap. It is not necessarily as simple as just reconnecting the wires.
Not having ever used those 7868s, I have done some poking around online since you first posted & it seems you are not alone with this issue, nor does there seem to be a good solution. There are a lot of HiFi guys in the same predicament.
From my reading, the Euro versions that are currently available are not really 7868s & have a thicker pin that stresses out both the tube & the socket, potentially creating further issues.
As you are a guitar player to begin with, it might be possible to swap to EL84s (more popular for guitar than bass), & it may be that the current 9-pin socket could be fitted to existing holes. I have no idea about power supply voltages & supporting circuitry without digging in.
There are many more knowledgeable here that can weigh in on the viability of that option.
But as previously mentioned; your amp, your decision.
Re: Ampeg SB 12 information please?
Boy, there's a few people trying to figure out how to get 7868 workable replacements. It ain't pretty out there everywhere ... different diameter pins, improperly labeled things, wowza.
I see that TubeWorldExpress has NOS RCA 7868's from 60's for basically $130/pair. In the original box ... we hope?
https://tubeworldexpress.com/collection ... rca-nos-p5
From just looking at the pictures, the EH7868's seem to have a visually thicker pin than original 7868s, if not as fat as some other 6 pin tubes. And some of the 7868 tubes from various suppliers also have thicker pins. Marvy.
I saw a forum post elsewhere that someone got some EH 7868 from Tube Depot that fit without any issue for the pins and were working fine in his amp ...
https://www.tubedepot.com/products/elec ... acuum-tube
And Tube Depot has these NOS 7868, pictured is a GE tube & box. But the text ends ... "variuos brands" Huh.
Thinking I might order the NOS RCA from TubeWorldExpress ... but first, anyone with any experience with them?
https://www.tubedepot.com/products/7868
I see that TubeWorldExpress has NOS RCA 7868's from 60's for basically $130/pair. In the original box ... we hope?
https://tubeworldexpress.com/collection ... rca-nos-p5
From just looking at the pictures, the EH7868's seem to have a visually thicker pin than original 7868s, if not as fat as some other 6 pin tubes. And some of the 7868 tubes from various suppliers also have thicker pins. Marvy.
I saw a forum post elsewhere that someone got some EH 7868 from Tube Depot that fit without any issue for the pins and were working fine in his amp ...
https://www.tubedepot.com/products/elec ... acuum-tube
And Tube Depot has these NOS 7868, pictured is a GE tube & box. But the text ends ... "variuos brands" Huh.
Thinking I might order the NOS RCA from TubeWorldExpress ... but first, anyone with any experience with them?
https://www.tubedepot.com/products/7868
Re: Ampeg SB 12 information please?
I also have read a couple posts by someone who's used the EH 7868s, with the slightly thicker pins, in old 7868 bases. Says that it's wise to use a cheaper tube (he gives an example) as your hole-modifying "fitter" ... I guess ... but slowly work the tube in with pressure going round the outside of the tube. And once done, the EH 7868s fit easily, and perform as they should.
And a couple others saying that yea, once they got the old base 'expanded' the EH tubes were fine. Although one or more may have lunched an EH tube during the process.
Hmmm. So much fun to be had it seems ...
And a couple others saying that yea, once they got the old base 'expanded' the EH tubes were fine. Although one or more may have lunched an EH tube during the process.
Hmmm. So much fun to be had it seems ...
Re: Ampeg SB 12 information please?
Hello —
Try reaching out to John Albion at pacifictv.ca. It’s been several years since I had need for any NOS tubes, however he was consistently reliable and great to deal with in the past. I see Sylvania 7868 on their baroque Y2K era website, though updates maybe lost or simply low priority.
Best ..
Try reaching out to John Albion at pacifictv.ca. It’s been several years since I had need for any NOS tubes, however he was consistently reliable and great to deal with in the past. I see Sylvania 7868 on their baroque Y2K era website, though updates maybe lost or simply low priority.
Best ..
Re: Ampeg SB 12 information please?
I've sent a request for specific info from PacificTV, but he doesn't currently list 7868 as available.
Re: Ampeg SB 12 information please?
And this is an image of the amp guts ... I'm seeing quite a few rather aged caps there ...
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Stevem
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Re: Ampeg SB 12 information please?
For sure replace all 3 brown paper ones and the 2 blue one, saving them just in case of a sale down the road.
Even though a EL84 and a 7868 are both power pentodes they do sound different and you might have to mod the bias circuit to run them.
84s also take a lot less signal drive, so in that Ampeg they will clip very easily which is something you might not want.
These amps also used 6L6 tubes so you might find that the holes punched in the chassis will accept a 8 pin octal socket as is.
In the PI section I would check to see how closely matched the two 220k and the 68ks are after all of these years and the 1.5meg and the 5.6 meg drift a lot also .
I always found it interesting that very few Ampegs used that 100 ohm wire wound cathode trim pot on the output tubes!
This pot I would check very carefully to make sure heat had not dry rotted it to point of crumbling and not functioning.
Even though a EL84 and a 7868 are both power pentodes they do sound different and you might have to mod the bias circuit to run them.
84s also take a lot less signal drive, so in that Ampeg they will clip very easily which is something you might not want.
These amps also used 6L6 tubes so you might find that the holes punched in the chassis will accept a 8 pin octal socket as is.
In the PI section I would check to see how closely matched the two 220k and the 68ks are after all of these years and the 1.5meg and the 5.6 meg drift a lot also .
I always found it interesting that very few Ampegs used that 100 ohm wire wound cathode trim pot on the output tubes!
This pot I would check very carefully to make sure heat had not dry rotted it to point of crumbling and not functioning.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Ampeg SB 12 information please?
From John at Pacific, he has only one NOS 7868 in stock.
From Tube Depot,
"Thanks for your email. At this time we have only a handful of these and one of them is GE and the remainder are RCa. From what I could tell by looking at the pins the GE is thinner than the RCA. Unfortunately I only have one of those. Let us know if we can be of further assistance."
At this point, it seems the thicker-pinned EH 7868 are the main thing available without a base swap-out.
Still hunting though.
From Tube Depot,
"Thanks for your email. At this time we have only a handful of these and one of them is GE and the remainder are RCa. From what I could tell by looking at the pins the GE is thinner than the RCA. Unfortunately I only have one of those. Let us know if we can be of further assistance."
At this point, it seems the thicker-pinned EH 7868 are the main thing available without a base swap-out.
Still hunting though.
Re: Ampeg SB 12 information please?
The 7868 is a MagNoval B9D base. The socket is physically larger than a noval B9A style. Look at the pic above to see the size difference between the 7868 socket and the 12AX7 socket.
A Belton octal socket would likely fit the chassis hole for the 7868, but may require elongating the mounting screw holes or drill new ones.
Re: Ampeg SB 12 information please?
That’s correct. Apologies, as I’d been sloppy in misreading the data sheet. That gut shot is very encouraging.sluckey wrote: ↑Wed Feb 05, 2025 5:16 pmThe 7868 is a MagNoval B9D base. The socket is physically larger than a noval B9A style. Look at the pic above to see the size difference between the 7868 socket and the 12AX7 socket.
A Belton octal socket would likely fit the chassis hole for the 7868, but may require elongating the mounting screw holes or drill new ones.
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