New Express Completed - Problem with Hum

[Jimy]

My just-finished Express has a problem with hum that I can't seem to figure out. The amp works well and sounds great with the volume set at 9 o'clock, but anything beyond 12 o'clock induces hum.

Symptoms: When I turn the volume up to 12 o'clock or beyond, the hum starts slowly and builds, so it seems to be some sort of feedback or self-oscillation. Hum is produced with or without a guitar plugged in. The chassis is sensitive to vibration, particularly around the volume pot. I can tap the volume knob and hear a "thump" though the speakers. Sometimes, if the hum does not start by itself, I can induce it by tapping the volume knob (or by playing a few notes on the guitar). The hum is not affected by the presence control or any of the tone stack controls. The hum can be stopped by turning the volume below 9 o'clock.

Build: Pretty much a straight forward clone of Francesca. I used KF style grounding, layout, and lead dress. I did modify the heater wire layout and I haven't yet installed the flyback protection diodes on the power tubes. Also, the purple wire from the OT needs to be trimmed to the proper length. I don't think any of these things would induce the hum. Pictures below.

[paulster]

That sounds like a particularly microphonic V1 tube.

There's enough gain in an Express to easily get the feedback purely through the microphony of a tube, which would explain how you can start it off by tapping the chassis.

Have you tried swapping different tubes into V1 to try to tame it, even if it's a tube that isn't best for tone at this stage?

It could also be a microphonic coupling cap, but you've used the PVC caps which aren't nearly as bad for this as tubular foil caps such as M150s or Sozos.

I'd definitely look at the tubes angle to rule this out before digging around inside.

Having said that, it could still be that you have the OT primaries round the wrong way, so you could also temporarily lift the NFB wire to see if the feedback problem goes away. If it does then you need to swap the OT primaries.

[stubbyfex]

Nice looking build! It looks very professional. My first Express build had the same problem, in fact it still does when I move it around. I've traced my hum down to the Output transformer lead dress. If I have either of the plate leads too close to the Phase Splitter tube wires, it will hum LOUDLY. I can also move them in such a position that it will squeal. If I have it "just right" it is fine, until I move it to practice or a gig, then it might hum again. I almost want to glue the wires down! Anyway, you might try using a chopstick and moving your OT and/or Phase Splitter wiring to see if it makes any difference.

[Jimy]

Appreciate all the suggestions. I smacked my head when I saw "microphonic tube." Of course! That must be it! Sadly, though, that was not the case. I've tried swapping out every tube, including the power tubes, and while some of the changes seem to reduce the hum (at least temporarily) nothing eliminates it. The chassis remains sensitive to "percussive maintenance" and I can tap the amp almost anywhere and hear it "bonk" through the speakers.

Some things I've tried:

Trimmed the OT lead to V4 to the proper length. No difference.
Moved the cathode ground wire for the second gain stage to the middle of the treble pot, as per KF layout. No difference.
Poked around with chopsticks. The wires that run across the chassis to the control grids on the power tubes seem to respond somewhat to prodding, but nothing definitive. I can't seem to push them into any position that eliminates the hum and I'm not convinced that this is the source.

At this point I'm considering shielded wire for the second gain stage grid. I also need to try lifting the NFB wire to see if the OT leads are reversed, but that seems really unlikely. I've gotten the leads the wrong way round on other builds and it doesn't sound like this. It just seems like some part of this circuit is oscillating. I've never heard a microphonic cap. Any tips on how to isolate one?

[Jana]

I've never heard a microphonic cap. Any tips on how to isolate one?

ping them with the chop stick. I guess opinions differ, I have had the most problems with microphonics in caps when using orange drops. I prefer M150's.

[M Fowler]

Jimy

I went over your build, very nice work.

I didn't notice anything wrong. You must be using the Heyboer 5200 OT with a blue center tap wired to the lower stby switch. At first that threw me but I have not build an express with this tranny but had a wiring diagrahm for it.

Just how close is that screw/washer in front of your filter cap the one wired to the stby switch? Pictures can be deceiving. I am only guessing but if it only happens when you turn up the volume perhaps you wrecked the pot soldering the bus wire to it.

[Jimy]

Jimy

I went over your build, very nice work.

I didn't notice anything wrong. You must be using the Heyboer 5200 OT with a blue center tap wired to the lower stby switch. At first that threw me but I have not build an express with this tranny but had a wiring diagrahm for it.

Just how close is that screw/washer in front of your filter cap the one wired to the stby switch? Pictures can be deceiving. I am only guessing but if it only happens when you turn up the volume perhaps you wrecked the pot soldering the bus wire to it.

Yep, I'm using the Toneslut vintage spec Express OT with a blue center tap. It's wired to the lower side of the standby switch.

The screw/washer is extremely close to the 40uf cap (C16 on the Worley layout). The body of the cap is almost touching. The (positive) lead of the cap is a good half of an inch away... so nothing is shorted. Is there a problem with the location of the cap?

You're saying the volume pot may be damaged/defective? That idea had occurred to me but I wasn't sure how a damaged pot would behave. The carbon traces might be damaged and the taper not behave correctly but how, in theory, would a damaged pot introduce a hum?

[M Fowler]

That cap should be fine with 1/2 inch distance. Pictures are deceiving.

Not sure how that pot would act if it was defective, hopefully, some of the more experienced builders and techs will review your tread and come up with some things to check. You have already covered the basic things to check for.

Mark

[Jimy]

This evening I figured I should try running shielded cable from the volume pot to the second gain stage grid, to see if that might help. But before I made any changes I decided to do one more chopstick test... Almost every place I touched the amp, it would go "thump" through the speakers, but when I poked the lead going to the V1a grid, it went "BONG!!" Aha.

I swapped out the plain wire for shielded cable. Problem solved.

[paulster]

I've actually managed to find microphonic shielded cable before, which took a bit of head-scratching for a short while.

I tried Sommer Onyx 2025 in a build as it has a nice low capacitance of 18pF/foot but it was unbelievably microphonic, making the entire chassis sensitive.

That soon got discarded in favour of regular old RG174/U.

[Oswald1958]

Have you tried using aluminum flashing as shield over the top. It will hum and pick up radio stations if the enclosure isn't completeley isolated. Try grounding the output jacks to the ground in front of the Big Caps. Try grounding a cap to the input jack (not sure what value)? Mine hummed at first, and these are a few of the things I tried. NOS tubes and shielding fixed it for me. Other than that, nice build.

[Jimy]

Have you tried using aluminum flashing as shield over the top. It will hum and pick up radio stations if the enclosure isn't completeley isolated. Try grounding the output jacks to the ground in front of the Big Caps. Try grounding a cap to the input jack (not sure what value)? Mine hummed at first, and these are a few of the things I tried. NOS tubes and shielding fixed it for me. Other than that, nice build.

Installing shielded cable on the second gain stage grid solved my hum problem. The amp is quite stable now, but it does seem to have a healthy amount of background noise when running at high volumes, so I'm definitely interested in the techniques used to tame the beast.

I haven't yet fabricated a bottom plate for the chassis but hopefully this weekend I can get that done to see if it helps. Every post I read here seems to indicate that the bottom plate is pretty important.

Regarding the output jacks: I'd have to isolate the jacks from the chassis with shoulder washers and run a ground wire over to the common ground point next to the cap stack. Is that correct?

Regarding the cap on the input jack: This is a snubber cap of around 0.01uF intended to kill RFI, correct? I've seen this used before on amps that don't have grid stoppers. Seems to work pretty well. Again, the jack would have to be isolated from the chassis.

Has anyone ever tried putting a 68k grid stopper resistor on the first grid?

[paulster]

Regarding the cap on the input jack: This is a snubber cap of around 0.01uF intended to kill RFI, correct? I've seen this used before on amps that don't have grid stoppers. Seems to work pretty well. Again, the jack would have to be isolated from the chassis.

Has anyone ever tried putting a 68k grid stopper resistor on the first grid?

That's the one. You shouldn't really need it though since you've got a Switchcraft jack which is grounded to the chassis right at the input point. I like Cliff jacks (insulated) so I use a 0.01uF cap between the chassis and the ground connection of the jack to kill off any stray RFI. I prefer this to adding grid stoppers.

You'll probably find that a lot of your issues will disappear when you get the bottom plate on, as this amp is very susceptible to external influences.

If you're getting RFI then I'd either swap the jack for an insulated one and add the cap as you described, or try the smallest grid stopper you can get away with to kill of the RFI without losing top end.

It will still hiss though, such is the nature of the beast. A lot of the changes that have been documented are either tube changes to lower gain varieties or changing the gain structure to reduce the gain throughout the amp.

[Jimy]

I installed a bottom plate last night and it really does make a difference. The noise floor on the amp is really quite good now. There's some hiss at high volume, but nothing more than what you'd expect from a relatively high gain circuit. The amp is very stable and hum free. So I think I'll forego any modifications and call this one done. Thanks to all for your input and advice.