forming caps in dual rectifier JTM45/100.

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forming caps in dual rectifier JTM45/100.

Postby Homebelly » Sun Apr 06, 2014 8:49 pm

Hi..

I posted this over on the amp garage forum.
I got a couple of replies and changed my diagram up to reflect the feedback given in that post.
here is the modified picture..

Image

One of the people that posted got me a little concerned about the grounding of the mains filters, shown here in the picture coming off of the rectifier.
I have used a version of larry's grounding that i had help putting together. The end result being this plan..

Image

I am hoping some one could jump in an confirm i have this set out correctly, and that i am on the right track for both the grounding plan and the method to form the caps using two 100K resistors..?

Thanks..
Homebelly
 
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Re: forming caps in dual rectifier JTM45/100.

Postby neikeel » Mon Apr 07, 2014 12:27 pm

Glad you dropped by here as I was a little concerned at the advice there (I am not a member at Amp garage).

The position of the 100k resistors is correct (at the apex of each rectifier grid where the B+ comes off)

What is not correct is the ground in black on one pic and the orange wires going to the grounds on the sockets. Your diagram shows them hooked up to the +ve of the filter caps - absolutely incorrect. The grounding rail of these power caps is the negative on the wire next to the switches. I have an original of one of these amps of my own and have one that I am tidying up for someone else at the moment:

Mine (67 45/100 Superlead):http://vintageamps.com/plexiboard/viewtopic.php?f=1&t=88501&hilit=early+superlead
Image


Current work (67 JTM100 Superbass):http://vintageamps.com/plexiboard/viewtopic.php?f=1&t=106855
Image
Scroll down and you will see me trying out some caps for size, more pics on my PB album.

So you see I might be cap reforming soon too :wink:
Neil
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Re: forming caps in dual rectifier JTM45/100.

Postby Homebelly » Mon Apr 07, 2014 5:23 pm

thanks neikeel...

I have a sick baby and a heavely pregnant and very tired wife on my hands at the moment.
i'll change everything up and post the fix later..

are those RS filter cans in your build..?
Homebelly
 
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Re: forming caps in dual rectifier JTM45/100.

Postby dai h. » Mon Apr 07, 2014 7:27 pm

the connections for the board look truncated in the first pic (no parts drawn in). If this is a wiring guide, I think you should be REALLY careful with this (might even be better to take the time and draw in the parts, connections accurately, correctly) so you don't inadvertently make a mistake (you could be tired, distracted so working from a wrong diagram (with assumptions) could be dangerous). For example, the connection for the feedback side of the phase inverter looks like it's going to the B+ (where the choke, 2nd filter after 1st off of rectifier out, and screens are connected together)!

as far as grounding schemes, different physical configurations will work as long as they make sense (in terms of theory). If you are following someone else's scheme I suppose that is okay as long as it's a good one (so make sure it is correct).
dai h.
 
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Re: forming caps in dual rectifier JTM45/100.

Postby Homebelly » Mon Apr 07, 2014 9:30 pm

neikeel wrote:What is not correct is the ground in black on one pic and the orange wires going to the grounds on the sockets. Your diagram shows them hooked up to the +ve of the filter caps - absolutely incorrect. The grounding rail of these power caps is the negative on the wire next to the switches. I have an original of one of these amps of my own and have one that I am tidying up for someone else at the moment:



Thanks for the help and support with this build neikeel across all of the various forums 8)

Any hoo..

how is this version..?

Image

Next set of questions regarding the cap forming are what is the final reading across both 100K resistors? I had all this stuff saved, but kind find the document :oops: :roll:
Homebelly
 
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Re: forming caps in dual rectifier JTM45/100.

Postby neikeel » Tue Apr 08, 2014 3:43 am

That is better, at least it is not wrong, not sure if that is the exact correct 'Larry' layout but it will work.

Typically for new caps you might get 40v across the 100k for a few minutes, then to 20 and 10 and reform (less than 3-4 v dc after a few hours. Old caps will take longer (overnight?).
Neil
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Re: forming caps in dual rectifier JTM45/100.

Postby Homebelly » Tue Apr 08, 2014 3:18 pm

Will i be measuring across both 100k's ?

I'm also a little concerned about what dai h has pointed out.

dai h. wrote:For example, the connection for the feedback side of the phase inverter looks like it's going to the B+ (where the choke, 2nd filter after 1st off of rectifier out, and screens are connected together)!


It all seems okay to me based on layouts and sceme's i have for other JTM/SL.
is there something wrong in there?
Both sets of filters should meet the 82K bias resistor along with the choke and the screens?

Both 100k should be the same approx

Dai's point is sound, the 8,2k dropper is where the choke is hooked up to (the other end is the HT feed to the choke and tied to the OT primary centre tap and B+ from rectifier and mains filters (via the HT fuse and HT switch) it is not 82k or a bias feed.

Post a detailed hook up diagram and we can vet it :wink:
Homebelly
 
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Re: forming caps in dual rectifier JTM45/100.

Postby dai h. » Tue Apr 08, 2014 6:31 pm

Image

Image

(maybe you didn't use the top diagram to wire?) Anyway, (in the phase inverter where three lines (resistors) come to a point, then go to a single wire towards the top, then to a wire over to the left side) can you see how the short wire sideways goes one lug too far? If wired according to the diagram, the cathode side of the phase inverter connects to the B+(!). Check out how the diagram compares to the actual amp. (I think small errors like this can just happen (someone has a brain fart or whatever), so it's good to try to be aware.)
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Re: forming caps in dual rectifier JTM45/100.

Postby Homebelly » Tue Apr 08, 2014 6:45 pm

Oh.. okay, yeah..
I can see where i screwed up in the diagram, and i can confirm it is not like that in real life.
My bad..
If i get the time i'll fix the picture and take these down.

File that under brain fart indeed.. :oops:
Homebelly
 
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Re: forming caps in dual rectifier JTM45/100.

Postby dai h. » Wed Apr 09, 2014 7:08 pm

ah, that's cool. Just a bit worried about safety.
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