Old tube amps ... replacing filter caps!

[doveman]

I always thought that the only thing you needed to replace on tube amps ... were the tubes. Not the case. I found out that these high voltage filter caps are only supposed to last about 10-15 years. They can make life difficult if they fail while still in the amp.

In the photo below ... you'll notice some small dimples at the ends of these 30uf/500v caps. When these fail they can explode the contents (electrolyte) on the amp ... making a real mess. :doh:

My 29 year old boogie amp was occasionally losing volume mysteriously. That is one of the symptoms ... along with ghost notes and several other things.

The tech-types over at Mesa/Boogie forum helped me out ... I replaced (3) 30uf/500v and (2) 220uf/285V filter caps. Amp sounds tighter ... better tone. Who knew? Just sharing.

 

[muttonchopsrule]

Re: Old tube amps ... replacing filter caps!

them there things can be scary! that sure is a lot of voltage.

congrats on the success though. they really aren't too hard to deal with, once you're sure they're discharged.

 

[doveman]

Re: Old tube amps ... replacing filter caps!

them there things can be scary! that sure is a lot of voltage.

congrats on the success though. they really aren't too hard to deal with, once you're sure they're discharged.

Yeah ... IMPORTANT POINT! These caps can KILL you with stored energy you even after the amp has been off for a while. Those same techs suggested that I discharge the cap carefully until it was below 9V before handling. The way I did it ... set up completely insulated 10W resistor with two insulated alligator clips. Clip on ends of each cap individually for 30 minutes. Check with voltmeter ... under 9V then you can handle. To be safe I left my amp off for a couple of days. MAKE SURE YOU KNOW WHAY YOU ARE DOING!

But think about getting a tech to replace them if you don't feel comfortable handling them ... it may make a serious mess inside the amp if they fail. Plus ... the amp sounds better!

 

[krankguitarist]

Re: Old tube amps ... replacing filter caps!

Did a cap job on my '68 showman reverb when I first bought it. They had never been changed. Crazy.

The amp sounded 110% better afterwards. I swapped out all the carbon comp resistors on the cap board as well in favor of some metal oxide resistors. One of the old resistors looked like it had seen better days.

The result...a friggin awesome vintage fender.

 

[doveman]

Re: Old tube amps ... replacing filter caps!

Reportedly, you can have shorter lives with amps that haven't been played for a while. If they sat in a closet or strorage without being played then sold and played all of a sudden.

I played mine just about every day ... for the last 30 years. But I think mine were about to go. But I was probably riding the "hairy edge".

 

[Rushfan2112]

Re: Old tube amps ... replacing filter caps!

Hell, I'm pretty sure changing filter caps even on new (and cheap) amps will improve tone a bit. Glad it worked out!

 

[WhoFan]

Re: Old tube amps ... replacing filter caps!

when i bought my 65 Fender Deluxe amp it was dead.... plug it in and it blew a fuse right away.... I had it serviced and they changed the tubes and got it working but the tubes were gone in less then 20 hours playing time.... took it back and the caps had to be changed.. something was allowing more then normall voltage to the tubes and there was noise...

 

[Rich_S]

Re: Old tube amps ... replacing filter caps!

Something else to keep in mind - filter caps (and their little electrolytic cousins, the cathode bias bypass caps) age and wear out even when the amp is not being used. Unlike tubes, which wear out from use, the caps' electrolyte dries up whether or not you use the amp.

 

[doveman]

Re: Old tube amps ... replacing filter caps!

Update: Been playing the amp for about a week now. It really sounds a lot better. At first, I was just replacing these caps because there were symptoms ... losing volume. The amp is tighter with better tone. I recommend doing this upgrade.

Important: Just educate yourself on how to handle these high voltage caps (again they can kill you even when the amp is off).

 

[bigbob]

Re: Old tube amps ... replacing filter caps!

The caps on the board almost never fail and can last nearly forever. Many believe that the bypass caps in the tone stack contribute to the amps tone and should never be replaced, at least on 50's and 60's fenders and vintage Marshall's. It certainly will hurt the resale value. The BIAS caps going to the output tubes should be replaced however as they are under higher voltage than the tone caps.

 

[WhoFan]

Re: Old tube amps ... replacing filter caps!

The caps on the board almost never fail and can last nearly forever. Many believe that the bypass caps in the tone stack contribute to the amps tone and should never be replaced, at least on 50's and 60's fenders and vintage Marshall's. It certainly will hurt the resale value. The BIAS caps going to the output tubes should be replaced however as they are under higher voltage than the tone caps.

when i had my 65 Fender recapped i asked for all parts to be returned to me... so i have a bag with all the caps and stuff in it... i thought it would be best to hang onto that stuff

 

[doveman]

Re: Old tube amps ... replacing filter caps!

This is what they look like when they fail. If you look at post #1, you can see the slight bulges starting on my caps ... similar to the 2nd from the left in this picture. I was beginning to get volume dips occasionally ... even after retubing the amp.

These caps can explode their electrolyte contents inside the amp ... if that happens ... you may have several non-original parts in that old collectible amp.

If you bag the old caps and never run them, then put them back in years later, they are more likely to blow and make the mess ... because they have not been used. I pitched my old caps.