Tube Amps / Music Electronics
|For current discussions, please visit Music Electronics Forum.||New: view Recent Searches.
New: visit Schematic Hell!
The sunn still shines online!
|Listen to great tunes streaming live right now!|
|3/25/2006 1:47 PM|
||Ampeg SB-12 re-cap.. Should I?|
I have a mid 60's Ampeg SB-12, and it sounds OK.. I recently peeked inside of it, because I want to put a 3-prong plug on it, and also to make sure it didn't have a "death cap" in it, in an attempt to make it safer to use.
When I looked inside, I noticed a couple of things:
1) It is point-to-point wiring, and it does not look easy to identify the parts in here whatsoever- I am having trouble telling if there is a death cap or not. I guess I need to go over it more closely with a schematic in hand..
2) All of the big capaciters looked rusty!
So, I'm thinking- I can just leave the amp alone as it currently works, or I can fix it up right by re-capping, etc.
What do you folks recommend? I've built an amp over at AX84.com, and I feel competent as a parts changer.. But, I look to the experience here to see what you folks would do. Is rust a common site on old amp parts, and not something to be concerned with? Or is this a sign that these old parts are done and need to be changed..
Above all, I like the sound of the amp, so I'm not looking to tweak or anything like that- my ultimate goal is 1)safety, and 2)reliability. So, basically the question is this- should I just leave the amp alone or recap it? Is it safe to play as is without a 3-prong plug?
|3/25/2006 3:23 PM|
Having an original Ampeg factory service book I can address some of your questions. Yes, you do have a "death cap" it's labeled "C14" and is the last componenet on the PCB adjacent to the 5AR4 - should be a 0.047/1000V unit. Oh, wait a minute - you mentioned point-to-point - hmmm, what year is yours? Anyway, in 1969 - the year of my service manual - it was a PCB amp with the death cap so I'm sure you've got one. I'd replace it with a three wire grounding cord not only for safety sake but to reduce noise. As far as the caps looking "rusty" - if this is on the positive end that may not be rust but instead electrolyte that has sputtered out the pressure release seal. In any case I'd suspect that they need replacement. The SB-12 that's in my manual lists the following electrolytics. 25/25, 30/600, 3X40/500, 100/100. The first filter, the 30/600 only sees around 475VAC from the 5AR4 which ramps up the voltage slowly so I'm not sure you really need a 600V unit, Ampeg engineers were being conservative and 600V units were more common so a 500V unit "may" be OK (but not if you put in any sort of SS rectifiers). Quickly looking over the circuit, this is a well designed basic amp with an anode follower PI.
|3/25/2006 3:46 PM|
Thanks for the info Rob!
I forgot to mention in my original post, my SB-12 is the 7868 power tube version- slightly less complex (thankfully for me!). I can't remember for sure, but I think it is a 1967 amp..
Yep, it works pretty well, but I do suspect that it will need a recap. It looks like the guts of this amp are all original!
So, I am curious- to get rid of the death cap, all one needs to do is just remove it? If that's the case, it certainly sounds doable! Also, if I replace the power cord with a 3-prong unit, what do I connect the 3rd prong wire to? Just one of the mounting bolts for the power transformer?
|3/25/2006 6:29 PM|
Yeah, attach the grounding lead to the chassis near the PT - I like to get an actual solder contact to the chassis but I've got a 300W soldering gun to do that wiht. The death cap was used to move the relative ground from one side of the plug to the other when most houses didn't have grounded outlets and the power plugs weren't polarized. Now the ground in most houses is more or less "absolute." Make sure that the "hot" lead from the power plug - black in US coding/the narrow prong - is the one connected to the fuse/switch side inside the amp.
|3/26/2006 10:59 PM|
OK, so I don't want to beat a dead horse here, but:
So, I am clear on the 3rd prong plug, the 3rd prong will go to a bolt on the power transformer, and soldered to the chassis at that spot as well. Also I will line up the "hot" wire from the plug with the fuse/switch side of the power trans inputs.
The only thing I wanted to be extra clear on is the removal of the "death cap".. Just cut it outta there, right? No need to re-wire anything or replace any other parts?
|3/27/2006 12:46 AM|
Basically correct. But I tend to leave the cap lead soldered to ground and then cut and tape off the lead that went to the switch and leave the cap in chassis. Why? Foolishness perhaps but every now and then you find a "collector" who wants to "restore" an amp to original condition and who loses sleep cuz the death cap is gone (these are the same ones that cause me to save old filter caps and slip the cardboard sleeve off 'em so that they can conceal a newer cap when I recap an old Fender) - if you don't expect to fall victim to a "collector" then don't worry. **Nother Rob Editorial Note: While amp owners generally endeavor to retain as much value in their equipment and keep it attractive to "collectors" I've never been comfortable with anyone who takes a "tool" - be it amp, axe, or monkey wrench - and possesses it only for "show." Tools - musical equipment specifically - should be used. It's a travesty to the the maker of a guitar or amp - and to all the users who can't afford a good one - - to hang useable tools on, or stack them against, a wall and have them never produce music!
|3/27/2006 3:10 PM|
Just to reiterate what was already said - Yes, you should recap and replace the cord. I also have a 7868 model SB-12 that I had sitting around for a few years until I decided to recap. And I'm kicking myself every day for not doing it sooner. That is the finest sounding amp I own (and I own too many). I'm in love with that little guy. It has a wonderful breakup around 7 and it's not excessively lound. Mine has an intermittent problem that I couldn't seem to find so it's in the shop now - which is driving me crazy !!
There is a schematic is here
If I remember correctly the "death cap" on mine was a Black Cat (?) and was located on the far side of the board near the input jack. Just yank it (drain it first) and put in a bag for some sucker... I mean "collector".
There are some odd caps in there that are tough to find (33uf/33uf dual cap) but you can get the parts here
I've used them a few times and had no problems. Good service. Good luck and let me know how it goes.
Mine also had some Friday afternoon wiring going on inside so make sure you check *all* the solder connections for cracks and cold joints. Also, the wires inside are kind of flimsy so check those too. I have a Gemini 1 that had a few of the wires crack off due to the smaller size.
Good luck and let me know how it goes. GREAT AMP !!
|Page 1 of 1|