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Thread: Ampeg BT15 capacitors

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    Ampeg BT15 capacitors

    I recently bought an Ampeg BT15 and took it to my amp tech. He said that all the capacitors are fried and need to be replaced. Does anyone know a good supply house that might carry old caps? I'll have a list of them very soon. Thanks, I'm new to this forum.

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    Lifetime Member Enzo's Avatar
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    Some thoughts.

    You don't need old caps, you need new ones. I am not being a smart ass. Well, no, I am a smart ass too, but that isn't my point. 40 year old electrolytic caps are probably dried out and need general replacing. I am going out on a limb here to say, in this low voltage circuit, your film caps are PROBABLY OK. What you don't need is the exact same caps that were used when the amp was built. They probably don't exist anyway. 1969 was a VERY long time ago... I was there.

    Today cap values are more standardized than they were then. A lot of the values in these amps were non standard. Looking at the preamp, I see some 1uf 40v caps, eg. C2,4,11,13. 1uf is common enough, but 40v is not. So don't waste time looking for a 40v cap, look instead for the very common 50v cap, or even the 63v or 100v. The circuit won't care that the caps can handle higher voltage. Moving along to the summing stage I see 6.4ud 40v C23. Really? Good luck finding that. Aside from automatically making it a 50v or higher as in the last example, 6.4??? 6.8 is a more likely standard value, but a plain old 10uf should work just fine there.

    For reasons not worth going into here, low voltage caps were more common then. So we see C24 is a 1000uf 2.5v cap. Today that is silly. 1000uf is common, but just get a 16v cap or most any voltage you see. Same with odd value C24, 124uf 4v. You won't find that. Make the search for 16v or higher, and 125uf is odd, so 150 would be more likely or even 120.

    You can find low voltage caps, but commodity caps pretty much start at 16v and go up from there, so why beat your head against the wall looking for 4v caps? And just so you know, caps are physically a lot smaller than they were then. Parts technology has progressed over the decades.

    Now in the power supply up top C28 is 640uf 25v. 25v or 35v is common, 640uf is not. 680uf would be a standard value, and 1000uf even easier to find. Right next to it is C27 64uf at 64v. No way. 63v is a common standard, and the amp won't miss one volt, but I'd prefer a 100v cap there anyway. 64uf is not going to happen. 68 is a common value or 100uf.

    Out in the middle C32 C33 in series. Note they are across a diode, so unless the diode burns open, those caps never see more than about a volt. Note also they are back to back, so they were making a bipolar cap. Well today bipolar caps - also called non-polar caps - are common and easy to find. SO one 100uf 16v bipolar cap should work great in place of those two caps..

    And then the main filters are 2500uf 80v, and also the speaker cap is the same value. You might actually find those, but if not, look at 100v caps, and if 2500uf isn't coming up, 2200uf might be more common, or 3300uf probably better anyway.


    Now before you throw me across the room, remember that back them, electrolytic caps often had value tolerances like -20%/+80%. That means a perfectly good and within spec 100uf cap could measure from 80uf to 180uf and be good. SO when I suggest cap values off the schematic mark, I am still within reasonable bounds.


    Now here is another concern I have. These parts are all extremely common types, and are available from most any general electronics parts suppliers like Mouser or Digikey. And unless your tech is a complete amateur, he ought to know where to find these parts. No offense to him, we all started somewhere. But I don't know you or him, so it is possible you are trying to "help" him find parts that he is perfectly able to find. In other words, imagine someone walks up to you and asks you to play some song, then proceeds to try and show you how to finger the chords.


    If he told you to go get caps, well, more power to you. But if you are planning to save money by getting parts to hand him, unless he asked you to help, that is really only going to same you $10-20, and it screws him out of making a few bucks on parts. And if someone brings me parts to use, I will tell them up front, my shop warranty will not cover any of those parts that fail or the damage they might cause.

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    Old Timer soundguruman's Avatar
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    Quote Originally Posted by Tele54321 View Post
    I recently bought an Ampeg BT15 and took it to my amp tech. He said that all the capacitors are fried and need to be replaced. Does anyone know a good supply house that might carry old caps? I'll have a list of them very soon. Thanks, I'm new to this forum.
    " all the caps are fried"...not necessarily...they might still be good.
    I just fixed a 1956 Fender Champ, and the original caps (and tubes still work fine).
    You want NEW caps, if they really are bad. leftover stock from long ago, OLD caps are basically JUNK.
    In these old Ampegs, more likely a bad tube, or a bad hum balance control...before you change capacitors.
    Capacitors are often changed unnecessarily...
    Are they really "ALL" bad? Not likely.
    Most of our capacitors come from CED / Antique Electronics Supply.
    AND can you get good tubes for the amp? or are they extinct? Check THAT before you go and put a bunch of money into it.
    I mean, replace all the caps, and there's no tubes available...what for?

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    Noodle of Reality Steve Conner's Avatar
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    The BT15 is solid-state. (T for Transistor I guess.) No hum balance control and no tubes to go bad either.

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    Supporting Member Jazz P Bass's Avatar
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    "All the caps need to be replaced" may have been an offhand comment.
    The tech may simply have been refering to the electrolytics.
    I would ask.
    Saying that all of the capacitors are in need of replacment is a bit of a stretch.
    If a true statement, it does make one wonder how the tech came upon that specific conclusion.
    Schematic link: http://usdn3.nodevice.com/preview/big/365/365767-1.jpg

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    Lifetime Member Enzo's Avatar
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    Yes, this is a solid state amp. All transistor preamp.

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    Thanks everyone for the quick and detailed responses. Much Appreciated. My amp tech is primarily a tube guy and he tries to keep things original if at all possible. He did present the option of using newer caps, but said that it may sound slightly different. He is very respectful when it comes to old equipment and is very particular about what he does to it. That being said, he also said that since we don't have a completely functioning amp for comparison, we probably won't be able to tell the difference, and it will still sound great. He told me that he had been unable to locate the exact replacement caps and I offered to help locate them. I have a passion for this old amp even though it isn't the highly sought, and high dollar tube version.

    Enzo, I sent him the info you supplied and he completely agreed with your assessment. I'm not a tech guy and your evaluation was much more detailed than his initial explanation to me, but essentially the same. I gave him the green light to go ahead with the repair using new caps with the different values as it appears to be the only viable option. He did locate the power supply caps but only in lots of 20.

    Jazz P Bass, I believe you are correct. He said that it needed the electrolytics caps replaced. I believe there are 15 of them. He sent a video showing the damage, and I stopped by to see it in person. As I said, I'm not a tech guy, but they sure looked "fried" to me.

    Soundguruman, my amp guy is really great with the old tube stuff and he is much more comfortable in that arena. He rebuilt a 1953 (I think) Bassman, and an old 50 something tweed Deluxe recently that are absolutely smokin' (in a good way)! My gig amp is an early 90's Peavey Classic 50 that he tweaked out. It reminds me of my old small box Marshall 50 with the 8x10 cab and it is amazing! He and I go back a ways and I completely trust him. I'm basically a guitar player, but love "pullin' on the ropes" from time to time and I love the sound of early Ampeg bass amps. The BT15 is probably as close as I'll ever get to a coveted tube B15 and I'm looking forward to many years of thumping in the night with this rig. Thanks again.

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    Lifetime Member Enzo's Avatar
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    So tell your amp guy to come here and join us. We are a friendly bunch, and if he is interested in amps and their innards, we certainly cover most aspects of it. He might find it interesting, and he might also find a valuable resource here.


    I just recently had an old B18N chassis in here for overhaul. Oh dear lord are they nice sounding amps.

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