Silicone conducting?

[mrn1ngstr]

Ive finished wiring an express. I used the Francesca schematic, and photos for reference, but there are a couple slight deviations. I used an off-stby-on switch in place of separate switches. I used cliff jacks and grounded the input on tone stack bus.

Here's where the issue is, as far as I can tell. I used silicone between the two bright caps to stabilize them. When I turn it on, theres hum, as expected. When I wave my hand over the tone stack, it gets louder the closer I get to the volume and bight switch. When I touch the pot shafts or bright switch toggle, hum disappears. When I chopstick the 100pF cap, nothing. When I chopstick the 500pF, I can hear the taps. When I touch my multimeter probe to the silicone between the two, the hum gets a good deal louder and buzzier. Sorry the cap pic isn't clearer- its as good as I could get it. The only other issue Ive found so far is that the volume pot has some crackle. This may or may not be related to the cap issue. The plan right now is to remove the caps and switch and replace all three and not use the silicone between them.

I didn't think silicone would conduct electricity. Did I miss something here?

[xtian]

I don’t think there’s an issue. You need to close the chassis with an aluminum plate, and that should solve it. This happens with most if not all amps.

[Colossal]

I see a few solder joints and lead dress that looks questionable. The Express can be an unforgiving mistress. While a cover plate is important for normal operation, the true test of a newly built Express' stability is that you should be able to dime all the controls without the cover on. It should hiss but it should not hum or buzz. If you've got your NFB lead wrong, it will squeal. Tapping the chassis (especially at high volume) and you should hear it loudly. You can even yell into a dimed Express and you will hear it. Be sure you are testing the amp in an area free from fluorescent lights (and their noisy ballasts or electronic drivers), cell phones, wi fi routers (they will induce a rapid ticking noise as they check in with the nearby tower), and dirty power. To get it right, you need clean lead dress, good solder joints, and good tubes. Also, I would get any tone stack leads away from those bright caps.

[rooster]

No, silicone does not conduct. You're volume pot ground lug and the bright cap attachment look to be connected. The bright cap goes across the non-grounded lugs of the volume pot. Bad pic?

[mrn1ngstr]

Thanks guys. Theres also a nice pop when I flick the bright switch.

Xtian, when I place the cover on, it does shield from most of the hum, but I think one of those caps is bad. Im going to replace it and see how that goes. Ive de-ox'd the volume pot but its still crackling. Im going to replace it.

Colossal, I've gone through and reflowed any joints that might have been cold since this photo was taken. Ill go back through and check again though. The neg feedback doesn't squeal but the when I turn that pot, through the speaker it sounds like something is being rubbed or dragged. I tried de-oxing that as well, to no avail. Im going to replace that pot as well. None of the other pots give off any noise at all. Its entirely possible I overheated the two pots when soldering the bus to the backs. I definitely melted some insulation on a couple wires. I let my tip go too long without tinning it and it got oxidized enough that it just wasn't transferring heat well. Bummer too, cause I'd kept that tip going for nearly a year.

[mrn1ngstr]

Rooster, looking at the pot from the rear with the lugs up, the left lug is soldered to the pot. The bright caps each have one leg soldered to a wire going to the wiper. The center lug of the switch is tied to the right lug of the pot. I've checked and I cant find anywhere that theyre touching where they shouldn't be. I know that Francesca's bright switch is wired differently than the others. Ive also read people saying this is incorrect but I haven't seen an explanation why, and from what I can tell it matches the schematic. Any insight here?

[rooster]

Funny, that Francesca error was presented by newbee me back when I first got into the Express build. And it wasn't that I was so brilliant electrically, but that I heard noise with the cap hanging off the input of the tube (the wiper). Also it was not something done by Fender who hung the open cap off the non-wiper lug of the pot. So I reversed it's position and commented here. I think Glen K tried it or maybe he found that the one he had was like mine (I can't remember) and soon the build schematic was changed. The first build schematic was the Francesca as far as I recall and it had this error in it..

So, this is your problem I think. reverse the cap connections to the lugs. DO NOT attach the caps to the wiper.

[erwin_ve]

Have you tested the caps going to the tmb pots on leaking DC with a voltmeter?

As far as the hum thing: is the amp earthed? Coming close with any body part to the first stage induces a lot of hum. I had this recently with a Liverpool build and found the wall ac wasn't earthed.(earthed wire wasn't connected in the AC wall outlet).
Connected to a earthed Ac there was simular behavior but not as much.

[M Fowler]

I posted and it disappeared weird. I'll try to post again.

The scratchy pot is dc on the pot causing that.

Most problems in an Express is Trainwreck grounding and second is the wire from presence pot to speaker jack, move that wire around until it sounds better.

Mark

[rooster]

Mark, I posted something as well and it did not post. I think the site is acting up....

[pompeiisneaks]

Mark, I posted something as well and it did not post. I think the site is acting up....

Have you guys gotten any specific error messages? I've not had problems posting myself.

~Phil

[rooster]

Phil, no, no error message. I wrote a post, added pictures to it, submitted it - so I thought - and then closed my browser. When I came back I saw my post had not been posted. So I wrote another, added the same pics, and submitted it, but this time it posted. Anything is possible, but I think I did everything correctly.

[pompeiisneaks]

Phil, no, no error message. I wrote a post, added pictures to it, submitted it - so I thought - and then closed my browser. When I came back I saw my post had not been posted. So I wrote another, added the same pics, and submitted it, but this time it posted. Anything is possible, but I think I did everything correctly.

I've accidentally hit the 'preview' button instead of the 'submit' button before and you see your post show up, but it's above instead of inline with the message. My only guess is that may be what happened. I think this is also why a significant number of double posts happen is that when you hit submit sometimes it stays at the submit window. I've seen it and caught myself before hitting it again.

~Phil

[mrn1ngstr]

Rooster, I swapped the two wires on the volume pot, and I got the same result. Volume is still scratchy once it gets to a certain point. If I dime it, it gives a nice "BRRRRRRRRRRRRRRRR." I started checking for leaky caps. Im not reading any DC on any of the pots except the presence pot. I then checked and theres 23.5V DC AFTER the .1mF coupling cap from V2 to the PI, which puts DC on both grids of the PI and the presence pot. I swapped the cap out and Ive got the same thing. I swapped the tube just in case, same thing. Now what?

[rooster]

Your getting ahead of yourself here. Pull V1, turn the amp on. Does it Buzz? If yes, Pull V2. Does the amp still buzz? Pull V3 - and things better be very quiet....

OK, assuming the buzz stopped after V1 was pulled, this is the problem area. Check all pin connections to V1 and the caps on pins 3 and 8 for starters. Arrg, but I will stop because you need to tell us what pulled tube stopped the buzzing.... I hope it was V1 actually.

Last question, where are you grounding the cathode caps on V! (pins 8 and 3 )? Someone has suggested the ground path here so please check it out. Honestly, if the bus bar ground gives you any doubt, attach pin 8 cathode ground to the ground of the input jack.

Also another thought, looking at the build guide, it shows the bus bar attaching to the Presence pot? This is really not a good idea. The Presence control needs it's own ground away from the Volume and Tone controls. Electrically it is separate, more PI and Output section oriented. Oh, I know this rather breaks with the Ken build idea but you should try it. It's a simple move, cut the bar at the Bass control and add a ground point right at the base of the Presence pot. Fender and Marshall just used the pot itself as a ground point.

Alright, report back on the tube pulling expedition....