Power supply capacitors,, What's up?

[Hub 2]

Hi,

Thanks to everyone for your help & advice.

I am going to replace all 6 caps and maybe upgrade some of the resistors.

When I said the 2 main caps were leaking, it is because the electrolyte has leaked out and is visible at the ends of the 2 main caps, I did not assume that a 6 volt increase in voltage, at a test point of 440 volts was a leak.

Do I need to use a variac to supply power to the amp after changing the caps ? I do have a mains socket dimmer switch, but I know it is not the same thing.

Andy

[jrfrond]

Besides the fact that "they just don't make 'em like they used to", there are a lot newer amps I've seen in which the physical relationship of the caps to the power tubes (all of the tubes actually) is just plain wrong, compounded by the fact that most of the caps utilized are rated at 85C, and exposed to a lot of heat for extended periods of time. So this is a "Three Strikes Yer Out" deal here, a veritable Hat-Trick of issues which I personally believe is responsible for premature 'lytic cap failure in many newer amps.

Incidentally, when it comes to filter caps, my mantra is "One bad, replace all", unless it is a physical break. Is it overkill? Perhaps, but for me, it's cheap insurance to keep the customer happy.

[greekie]

Also, 450V 105C variants are SO cheap now a days! So I can definitely agree to that!

[stokes]

In one instance,my friends less than 2 year old Blues Deville,I took the offending 22uf from the input AX7,which had 17vac on it in circuit and was causing the oscillation,out and tested it with my cap meter,it read 28uf.I then placed it in my leakage tester,which has variable dc voltage to 500vdc connected to a Fluke 8000A miliameter and it showed only .07ma's leakage at 450vdc,you would think this acceptable,no?But there you have it,that cap was causing oscillations.I say damn the specs,crap is crap.I will only use Sprague or F&T,period.And like John says,if one is bad,change them all.

[tedmich]

capacitor "forming" with a variac is less of an issue with fresh caps bought from a reputable component dealers; (Mouser, Allied, Newark, Digikey, Farnell?) as they should be ready to go. Just leave the amp on for a day before cranking it up IME. DO NOT use a dimmer as they are noisy or worse with audio equipment. Resistors do not "upgrade" just replace faulty ones IMO.

[Chuck H]

I've always used caps as they arrive. I buy fresh for each build (I don't build much). I've only
had "stale" caps twice. Once was back in the day when you could walk into an "electronics" store and buy components. Seems they weren't rotating stock. Those caps hummed terribly and were promptly replaced with known "fresh" ones that were fine. The second time was a more recent build so the caps were ordered from Mouser. There was some minor humming and ghost notes in a build I knew to be better. I simply left the amp on for a few hours and it improved. I continued to leave the amp on overnight and by morning the amp was performing as it should. Other than those two incidents I've done fine just sticking the caps in the board and letting the amp burn in for two hours at initial start up. But...

It's not unusual for an amps B+ to meet or slightly exceed the caps rating when the circuit is unloaded. Once the caps are broken in they will handle the brief periods of abuse just fine. But I always have tubes installed for the burn in. I know that current is bad for forming but I think too much high V is worse. This is my rougue method and, while not ideal, seems to work just fine.

Chuck

[Enzo]

All your Fender, Peavey, Marshall, Ampeg, whatever amps are assembled and tested without bringing them up on variacs. Any additional forming the caps might need will occur as the amp runs its first few hours, as you discovered.

The ratings on the e-caps in the amps are working voltage ratings, they have also "surge" ratings. The caps can withstand higher voltages, just not all the time. It ain;t like a 450v cap will catch fire at 451 volts.

[jrfrond]

If you suspect a bad cap, don't bother "overtesting" it, just swap it out. This goes for ALL components. You can overthink it to death, when swapping would have solved the problem and saved time.

As Enzo said, no manufacturer does cap forming, hence, neither do most techs, unless using NVOS (New Very Old Stock) caps, and THAT is just a plain bad idea anyway.

[Hub 2]

Hi,

Thanks again for your help, I have now ordered 6 replacement caps for my Fender Deville, the original ones are Illinois caps :
2 x 22uf 500v : 2 x 100uf 350v : 2 x 47uf 350v

I wanted to try a different brand as the amp is only 4 years old and has not had much use, not like some of my amps back in the 70`s.

I chose F & T Axial lead caps : 2 x 22uf 500v : 2 x 100uf 350v : 2 x 47uf 500v

The 47uf caps are a higher voltage than the original Illinois caps, it was the closest I could find.

Is it ok to use these to replace the originals ?

Andy

[Euthymia]

The 47uf caps are a higher voltage than the original Illinois caps, it was the closest I could find.

Is it ok to use these to replace the originals ?

Yes!