Forming filter caps

[Strat78]

Take out all the tubes. Use a variac to keep the voltage from exceeding the max voltage rating of the pre amp couplers and filter cap as all the filter caps form. I start around 100v and increase to 110v all the while taking readings from the couplers and filter cap. Measure the volts across the 100k resistor, it should drop down to under 5v after 3 to 12 hours depending on how fast the caps feel like forming. After a couple of hours If you measure 30v across the 100k resistor and the voltage is creeping up around 375v at the couplers and pre amp filter cap, make sure you keep an eye on it because by the time the reading drops to 5v your voltages will be up around 475v which will fry your caps. Make sure you keep that voltage down below 400v with the variac.

[axeman]

Do I replace the red jumper wire that's going from the rectifier diodes to the filter caps with 100k resistor. Just to be clear. I get everything else.

Thanks

[Strat78]

Yes, but note that most 12xxx layouts have the HT wire going to the diode first then to the cap. In that pic the HT wire goes to the cap first then to the diode. The resistor goes between the HT/cap and the diode, I put little red dots to show where the red jumper goes from the cap to the diode, that's where the 100k goes. What kind of caps are you forming?

[axeman]

F&T capacitors, so just to be clear the 100k goes between the rec diodes and the first 100uf filter cap, and remove the red wire from under neath the board that would normally go from the rec diodes to the first 100uf cap.

[Strat78]

Yep, just connect that wire from the HT permanently to that first filter cap and when you are done with the 100k resistor, just run a little jumper from that cap back to the diode on the top of the board so you can access it easier for future formings. It's just a more convenient set up so you don't have to go under the board to disconnect the HT wire every time you need to form a new cap.

[axeman]

Cool, Thanks. I'm making a video on my build, now I can add forming the capacitors.

[youtube] https://www.youtube.com/watch?v=KWsJtr8N79M [/youtube]

[Strat78]

I know I'm being redundant, but here is how I wired it with my last build for easy access. The little red wire connects with the diode under the board and the HT wire connects under the board at the filter cap. It's so easy and clean looking this way. Just unsolder the red wire from the cap and put the 100k right there. The center tap is on the left fully accessible.

[axeman]

That's easy.

Thanks

[danwelsh]

It ghost notes really bad past 4. Like we talked earlier how I hate ghost noting!

It's senseless to form filter caps, which already were fired up once - they now are how they are.

Forming filter caps must be done BEFORE the first time firing up the amp.

Larry

Wish I would have read this about 24 hour ago......oh well next build they'll be formed

[Allie]

Hi folks... sorry to crash in here all of a sudden but I just wanted to thank Larry for the awesome write up of his cap forming procedure, and all the folks that asked questions and gave input. Someone on another site pointed me at this place when I mentioned that I was going to try to resurrect an old Guild T-112 that I shelved years ago because it was getting noisy. After reading this thread forwards and (literally) backwards I felt confident enough to dive in and cook me some caps. My little Guild is a leetle different than the Marshalls inside so I had to figure the particulars on my own, but the principles were well explained in this thread.

I ended up disconnecting the first capacitor lead AND a red wire from the cathode pin of the rectifier in order to limit current to all the caps, and tacked them to the lead of the 100k resistor. Put the amp on a Variac to limit voltage to ~100v, and away I went. The Variac was probably not necessary after all on that amp, even at 120 volts and all the tubes pulled nothing went higher that about 360 volts, well under the limits of the lowest rated caps. What I missed at first was basically a bleeder resistor. None of the resistors in the PS went directly to a ground, but after the thing stalled out at 50v for a few hours I knew something was up... one resistor went to feed a pilot lamp, and from there the lamp went to the chassis... durr! And yeah the voltage drop there was something in the neighborhood of 50v. Unsoldered a lamp lead and things got moving again. The last few volts went really slow... It seemed to stall out again at 9.5 volts, so I tried what was described in a similar write up on an antique radio site, subbing a 10k resistor for the 100k when the voltage gets lower. The amp is only 12 watts, not too much more than some radios, maybe that's necessary on lower powered equipment? Either way it seemed to get things moving again, and this morning I ended up with 1.5v DC at the 10k resistor, putting the 100k back gave 6.5 volts. Good enough, I had to go to work and didn't want to leave it plugged in all day.

The amp sounds nice and clean, I'm a happy camper, and can move on to the next project, a 1987x that I also shelved years ago because it started blowing fuses.

Awesome site guys!

[marblues2]

Hello, this is my first post here and I have learned a lot just reading thru the various sections. Thanks to everyone for all the advise. If I missed this somewhere I apologize but I built a 50 watt plexi using the metro layout and was wondering if anybody would know how to setup this to form the filter caps? (ie: placement of the 100k resistor)

Thanks

[neikeel]

Remove all tubes. Disconnect from mains

Take your 100k resistor and mount a croc clip on either end.

Clip the resistor across the standby switch terminals securely so that it cannot short on chassis/anything else. If in doubt insulate the croc clips

Plug in and switch the mains on, don't switch standby as the R is doing the work.

Measure voltage drop until it drops to < 5vdc and you are good to switch off, unplug from mains, drain the filter caps with your other croc lead (where the V of v1 plate resistors with one end and the other to the chassis) and then remove the clip on 100k.

You are now good to go.

[marblues2]

So only one end of the resistor is attached to the standby switch via croc clip?

[neikeel]

So only one end of the resistor is attached to the standby switch via croc clip?

No, not sure you have entirely understood the princple of what we are doing.

The resistor is clipped to bypass the standby switch (which if it is a Metro 50 will be a Carling switch with only two terminals. In the off position the switch is open circuit, when in the on position the terminals are made and current flows.

If you clip the resistor across the two terminals you have made a circuit so all the HT from your rectifier diodes flows through the resistor and on to the filter caps and the rest of the circuit, do not switch the standby on or the current will flow through the switch not your resistor.

The resistor acts as a limiter to keep the current down whilst the capcitors reform, when they are fully reformed there is minimal voltage drop across the resistor and you know you are done.

[marblues2]

Got it. Thanks a million!