Capacitor Forming (Before you turn that old amp on...)

[MrCreosote]

I believe this is a topic critical to every tube amp owner.

For one thing, it deals with the tendency of an amp to smoke its filter caps when you turn it on after sitting OFF for 20 years.

I am a novice to Caps Forming and it would be nice if others that know a lot more than me, would add to this thread.

In short, there is a molecular oxide layer that exists on the foil of a functioning electrolytic capacitor.

This layer dissipates slowly when the cap is not used and is also not present in a new cap.

Therefore, it is important to form a uniform (as opposed to a splotchy) oxide layer before the cap is fully loaded.

This is caps forming.

The process is very simple:  slowly increase voltage on the cap until you reach your operating voltage.  This should be done slowly so the cap does not increase in temperature.  There are variously opinions of how long.  Some as short as a couple of hours up to a day sometimes depending on circumstance.

The easiest way to accomplish this is with a Variac although there are methods on-line to do this without one.

What seems to be a basic method is to remove all the tubes except for the rectifier, then start your voltage build up profile.  What seems reasonable is to increase in 10% intervals with a dwell time at each level.

There are other tests you can perform while forming such as checking grid voltages at the various tube sockets where the tubes have been removed.  I am very unschooled on this procedure, but from what I have read, grid voltages should be zero for this test.  If not, then there is a leaky capacitor present that will produce excessive grid voltage when the tube is present and operating.

There is a similar test you can perform for the screen bypass caps but I will leave that as a homework exercise.

NOTICE:  PLEASE DO YOUR OWN RESEARCH ON THIS TOPIC SINCE I AM A NOVICE. 

We really need the experts to pile one here(!)

But if you're about to turn on an amp that has been sitting OFF for decades, please learn how to cap form it before you do!

Regards,
Tom

[bonosurf]

Sorry, in IMHO you need to replace 20 year old PS filter caps before they fail on you or your customer during a gig. If you are going for museum quality restoration for your collection who cares?

Ask any electrolytic capacitor manufactuer about reforming a 20 year old capacitor in a high voltage application and they'll politely refuse comment.

You can search this site if you want more opinions: http://music-electronics-forum.com/f22/

[MrCreosote]

Perhaps, but I can tell you that audio equipment loses value when even a famous expert does even some simple mod which actually improves the sound/performance. 

Maybe its different with vintage amps. 

I would think that the whole point is to get that vintage sound in which case it's debatable to modify anything that would change the sound quality.  Like upping the capacitance for tighter bass.

If someone is working gigs for a living, if particularly on the road, bouncing a vintage amp all over the country is risky.  There are cheaper, more reliable solutions.

The recording studio is another matter.  Controlled environment, attention to detail, get that classic sound. 

Of course, there is probably a class of musician that wants that Marshall head to be seen and really doesn't care if its been modernized internally.

I mean there is debate over whether to leave the disconnected failed quad cap mounted  for "aesthetics."  So there are some people that want their amps to "look" original down to the chassis(!)

There are so many choices and individual tastes I guess the question simply cannot be answered.

[Johnny]

Replacimg electrolytic caps is not a mod, it's maintenence. If you want to keep the collectable value of that amp, don't replace the caps. If you want to actually use that vintage amp, replace the caps.

BTW it did happen to me. Vintage amp being used regularly, a cap failed and took out the tubes, output transformer, and speaker.

If you have an amp that hasn't been used in 20 years, it would be wise to have it checked out completely bofore plugging it in.

[Soundmasterg]

Sure there are some people out there who will frown on replacing ANY part in a vintage amp, but these are usually collectors who don't play or have no knowledge of the fact that some of these parts do wear out just like the tires on your car. Try driving a 1951 Cadillac around with rotted tires from it sitting in a barn for 50 years.....its the same thing with electrolytic and paper caps and your vintage amps. If the amp has been sitting around without use, you can almost guarantee that it will have at the least a very loud hum when you turn it on, and at worst it will blow tubes, output transformers, fuses, etc. You can reform these e-caps sometimes and the amp may function for awhile, but you never know when those old caps will fail. Replacing the caps restores the amp to the condition it was in when new.

Now with the exception of defective coupling caps that are leaking DC, I personally don't recommend replacing the coupling caps in vintage amps until they do fail, as replacing those with new caps will change the sound quite a bit more than using exact value replacement electrolytic caps when those type need to be replaced. With the Sunn mods I recommended a couple years back for replacing the can with discrete caps, you can do that with exact value caps and it won't change the sound. For Sunn bass amps, I personally think it sounds better to go with larger values in the first two stages, and Conrad has heard my amp in person and he likes it better that way too. For the vintage snobs, that would be heresy, and they can change the caps back if they want. I only recommend this mod to vintage Sunn bass amps, and not the guitar amps. Personally I don't like the sound of Sunn amps for guitar so I haven't wasted any time worrying about modding them per se. Bass players will like the increased filtering because it makes the bass more present and tighter, and the amp is quicker. It also hums less. A lot of bass players today do not even like tube amps for bass as they are quite a bit slower than solid state amps, though with some tasteful mods, tube bass amps can be made quicker and still retain vintage character. The mods can be done to where no holes have to be drilled and the amp can be put back to stock if desired. Its not like the Sunn amps are worth a ton of money, and I doubt if they ever will be worth what the top notch Fenders and Marshalls are worth. If you're worried about it, then don't do the mods to your amp. Stick with the stock quad can cap and when it fails in 5 years due to over-voltage and poor QC, then you can replace it again, and then again in another 5 years, and you'll spend about 3 times the dough to do it each time. Technology moves ahead as time goes on and sometimes its a good idea to take advantage of it. Can caps are old tech and aren't used unless they have to be for space reasons these days as they cost more. Btw, leaving the can cap in place works for aesthetics, but it also covers a hole that would otherwise be there in the chassis and would have to be somehow safely covered with high voltage DC just under the hole.

I've done the discrete-larger caps mod to a Sonic 1/200S that I don't ever plan to get rid of, and for a friend's 200S and 2000S. The 2000S gets rave reviews by everyone who hears and/or plays it. I know of quite a few other people both here on this forum and elsewhere who have done it themselves and are very happy with the performance and sound of the modded amps.

Greg

[kerlkoenig]

If you've got a vintage amp whose value partly depends on bad maintenance decisions like a frayed old 2 prong power cord or leaky filters, better sell it soon. The fad won't last forever. In a few years those of us who are still buying amps to play rather than to collect will be able to pick them up cheap again.

[Paul of the Bastards]

The fad won't last forever. In a few years those of us who are still buying amps to play rather than to collect will be able to pick them up cheap again.

Can't wait!

[EdBass]

Sure there are some people out there who will frown on replacing ANY part in a vintage amp, but these are usually collectors who don't play or have no knowledge of the fact that some of these parts do wear out just like the tires on your car. Try driving a 1951 Cadillac around with rotted tires from it sitting in a barn for 50 years.....its the same thing with electrolytic and paper caps and your vintage amps. If the amp has been sitting around without use, you can almost guarantee that it will have at the least a very loud hum when you turn it on, and at worst it will blow tubes, output transformers, fuses, etc. You can reform these e-caps sometimes and the amp may function for awhile, but you never know when those old caps will fail. Replacing the caps restores the amp to the condition it was in when new.

Now with the exception of defective coupling caps that are leaking DC, I personally don't recommend replacing the coupling caps in vintage amps until they do fail, as replacing those with new caps will change the sound quite a bit more than using exact value replacement electrolytic caps when those type need to be replaced. With the Sunn mods I recommended a couple years back for replacing the can with discrete caps, you can do that with exact value caps and it won't change the sound. For Sunn bass amps, I personally think it sounds better to go with larger values in the first two stages, and Conrad has heard my amp in person and he likes it better that way too. For the vintage snobs, that would be heresy, and they can change the caps back if they want. I only recommend this mod to vintage Sunn bass amps, and not the guitar amps. Personally I don't like the sound of Sunn amps for guitar so I haven't wasted any time worrying about modding them per se. Bass players will like the increased filtering because it makes the bass more present and tighter, and the amp is quicker. It also hums less. A lot of bass players today do not even like tube amps for bass as they are quite a bit slower than solid state amps, though with some tasteful mods, tube bass amps can be made quicker and still retain vintage character. The mods can be done to where no holes have to be drilled and the amp can be put back to stock if desired. Its not like the Sunn amps are worth a ton of money, and I doubt if they ever will be worth what the top notch Fenders and Marshalls are worth. If you're worried about it, then don't do the mods to your amp. Stick with the stock quad can cap and when it fails in 5 years due to over-voltage and poor QC, then you can replace it again, and then again in another 5 years, and you'll spend about 3 times the dough to do it each time. Technology moves ahead as time goes on and sometimes its a good idea to take advantage of it. Can caps are old tech and aren't used unless they have to be for space reasons these days as they cost more. Btw, leaving the can cap in place works for aesthetics, but it also covers a hole that would otherwise be there in the chassis and would have to be somehow safely covered with high voltage DC just under the hole.

I've done the discrete-larger caps mod to a Sonic 1/200S that I don't ever plan to get rid of, and for a friend's 200S and 2000S. The 2000S gets rave reviews by everyone who hears and/or plays it. I know of quite a few other people both here on this forum and elsewhere who have done it themselves and are very happy with the performance and sound of the modded amps.

Greg

Components don't really matter to $ value, as long as you don't drill holes or structurally modify the piece. If it's a reversible mod, particularly a safety or maintenence issue (I am also in the "replacing filter caps is like changing the oil on your car" camp), I don't think it matters at all. As you know, Greg, I've modded the power supply on almost every "player" Sunn I have, if the time comes to ever sell one, I'll put it back to stock if the buyer wants, or give them the stock parts, no biggie.
Grill cloth, tolex, face plate, and other structural stuff is another story altogether; from a collector standpoint original wins over restored every time.
Even if it looks as ratty as all get out. Attached is a pic of a 2000S rig I "devalued" intentionally in order to have it look good!

If you've got a vintage amp whose value partly depends on bad maintenance decisions like a frayed old 2 prong power cord or leaky filters, better sell it soon. The fad won't last forever. In a few years those of us who are still buying amps to play rather than to collect will be able to pick them up cheap again.

What about people who collect vintage amps/gear, but also play them? Will those people be able to pick them up cheap again too?

Seriously though, I think the time to buy is NOW, IMO if you're waiting for the bottom to fall out, you're missing the boat! I watch these things pretty closely, and prices are lower now than they've been in a long, long time.
Check eBay completed auctions, or try making some seemingly ridiculous offers to Craig's List sellers. It's a sad fact of life, but some people are getting desperate.
Luckily for all these economic trends are cyclical, but when the market dictates... BUY, BUY, BUY!
In few years I suspect (i.e.; really, really hope!) that vintage gear values will be where they were a few years ago.

[Oli]

Hi,

for me it's a strage thing to "collect" amps without using them. The amp consits of electronic parts which change their parameters by getting older. Thats simply a fact. I can not understand why peope are gonig to risk thier amp just to be shure that it stays 100% original.
You may know "Spinal Tab", the guitar man has a guitar on tour which must not be played...."don't even look at it"....sounds the same to me. Amps are build to be played. If the value of the Sunn amp goes up...fine to me. But it's all about sound and music, not about money or "artifical vintage virginity"!    

....just a point of view....

Greetings

Oli

[Soundmasterg]

I completely agree about some of these collectors who don't even use their amps. The ones who won't even consider an amp unless it is 100% original and buy them as an investment have a screw loose. These things are meant to be played, and like anything else in this world, they wear out and parts need to be replaced. You can replace them as original if all conditions are the same and expect them to work the same, but all things are not the same. AC wall voltages are higher these days, which means the DC voltages inside the amp are much higher, often exceeing the cap's max voltage level. Sounds like a good time to upgrade the amp to ensure it will work well to me. You can do this with stock values, or go with larger power supply values for a better sound and less hum, and thats what I and some others choose to do. Anyone who is working on amps should be able to tell you why they are modding an amp and what the pros and cons are, and they should also be aware of the dangers of firing up an amp that has been sitting for years, or has vintage power supply caps in it, and not bringing up the amp on a variac if the caps are not replaced. If they don't know why you should do this, then they should not be working on amps for safety reasons.

Greg