Thoughts on forming NOS LCR 50/50s for my JMP. . .

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Thoughts on forming NOS LCR 50/50s for my JMP. . .

Postby drasp on Sun Apr 20, 2014 4:17 pm

SO I was very lucky in scoring a set of NOS LCR 50/50s (late '90s date codes) from a very generous member here, and I'd love to give them a whirl in my bone-stock '77 JMP 2100 combo. The amp has its original Daly caps & is in need of some fresh filtering parts. I've got a set of Vishay BC 118 caps for the parts on the PCB & had been planning to just use F&Ts for the 50/50s, but I'd love to give the LCRs a whirl, (even though I know some folks will say its silly to use anything other than the newest & best quality parts available.)

That said, I've done some reading here & on other boards about forming NOS caps slowly over a day or two with a Variac to bring up the voltage gradually. This seems like a great way to handle things, and I'm quite sure the LCRs should give many years of good service, BUT I don't have a variac at my disposal. I've also read a great thread on the Amp Garage board about "Larry's Method" (German amp builder who uses these NOS LCRs in his new amp builds). Here is the thread:

http://ampgarage.com/forum/viewtopic.php?t=18886

Any other thoughts on this? Anyone have experience using the last of these late '90s LCRs in recent times? Anyone want to give me a hard time for choosing old parts just because of their particular shade of blue & UK heritage?

:cheers:
If you have a way of contacting Steve Gibbs of the Audio Bros. or any information about what happened to him, please PM me!
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Re: Thoughts on forming NOS LCR 50/50s for my JMP. . .

Postby PeterS on Sun Apr 20, 2014 5:03 pm

I've used a method similar to that, no variac. Just take the tubes out of the amp, and install the caps but add a resistor of say 50k - 100k in series. Power the amp, measure the voltage across the caps. Voltage will rise slowly depending how leaky the caps are. Resistor needs a suitable power rating - if the caps are shorted the resistor dissipates a few watts.
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Re: Thoughts on forming NOS LCR 50/50s for my JMP. . .

Postby LUC.key on Mon Apr 21, 2014 10:03 am

I've used Larrys method succesfully in the past, simple and easy :cheers:
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Re: Thoughts on forming NOS LCR 50/50s for my JMP. . .

Postby Unit_1 on Mon Apr 21, 2014 4:32 pm

I'm lucky enough to own this - not only does it reform it but also can test if it was successful or not :

ouuuu, dials, buttons and meters - Oh My! :lol:
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Re: Thoughts on forming NOS LCR 50/50s for my JMP. . .

Postby drasp on Mon Apr 21, 2014 11:28 pm

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?

edit: 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!!!
If you have a way of contacting Steve Gibbs of the Audio Bros. or any information about what happened to him, please PM me!
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Re: Thoughts on forming NOS LCR 50/50s for my JMP. . .

Postby drasp on Tue Apr 22, 2014 11:09 am

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!!!
If you have a way of contacting Steve Gibbs of the Audio Bros. or any information about what happened to him, please PM me!
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Re: Thoughts on forming NOS LCR 50/50s for my JMP. . .

Postby Unit_1 on Tue Apr 22, 2014 3:48 pm

i love stories with happy endings! :jam:
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Re: Thoughts on forming NOS LCR 50/50s for my JMP. . .

Postby pckpat on Tue Aug 19, 2014 1:44 am

drasp wrote:SO I was very lucky in scoring a set of NOS LCR 50/50s (late '90s date codes) from a very generous member here, and I'd love to give them a whirl in my bone-stock '77 JMP 2100 combo. The amp has its original Daly caps & is in need of some fresh filtering parts. I've got a set of Vishay BC 118 caps for the parts on the PCB & had been planning to just use F&Ts for the 50/50s, but I'd love to give the LCRs a whirl, (even though I know some folks will say its silly to use anything other than the newest & best quality parts available.)

That said, I've done some reading here & on other boards about forming NOS caps slowly over a day or two with a Variac to bring up the voltage gradually. This seems like a great way to handle things, and I'm quite sure the LCRs should give many years of good service, BUT I don't have a variac at my disposal. I've also read a great thread on the Amp Garage board about "Larry's Method" (German amp builder who uses these NOS LCRs in his new amp builds). Here is the thread:

http://ampgarage.com/forum/viewtopic.php?t=18886

Any other thoughts on this? Anyone have experience using the last of these late '90s LCRs in recent times? Anyone want to give me a hard time for choosing old parts just because of their particular shade of blue & UK heritage?

:cheers:

I just got my 1974 Matamp GT100 back from my tech buddy. It was blowing fuses. Besides some bias resistors, he swapped out the old Erie can caps with a pair of 50uF/500VDC LCR's he had around, The date code seems to be 1989. He said the amp's response was too stiff with the OEM filter caps, so he changed them. I am sure he checked the used LCR's out very carefully, and used the proper procedure for their re-installation. The amp sounds incredible now-(it still has the original Mullard xf3's in it).Jeff, my friend who did the work has had a ton of nice vintage amps across his bench, plus builds his own when time and the demand are there. He says it is now one of the best sounding heads he's ever heard. He has done work for some pretty high-profile bands, so reliability is a priority, as well as tone.
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