Filter Caps Replacement

[iwantmypie]

Hello all,

Just wondering if replacing filter caps is possible without a Variac? Do new caps really need to get "broken in" or can I install them and turn on my amp without problems? I haven't had any problems self servicing my tube amp but this is the first case where I'm wondering if I should have a pro work on it if I can't obtain the proper installation equipment.

Thanks!

[JoeM]

You dont need to fire the amp up on a variac just to replace the filter caps. What do you think happens to new amps off the assembly line?

[tedmich]

the variac ramp up is usually most needed when using older EL caps which might need "reforming"

if you used newish EL caps you should be fine but you can always run it low for a few hours to warm things up.

people actually sell variacs on the Bay labeled as "Van Halen Marshall Brown Sound" LOLOLO

[ThermionicScott]

I like to remove all the signal tubes from the amp, plug in a Copper Cap rectifier, and work my Power-Stat up to 120V (or the B+ up to the voltage rating on the caps, whichever comes first) over the course of an hour. Does it help? Who knows, but it doesn't hurt anything.

- Scott

[Chuck H]

I have noticed a difference in performance with "new" caps on occasion. I don't charge new caps slowly. I just put 'em in and go. And there have been times when an amp was a little noisy or had some other minor issue until the caps charged up better. Then all was well. I figure that sometimes "new" caps have actually been on the shelf longer than their intended shelf life. Sitting there unused basically renders them old caps, more or less. So I think a slow charge is a good idea. I don't have a Variac so I just use a light bulb current limiter (100 watt lamp in series with one side of the AC mains) and burn in for a few hours. This has worked well for older amps too. In fact, I was at a friends house (a really famous friend who use to MFG amps) and he had two identical twenty year old amps in their original boxes. He thought he was going to just uncrate them and crank em up. I told him not to but he did anyway and after about seven or eight minutes the first one fizzled and smoked the main fillter. I plugged the second one into the light bulb current limiter for a few hours before playing it and it's still working fine, but with minor hum issues. I repaired the one that blew up.

Chuck

[R.G.]

NB - the simplest way to re-form caps if you suspect them is not the variac.

While that can work with solid state rectifiers, any amp with a tube rectifier does not start providing any voltage to the caps until the rectifier tube starts conducting, which doesn't happen until the filament voltage gets up there, so most of the careful, slow, walkup of voltage is wasted. The effective range on the variac starts when the tube filament gets hot enough to start emitting, probably at a line of 80-90V. Then it runs through nothing to full current over the remaining range.

It's simpler to remove the tubes except the rectifier tube, temporarily solder a 100K to 200K resistor between the rectifier and the filter filter cap, then just turn it on.

A leaky cap will still leak, but the current is limited down into the "forms oxide" range by the low current let through by the inserted resistor. As the oxide forms, the voltage across the cap goes up and the voltage across the resistor goes down. When the voltage dropped across the resistor gets down to some small percentage of the B+ voltage, then you know that the capacitors are supporting the vast majority of B+, and it's safe for the new caps to pull out the temporary resistor.

The resistor also keeps any Doh!! errors in the job from spraying electrolyte all over the inside the the chassis. How do I know this?

[MWJB]

"While that can work with solid state rectifiers, any amp with a tube rectifier does not start providing any voltage to the caps until the rectifier tube starts conducting, which doesn't happen until the filament voltage gets up there, so most of the careful, slow, walkup of voltage is wasted. The effective range on the variac starts when the tube filament gets hot enough to start emitting, probably at a line of 80-90V. Then it runs through nothing to full current over the remaining range." Sorry RG but this is a myth, any tube rectifier that I have come accross works fine from 1/3 of wall AC (40VAC in the US, 80VAC in Europe). You will see 1/3 of main B+ & 1/3 of negative bias dc (not enough current draw to let dropping resistors do their stuff, so voltages down the B+ rail won't make any sense at first). It is a handy way of bringing an amp up, whatever the work carried out, as certain issues will show up well before you get to full wall AC (wrong phase on OT primaries, back to front filter caps, easier to track negative bias voltage against B+ so you don't hit the tubes with masses of plate current on start up).

[iwantmypie]

we'll I'm not too picky about whether they are new or old cap. I'd personally have something new that's not NOS.

Any good suggestions? My amp is a Peavey Mace and the filter caps are 100uf/350v.

[stokes]

Its always a good idea to do it,as someone already said,you dont know how long they've sat on someones shelf.Sure,amp manufacturers dont do it,and I've seen way too many new fenders(less than a year old) with bad caps.I'm getting an amp in tommorrow from a guy who a year ago wanted to do his own cap job.He asked me to order him caps for his '68 Bassman.I warned him about the variac method and he said he heard it was a lot of BS and he could just install them.So now he wants it "done right",since the his main supply cap vented its guts.I told him I'll do it,but I aint cleaning any mess the cap vented.I was taught many years ago that forming them was the proper way to do it,so thats the way I've always done it.Is it absolutely necessary?I think so,I know a lot say it aint,but I've never had a cap job come back.

[R.G.]

Sorry RG but this is a myth, any tube rectifier that I have come accross works fine from 1/3 of wall AC

As you say, any tube rectifier that you've come across.

My sample space may be smaller than yours, but I have encountered tube rectifiers that didn't do much til the voltage was on up there.

We're all shaped by our experiences aren't we?

That's not to say that slow bring up on a variac can't be useful. I have and use one. But specifically for recapping, a series resistor uncovers problems as well as a variac *for me* when I haven't gone and mucked with anything before the rectifier. YMMV.

[soundmasterg]

I've used RG's method to reform caps, although I was using my Sprague Tel-Ohmike instead of an amp with a limiting resistor soldered in, and it works great.

Greg

[tedmich]

there is always (gasp) Gerald Weber's series lightbulb rig for initial startup.

At least its cheap, I have a little nichrome toaster unit made by Clarostat which does something similar, but its only 2 prong.

[R.G.]

there is always (gasp) Gerald Weber's series lightbulb rig for initial startup.

At least its cheap, I have a little nichrome toaster unit made by Clarostat which does something similar, but its only 2 prong.

Yeah, the light bulb thing probably does work, but I worry about it holding the leakage current down low enough for forming current on a cap that really needs the limitation. It's sure a great way to keep little curls of smoke from coming out!

Being an obsessive-compulsive sort where electronics are concerned, I like to get right down to what needs controlling, so personally I always just go put the resistor right in series with the caps. But there are other ways. There's always another way.

and as far as

I've used RG's method to reform caps,

I can't take the credit for this one. I read it in some musty, dusty old Golden Era service literature. It was the preferred way for that author at least back in the day when your TV repairman brought tubes to your house - back when there were tube TVs, and back when there were visiting TV repairmen.

[JHow]

Gerald Weber's series lightbulb rig

GW may have popularized this among amp builders, but I'm pretty sure that trick is way older. I have seen it in radio repair texts.

[R.G.]

GW may have popularized this among amp builders, but I'm pretty sure that trick is way older. I have seen it in radio repair texts.

Yep. That one is quite old, too.