Amp Help

[BlueAngel]

The famous Sprague Atoms for years have been a big cap case with a small and modern size cap inside. They tore some apart and posted pics, then got someone from Sprague to comment at the Hoffman amps forum, and basically Sprague only recommends these for vintage replacements and not new construction. They make smaller, more modern caps of better quality these days, in addition to other manufacturers like F & T, Illinois caps (I personally don't like these), Xicon, Rubicon, etc. Moreever, there was some theorizing that having a small cap inside a big case could make the caps not withstand heat as well since they wouldn't be able to dissipate any through the case, and not withstand vibration as well since a little cap could be moving around inside the larger case.

I didn't know that - I've never so much as bought a Sprague Atom let alone dismantled one since I think they're overpriced and I don't believe that filter cap brand has any significant effect on tone (although there are some I try to avoid for reliability reasons) - but it doesn't surprise me! I agree with mounting a small cap in a large case being a bad idea, too.

The big caps I'm talking about are the ones in older amps - especially solid-state - which I'm quite sure are genuinely large inside too since there would be no reason to 'cheat'. The ones in early 80s Trace Elliot and 70s Acoustic amps for example are absolutely huge compared to anything in more modern amps, and these have tremendous depth and punch for their rated power.

Maybe I should have said that modelling amps can sound better than some solid state, as of course not all amps are created equal. I really like the old Kustom amps that CCR used, and they're solid state. Some of the Vox solid state stuff was quite good, and likewise the RIC Transsonics....but these are old solid state designs which have much in common with tube amp designs of the time. Modern solid state.....I can't say I like much of it beyond the Roland stuff...and in all cases I prefer tube amps. The new modelling amps....well they're ok but still not up to tube quality. I could play a set with them if required and would prefer the Vox modelling amps to other brands because they seem to have a bit more feel, but for me, tube amps are where its at. They just sound more realistic in all respects.

I mostly agree, but I still do have a lot of time for conventional, analog solid-state - even some fairly maligned cheaper brands sound OK to me with careful dialing in, certainly better than any modeling amp (even the Voxes, which I agree are the best... or least bad!). There's just something more natural about the tone, and they all work better with a good overdrive pedal, which I tend to find sound better than the onboard distortion in many solid-state amps. I've never heard a digital amp that works well with a pedal or sounds really right in a mix, and I find it very puzzling that so many people seem to like them, or rate them higher than analog solid-state. I guess we hear things differently!

[johnallg]

Ken, interesting. I've been toying with the idea of adding power supply capacitance to my SVT-3 Pro for just that reason - more transient response (punch).

The great thing is that with improving technology, caps are getting a lot smaller in relation to their electrical ratings, so you can usually get a much larger value cap into the same space as the one you're replacing, without having to modify the board or the mountings.

But I think there is also some evidence (I would have to say anecdotal, since I don't know if anyone has actually measured this properly) that physically larger caps have more dynamic punch and sound better, even at the same electrical rating. I've noticed that a lot of modern amps with tiny caps just don't seem to have the sort of volume that you would expect from their (genuine) power ratings, whereas older ones with caps the size of soup cans often sound really great and much bigger than you would expect. A while ago I compared a new 600W Ashdown to an old Mk5 150W Trace Elliot through the same cab, and the Trace actually sounded almost as loud, and better with it. I would guess the size of the internal components in the caps could have a bearing on their internal resistance at high currents.

Just be careful you don't overload the rectifier diodes with the power-up surge though - an empty cap is almost a dead short to the rectifier, and the bigger it is the longer it remains that way.


Sorry for the technical thread diversion

Always check the ESR rating of the caps you'll be putting in - the lower the better. I have not looked at this for years (I published 3 articles on early cd player mods) so I can't confirm or deny this conjecture. Also make sure your rectifier(s) can handle the inrush current! Larger caps for a given value/voltage will handle more heat and last longer. 105C rated caps will last longer also.

[scott_s]

The story I've heard is that Sprague kept the Atoms in larger cases because that's what the vintage guys wanted -- something to occupy the space in antique designs and *look* right, even if they are no better than the smaller ones. While I'm glad that capacitor technology has advanced, you gotta admit that vintage amps look kinda funny with tiny little electrolytics in the power supply.

- Scott

[BlueAngel]

The story I've heard is that Sprague kept the Atoms in larger cases because that's what the vintage guys wanted -- something to occupy the space in antique designs and *look* right, even if they are no better than the smaller ones. While I'm glad that capacitor technology has advanced, you gotta admit that vintage amps look kinda funny with tiny little electrolytics in the power supply.

I don't think - or care, if I did - that the small caps 'look funny' in old amps... but you may need to take extra measures to stop them vibrating, since the spacing between caps (and under the foam strip in an old Fender cap tray) can allow them room to move.

I've always fitted smaller axials without even thinking about it, but there can be real problems with radial cans, as in old Marshalls - the modern caps are much too small for the clamp to hold them. Even the 'correct size' modern cans that Marshall themselves use in the reissue amps are very slightly too small, and you have to choose between winding a few layers of tape around the can, and replacing the clamp with a modern one as well.

Once when I was restoring a '68 Marshall Major, I actually had a specialist cap supplier fabricate me two caps to my specs, since the originals were both an unusual electrical value and and odd diameter, and I couldn't find anything even close - it was horribly expensive per cap, though not really as a fraction of the total repair cost. They did tell me they would be much cheaper if I ordered a thousand, but I'm not sure Marshall even made 500 Majors .