Forming Filter Caps in a JTM 45/100

[rip]

Larry posted this a while back:

Forming filter caps...

... isn't also a bad things for new caps and enhances their physically parameters, also when they've been (quick-) formed at the manufacturer.

But for new caps, which have been stocked on the shelf for more than 5 years it's obligation!

I'm describing the steps valid for a 100W Marshall, with the 45W or 50W models you can share some of this steps:

- pull out the power tubes and the preamp tubes

- disconnect the yello wire going from the PT between the first two filter caps, temporarily tape the unsoldered end for safety (100W only)

- unsolder each one lead of of the both 56K bleeder resistors across the both screen's filter caps and tape it temporarily for safety (100W only)

- disconnect the red wire from the rectifier diodes (the rectifier tube) to the first filter cap and swap in temporarily a 100K/1W resistor

Now fire the amp up completely (power & standby to 'on') and measure the voltage drop across the 100K resistor you've swapped in!

You'll notice, that first you'll have a voltage drop of about 200V, which within 15-30 seconds reduces itself to about 30-40V - then further on it is dropping slowly.

If so, all's ok and you can switch off your meter and do your other things, because now the caps require 4-24 hours time to form, depending on the cap's quality and the cap's age.

Only insure, that nobody else could get in contact with your 'open' amp - way the best is, to lock the room, where the amp is sitting and keep the key in your pocket. It's danger to the life for unskilled people!!!

From time to time you can check the process of the voltage drop - and as soon as the voltage drop across the 100K is less than 5V (five volts), your caps are formed.

Switch off the amp - and before you remove the swapped in 100K resistor, implicitly bleed the caps by connecting pin #1 of V1 with an alligator clip to ground for at least 2-3 minutes!

Now reset the temporarily tweaks and put in all the tubes again - you've done Fire up the amp and enjoy its (valorized) sound

Larry


BTW: Also when an amp hasn't been in use for longer than six months, you should form the caps, before you fire up this amp again! If not, the longevity of the installed caps might be significantly diminished!

And wanted to see if anyone has done this with a JTM45/100. I'm concerned because of the yellow wire (HT CT). It's not connected the same way as on a normal 100 watter.

[rip]

For a JTM45/100 is this correct:

- pull out the power tubes and the preamp tubes
- disconnect the HT CT from ground or remove the HT fuse from ground?
- if you have the bleeder resistors across the filter caps to keep voltages equal remove/disconnect them
- disconnect the DC wire from the rectifier diodes to the first filter cap and swap in temporarily a 100K/1W resistor
- fire the amp up completely (power & standby to 'on') and measure the voltage drop across the 100K resistor you've swapped in


does this seem correct?

Thanks
Rip

[shakti]

It's dead easy on a JTM45/100:

Remove all tubes.
Follow the wire that comes from the rectifier and runs to the standby switch. This is B+. At the other side of the standby switch, unsolder the wire (which should run to the mains filter caps) and insert the 100k/2W resistor. Re-attach the wire to the other end of the resistor. Presto. You can now start forming the caps once you switch it on and also flip the standby switch.

Remember to drain the caps before you unsolder the resistor again. I forgot in all the excitement...ouch!!

[rip]

So....

Nothing has to be done with the HT CT because the B+ circuit is not like the traditional 100W with the voltage doubler circuit.

And because your don't have the bleeder resistors on the JTM45/100 you do nothing, I suppose if you added them then you may have to disconnect them.

Thanks
Rip

[rip]

Is what I said correct?

Thanks for the confirmation.
Rip

[Roe]

yes, just a 100k across the standby switch

[rip]

awesome thanks

Rip

[timsmcm]

you do this with all of the tubes out and the speaker output hooked up to a speaker cab.

[Roe]

tubes out. speaker jack shouldn't matter

[shakti]

Tubes out, yes. But if you're using a Marstran PT, I'd put it on the low HT taps...if you use the 560V taps, you'll be hitting the amp with probably close to 600V, which won't drop all that much before the phase inverter and preamp, where you'll have single caps. On the mains and screens you have caps in series which means double their voltage rating is safe. But in the phase inverter and preamp you "only" have the cap's rated voltage to go by. At 450V, you may be at the limit if you have the tubes out and HT taps @ 560V.

However, if you're using a Metro PT or the Marstran @ 490V you'll be safe, I'm sure.

[rip]

Just curious what is the recommened voltage rating of the preamp filter caps? I currently have the blue BC 22uF 450v caps there. I'm using the metro 1204-43 PT which kicks out 500v.

[shakti]

A 450V rated cap in the preamp will be fine.

[rip]

Thanks

[Dom]

Just formed my filter caps in my JTM45/100 build.
I'm using Rifas and a BC/Vishay in the preamp.
It took quite a while.
Here are the measurements:
4h: 11V
11h: 8.5V
21h: 7V
28h: 5.8V
34h: 5.7V

After 34h I shut the amp off. Hope 5.7V is enough. Does somebody else has experience that Rifas take much longer than let's say F&T ?

Cheers

Dom

[Roe]

5.7v means ok but they form way slow. seems strange for new caps