Experiences forming NOS LCR caps with 'Larry's Method'. . .?

[drasp]

Hello all - first post here!!!

I've got a bone stock '77 JMP 2100 that I'm planning to re-cap & put into regular service as my primary practice & gigging amp. Already got a set of fresh Vishay BC 118s for the small e-caps on the PCB, had planned to just get 3x F&T 50/50s for the filter caps, but had a gift from a very cool forumite of 3x late '90s/end of run NOS LCR 50/50s.

I've read the thread on "Larry's Method" here:

https://tubeamparchive.com/viewtopic.ph ... cr&start=0

Great read. I don't have a variac, and if I'm reading correctly, wouldn't need one for this method.

I know a lot of folks will simply say not to waste my time trying to form old caps, even desirable caps such as the LCRs, but its my own time I'm wasting if it doesn't work out, so as long as I'm taking steps to do it right & minimize risk to the amp, I'd like to give it a whirl.

Anyone been down this road with the late '90s NOS LCRs & wish to share their experience?

THANKS!!!

[danman]

Hello, this is also my first post and I thought I would share my experience with the "Larry Method". I recently finished building a JMP 2204 clone and decided to try this method on the set of ARS 50/50 caps that I used. For this 50 watt circuit all I had to do was attatch a 100k 2watt metal film resistor across the standby switch and turn on the amp. The voltage initially read in the high 300's, but steadily dropped over a period of about thirty mins until it leveled off at about 4volts. Very easy procedure on the 50watt circuit and I will definitely use again if I am unsure of the caps condition. I believe the procedure for the 100watt is slightly different but worth trying. Hope this helps!

edit: Sorry, I forgot to mention that I did have all tubes removed from the amp and brought the voltage up slowly at first using my variac. I believe in Larry's article that he never mentioned using the variac but I just felt more comfortable bringing the voltage up slowly to be sure I had not made any mistakes.

[drasp]

Thanks for the reply danman!!! I'd read about using a variac, makes good sense, but I don't have one. . .how essential is it for the Larry method?

[Roe]

no need for a variac. the larry method is better than the variac method since it controls not just voltage but also current

[Mescalero]

It may well be appropriate to refer to MIL-HDBK-1131B rather than to "Larry." Nothing against Larry, never met him myself. However, procedure, acceptance/rejection criteria are all well documented. Here's a link to a copy:
http://www.dscc.dla.mil/Downloads/MilSp ... hb1131.pdf

[Paultergeist]

wow......that is a pretty solid MIL-SPEC reference for this......

Thank you for sharing.

[danman]

As Roe mentioned, the variac was not necessary for "Larry's" method. I was just being over cautious because this was a new build and I wanted to be sure I had not made any mistakes.

[drasp]

Way cool!!!

Is it necessary that the resistor be 100k or similar? Just realized the largest 1W resistors I have around are 10k - will that allow too much current?

[martin manning]

Just realized the largest 1W resistors I have around are 10k - will that allow too much current?

Ten times as much. Got a couple of 220k 1/2W you can parallel?

[talbany]

I used Larry's method to reform some old Yellow Sprague Atoms for a Dumble amp several years ago.. I used Mil spec 100k's.. Took about 12 hours for the caps to finally get under 5v but worked like a champ.. Amp sounds great too!!

Tony

[drasp]

Just realized the largest 1W resistors I have around are 10k - will that allow too much current?

Ten times as much. Got a couple of 220k 1/2W you can parallel?

Good call! I didn't have any 220ks, but I had 3x 390k 1/2W parts that I wired in parallel for ~130k & figure the 3rd 1/2W part should help dissipate extra heat.

My setup is a bit different from the 100W amp illustrated in the .pdf linked in my OP - the red wire from the diodes goes to the H.T. fuse, then turns yellow & goes across both sections of one 50/50. This is where I put my resistors in series and I'm monitoring my Vdrop there. Started out @ ~250Vdc, has been slowly dropping, down to ~150Vdc now, maybe ~10min later. In Larry's original write-up, he suggests the voltage drop should be down around 30-40Vdc after only ~30seconds, but the .pdf also has some back & from what I think was an old MetroAmps thread. Someone there was forming old Dalys & took 4hrs to get down to 40Vdc. Going to keep monitoring these & see how they do, will report back!!!

[drasp]

Also, dumb question & too late to be asking, but i assume chassis on the bench, tubes pulled, no need for a speaker to be plugged in, correct!?

[martin manning]

Correct, and as long as it's still dropping, let it keep going.

[drasp]

Yup, down to ~70Vdc already.

[drasp]

Happy outcome! Woke up this AM to ~4Vdc drop across my resistors. Installed my tubes, drained the caps through V1 & then, because I still have the resistors in, decided to see how quickly the Vdrop would get back into the "happy" range - within ~2min. I'm back below 5Vdc - looks like these LCRs have formed up just fine!!!