Forming filter caps

[novosibir]

... it was soldered to the red cable coming from the diodes, where the 100K is added. So the red wire is soldered to both resistors, one going to ground the other to the first cap. Is that not right ? Should I add a wire between the 220k and the 100k ?

The wiring like you did is correct, but I'm astonished, that you've had a voltage drop of 100V and this shows, that every PT is reacting different, depending on how stiff or spongy it is. Have you tried bigger resistor values? 470K, 680K?

Larry

[bill bokey]

No I only tried 220k this time. I'll try again, replacing all caps and with 470k or higher.
Thanks for your help

[Dom]

Hi guys,

I'm just forming some old Erie 100uF 350V caps. Maybe they're from 1975-76 becasue the 4 digit serial number starts with 75** and 76**.

I made kinda forming rig with my 50W Plexi. I pulled all tubes and put an alligator clip from rectifier to 100K and from there on I hook up the caps in series to ground. Voltage is 450V.
The caps seems to be good.
1st pair 8 hours 2.5V across 100K R
2nd pair 11 hours 0.9V
3rd pair 2 V
.... then I stopped counting hours. But all are below 4V.

But there were big differences when checking voltages across the caps themselves.
I.e.
#1 cap 330V - #2 cap 120V
#1 cap 280V - #3 cap 170V
#1 cap 270V - #4 cap 180V
#1 cap 400V - #5 cap 50V
#6 cap 230V - #7 cap 220V (best match so far)

So from the voltage to voltage ratio can I find out which caps are the better ones?
Do I only have to look for a balanced pair ? is higher voltage less or more capacitance?

Cheers
Dom

[MajorWatt]

Larry, is it OK to simply pull the tubes (ALL of the tubes, unless you have a tube rectifier that is) and simply variac the amp? Start low and slowly increase every hour or so?

Indeed....

[jbhunn]

Here's a cap forming rig I built based on Larry's method using a pedal enclosure mounted on top of a Drake 50W Marshall PT I had laying around - along with a variac. I formed some 50/50uf JJ's first - took FOREVER. New F&T 50/50's formed in just an hour to under 2 volts.

[SteadyEddie]

Had a Ceriatone "Holy Grail" cap fail, so I ordered some ARS cap cans. Got a 50+50 for the mains (100uf total) and 16/32 for screens/PI. Leaving the preamp caps alone for now but I have two separate 16uf for the preamps.

The ARS caps came in today, I soldered them in, then soldered in the 100k 1w resistor. I figure if I'm going to do this, do it right. Fired it up, and it was over 90v, but dropping steadily. It's down to 23v right now, I guess about 20 minutes after I started it. Looks like it's dropping voltage a bit slower now.

ARS caps are supposed to be quality, but I figure that they're sitting on shelves there for a while in Van Nuys, CA. Can't wait to hear my amp tomorrow!!!

[SteadyEddie]

Right now, it's sitting at around 6V, doesn't look like it's going any lower. Is it possible that I'm reading some leakage from the existing preamp axial caps, which were never formed like this?

Edit: Update, it's about 4.9v right now. It's been sitting around this for a couple hours now. I think I'll call it good, anything less than 5v is supposed to be good, right?

[fredcapo]

Hello ,

New member here. This is an excellent site. Thanks.

I have followed the reforming procedure of page 1 and after 14 hours am only down to 36 VDC
across the 100K resistor. it started at over 200 V and took 2 hours to get into the 60 V range.
I am applying this to a 1973 Super Lead 100 that most likely has not been powered up in a decade or two, and that
appears to be all original - no mods or previous repairs.

My question is - if after 24 hours the voltage is not down to around 5 VDC, how long do I let it run or at what
point do I declare there is a bad cap and try to isolate the leaky cap?

thanks

[novosibir]

If you aren't below 10V after 48h

Larry

[fredcapo]

Thanks for that guideline... At 24 hours the voltage seems to be cycling between 31 and 27 VDC on the meter, as if the forming is not holding. Assuming it holds at around 30 volts, can the amp be safely operated without risk of catastrophic cap failure?

Just for the record, if I set meter to VAC it reads about 40. Not sure if that is just normal ripple or indicates bad cap.

Thanks

[novosibir]

40mVAC would be ok, but 40 VAC is way too much and IMO impossible. If this is the case, then maybe you haven't disconnected the yellow cable, what goes between the 1st filters?

A Marshall, what goes below 5V during forming is great.
A Marshall, what goes below 10V during forming is ok.

But a Marshall, what stays beyond 10...12V is a risk and earlier or later at least one cap will say 'bye'

Larry

[SteadyEddie]

40mVAC would be ok, but 40 VAC is way too much and IMO impossible. If this is the case, then maybe you haven't disconnected the yellow cable, what goes between the 1st filters?

A Marshall, what goes below 5V during forming is great.
A Marshall, what goes below 10V during forming is ok.

But a Marshall, what stays beyond 10...12V is a risk and earlier or later at least one cap will say 'bye'

Larry

I was also reading some AC voltage during forming. It would decrease as the DC voltage decreased. I didn't get a final AC reading once my caps got below 5vdc.

[fredcapo]

Thanks for the comments. The center tap wire is disconnected as well as the screen resistors.

The voltage drop across the 100K resistor is appx 30 VDC and yes reads 40 VAC. I am not enough of a circuit wonk to explain.

However the voltage at that point reads appx 460 VDC to ground with 16 mv AC component. Pretty normal perhaps except that maybe the DC should be higher.

So the voltage drop sticks at around 30 VDC - with a few volts here and there - from 18 to 36 hours. It was then powered down few hours and powered up and the voltage drop went from 200 back to 30 in about 2 minutes. I'm going to continue to keep it powered up but it seems to be stuck in the 30-27 vdc range.

Will more or less current be a possible fix? or replace one or some caps?

What do you recommend to replace these DALY caps?

Otherwise, the plan is to sell this amp for a friend. Maybe best to sell it all original, disclosing the leakage and let the new owner fix as desired. I'd like to list it on this site once a decision re caps is made.

Thanks

[novosibir]

By any means at least one of the DALY's is faulty, but if only one, the next one will follow soon.
Originality up & down, but no other way, than to replace ALL DALY's - otherwise it's just a question of time, until...

About the 40VAC I can't say much concrete, because I've never had the idea to measure somewhat different than VDC across the 100K

Larry

[fredcapo]

Larry

Thanks for your response. I edited/tweaked some of the data in my post. I'm going to let it run another 24 hours.

I have seen on some ham radio tech sites that some guys increase the reforming current at the later stage of the process.
What do you think?

Right now at 28 volts across 100k. We have 280 micro amps... Kids stuff right?

Thanks.