Charging an amps e-caps

[dude]

Putting a variac on an amp that hasn't been played in a few years (that you knew had played well before) and bringing up the acv slowing to reform the caps, would this damage the tubes, the low voltage at first? I have an amp I haven't played in a few years, I set the variac on 50vac and planned on turning it up about 25v every five hours or so to reform the e-caps till I got to 120vac. Will this cause any damage to the tubes filaments? Or should I pull (except the rectifier tube) the tubes first..? 

[sluckey]

I would not be concerned. But if you have doubts just pull all the tubes, including the rectifier. Use a ss recto.

[dude]

I thought so at first but google didn’t produce answers. Thanks

[dude]

I just read that an amps tube rectifier doesn’t pass dc voltage till the voltage to the tube hit 75 to 80 vac..? If this is the case then a SS plug-in should be used and not the tube when bringing up e-caps on an amp sitting for years.
Any thoughts on this..?

[pdf64]

See http://geofex.com/tubeampfaq/TUBEFAQ.htm#forming

[tubeswell]

What everyone else already said. And I wouldn't bother trying to reform caps if they look bulgy or have corrosive looking gunk coming out of them.

[dude]

pdf64, thanks for the link. Tubeswell, yeah l’m with you, any evidence of a bad e-cap, replace. I have a few amps l stored for over a year or more, two haven’t been turn on for three years at least. Since l just got this new toy, the old General Radio Co variac from the 40’s and serviced it, l figured why not
bring up the caps slowly. Amps were builds l did years ago. But l agree with you. Thanks for posting.

[PRR]

...tube rectifier doesn’t pass dc voltage till the voltage to the tube hit 75 to 80 vac..?....

The 'detector' in a tube radio works on part-Volt "AC".

I have seen 12V batteries charged with tube rectifiers, both Mercury (works good) and vacuum (not ideal).

I don't know a heap about power rectifiers near 80V because in any normal operation we come up from zero to 400V faster than the meter-needle can move (or digits dig it).

But nevermind speculation. You don't have another tube amplifier near the bench? Bring the 400V of DC out through a 100k 2W resistor to your victim. These values will supply at-most 200V 2mA or 0.4 Watts to the cap being charged. If it is bigger than your thumb, it won't blow-up real fast.

[tubeswell]

I have a few amps l stored for over a year or more, two haven’t been turn on for three years at least.

2 or 3 years sitting idle shouldn’t be a problem if it was recapped within the last 10-15 years. I’ve left amps idle for that amount of time and they still worked fine. But I get the allure of trying out a new gadget. :-)

[bmccowan]

2 or 3 years sitting idle shouldn’t be a problem if it was recapped within the last 10-15 years.

That's been my experience too, and I have read (can't question that, eh?) that newer chemistry caps have much longer shelf life than older ones. I've tried the slow reforming thing with a variac, but I am way too impatient. I plan on leaving it at a low voltage for an hour and three minutes later I am turning up the knob.
In general, I think good quality lytic caps last much longer than rumors suggest.

[HotBluePlates]

... shouldn’t be a problem if it was recapped within the last 10-15 years.

That's been my experience too, and I have read (can't question that, eh?) that newer chemistry caps have much longer shelf life than older ones. ... In general, I think good quality lytic caps last much longer than rumors suggest.

A few years ago I bought a Sencore LC75 that measures leakage current while applying a high DC voltage.

Shortly before, I bought a 1964 AC30 and had replaced the power supply caps.  I tried out the Sencore on one of the removed caps and found its leakage was less than on some modern electrolytic caps!  I haven't re-installed the old caps yet, but it proved that they sometimes last very much longer than we think.

OTOH, the "10 year cap replacement rule" was espoused by a guy that serviced old amps.  When infinite time is available we can test every item to find the one individual part that failed.  When Time is literally Money and you get a "pay raise" by working more productively, you can't afford to test every individual component.  And your reputation suffers when a "repaired amp" comes back next week for another one-component failure, then the month after for another one-component failure.  So you develop "rules" that replace more than absolutely necessary, earlier than absolutely necessary, to minimize troubleshooting & repair time and avoid the risk of "Callbacks."

[dude]

I get all these replies, to one degree or another.  Maybe the best thing is "don't fix it till it's broke".  No symptoms, no new e-caps..?

[sluckey]

I get all these replies, to one degree or another.  Maybe the best thing is "don't fix it till it's broke".  No symptoms, no new e-caps..?

I agree. But you're not likely to do any harm. Go ahead and play with your variac if you want to.

[HotBluePlates]

I get all these replies, to one degree or another.  Maybe the best thing is "don't fix it till it's broke".  No symptoms, no new e-caps..?

Putting a variac on an amp ... and bringing up the acv slowing to reform the caps ... I set the variac on 50vac and planned on turning it up about 25v every five hours or so to reform the e-caps till I got to 120vac. ...

I read this bit in a Gerald Weber book, too.  I tried it about 27-28 years ago.

I did not wind up with a result that was any better than simply soldering in new caps and flipping the power on.  So I never "slowly formed filter caps" after that.

If I was concerned about an old amp that hadn't been used in a long time, I'd either use a lightbulb limiter or insert a 100kΩ resistor between the rectifier output & 1st filter cap (that with definitely slow-trickle current to the caps).