FPIII Resistance Check Results

[Doc B.]

I believe that this is the kind of thing best learned if you learn from experience. Nope, I don't mean touch your tongue to the terminals...connect your voltmeter across the terminals of one of the filter caps and turn the amp on. Leave it on for a minute so the cap charges up, and then turn it off. Watch how long it takes the voltage to get down to, say, 10 or 20V as it bleeds off. Once you do that you will have a good sense of how long you need to wait before the bleeder resistors have completed their job.

[Grainger49]

Mike,

The bleeder resistor in the power supply will remove the voltage quickly.  

If you want to be sure throw your meter across one of the big power supply capacitors before getting your fingers in any dangerous places.

The outside of the capacitor is safe.  It is not connected to either lead.  Edit: see below.  Although none of the capacitors I have seen on Bottlehead equipment has a capacitor lead connected to the can it is not a guarantee that it may be connected in the future.

You got it, you have to touch both leads, or the + lead and the chassis at the same time to get hurt.

[Paul Joppa]

Grainger, that's not always true. Many capacitors, especially electrolytics, have the case connected to the negative terminal. Since it's a voltage doubler power supply, some of the caps have above 200 volts DC on the case.

Gun people say "always assume a gun is loaded until you have personally checked, and then only until you turn your back". Same for high voltages is my rule. It takes only a moment to check; if you have the amp upside down you probably also have the meter at hand!

[Grainger49]

I know that the multi-capacitors in cans on old tube amps use the outside for the common of 2-4 internal caps. 

Are you saying the caps in the FP III use the can for the common?

[Frank Breech]

Thanks. 
I will do the test and watch the caps charge up, then watch them bleed. 

So, now I know that it can be dangerous to touch the case of certain caps.  Specifically, is the FPIII one of those cases?  I promise to proceed with caution no matter what the answer.  And I'll never turn my back on them. 

I tried contact cleaner on my SW a little bit ago ... no luck.  Any suggestions about what to do to quiet them down?

Thank You
Mike

[Paul Joppa]

No, I'm saying you don't know if you don't check. I looked at two data sheets and found no specification for the can connection; I tested three electrolytic caps that were at hand and found the can to be insulated from the leads. I have some non-electrolytic caps within easy reach rated 36uF/3800vDC, on which the negative terminal is welded directly to the can.

But we do not check for this, and it is not always specified by manufacturers, so there's no guarantee that the can is isolated or will remain isolated if we change the source of the part or that the manufacturer won't change the part in production - it would still meet the specs, after all.

[Paul Joppa]

Oh yes - the switch noise could also be a poor solder connection, either on the pot itself or on one of the wires connected to it.

Bottlehead will replace the switch if it is defective, but it's a lot less hassle to re-flow the solder connections!

[Frank Breech]

Ok.  WIll reflow all solder connection to that pot and get back to you. 
Thanks

[Laudanum]

Mike ... regarding your 12AU7 tubes, I am assuming that you have the EH tubes as shipped with your kit.   I could be wrong but I believe that they have spiral filaments and generally dont glow.  Or, atleast not as brightly as most tubes.  If you look down from the top, you probably will be able to see some glow if you dont see any from the side.  But the FPIII wouldnt work if those tubes werent.   Funny but most of the 12AU7's I have tried in my headphone amp (Crack) have been pretty bright including the one that's in there now (it's an old stock tube).   I built the FPIII with 6SN7 tubes and the Sylvania tubes in there dont glow much compared to the other tubes on the rack.   Different tube, same "problem".  I like the glow but the sound is more important   

[Frank Breech]

Laudanum, this could make sense ... when I did the smoke test and let everything go for 30 sec as indicated in the instructions, I was looking down on the tubes.  Now, in my rack, there's no space for me to get my head over them when on, so I'm only looking at them from the side.  I'll look into this and see if I see glowing ....

However, I have a bigger problem.  I was 'messing around' thinking about the dirty pot sound that I'm getting out of my right SW pot.  My rack shelves are glass, so I can actually look under the FPIII.  I noticed that the noisiness seemed to be related to how the right 12AU7 was seated in the socket. The LED on the underside would flicker on and off with the slightest touch of the tube and each touch also incited the same noise and some nasty popping i would hear when adjust that right channel SW.  I turned my amp off and investigated a little further, wiggling the tube in the socket to get it seated in there alright.
Then - and I can't remember exactly what happened, but at some point I turned off the FPIII and turned it back on - and after that nothing.  The LEDs at the underside 12AU7 sockets are not lit.  The OD3 still lights up just fine.  But, no music. 
Shoot ... what the heck?