humming overdrive

[phsyconoodler]

I just built an non-HRM build and the overdrive hums.What I mean is,there is a rather loud hum when the OD is engaged.I used shielded cable everywhere and it's definitely related to the gain.If I crank up the trimmer it gets louder.Turning up the master makes it louder.
I'm thinking it's just amplifying the small hum I have even in the clean channel but I'm not totally sure.
I used an older Traynor PT and it hums a bit even before I switch the standby on.It could be the PT.I have built quite a few amps and all the rules apply to this one too,it's just the older iron that's different here.
Any ideas?I tried tubes,lead dress,shielded cables everywhere,i haven't swapped any grounds yet,but haven't deviated from any previous builds that were dead quiet.
Kind of annoying.any insight would be welcome!

[10thTx]

Is it mechanical hum that is transfered thru the speaker? Is there something vibrating on/in the chassis? Is the speaker vibrating something .........OR the PT or OT vibrating & causing a hum?

Or an electrically induced hum? Can you isolate it to see which it is?

With respect, 10thtx

[phsyconoodler]

It's not mechanical.It's definitely electrical.Turns into a buzz rather than a hum as the gain or master goes higher.

[ChrisM]

You following the grounding scheme per #124?

[phsyconoodler]

No,I followed the d'Lite scheme.It has never been wrong in previous builds.
It hums even before the standby is on.I'm thinking it's the PT.
It doesn't have a faraday shield and I didn't put the OT too close and it's perpendicular.

[guitardude57]

I've had a PT do that before. Power up and you could feel it hum.


I would sub in another, before you drive yourself crazy.

It is what I did and solved the problem. Can't imagine the vibration with the hum............ thought it was a sign to retire it. Strange. Don't hear about this much.

[heisthl]

probably hum from the relay, disconnect the control wires and use a battery as a test.

[phsyconoodler]

Humm....from the relays! That's possible!I'll give it a shot.
Wonder how to cure that?I'm using the filament supply with a bridge rectifier and 4700uf caps with a 180 ohm decoupling resistor.

I guess I might have to get a separate transformer for the relays.

Thanks for the tip heisthl.

[Structo]

That is strange.
I notice on my D'lite that I get a bit of mechanical hum before I flip standby.

One thing I found on my amp was that the routing of the standby wire to the filter board was pretty touchy.
When I first built the amp I didn't do a real good job of harnessing the PT primary wires and so when I ran the standby wire it was too close and picked up a bit of 60 Hz noise there.

Later I also wired it so the first filter caps (the two 100uF) were before the standby switch so that they charged when power was applied.

Double check your OD relay and make sure that the pins are wired correctly so that it grounds the OD grid when not engaged.

Also could be a grounding problem in and around the relays or power supply to the relays.

[guitardude57]

I out of habit, wire the first power filters before the standby too.

My old "hummer" tranny must of had some weird winding in there. The darn thing vibrated and hummed......even though it had tight laminations, and electronically measured OK.

Since I had another old one laying around, just swapped it, and had no further problems.

I haven't encountered real bad unbalanced filaments, so cannot comment on how that translates into electro-mechanical hum and vibration.....other than typical heater hum we all deal with time to time.

Relay chatter........read some folks have had this.

Relay hum.....bad ground, or weak coil, bad diode or cap on the relay, wrong wiring to footswitch.......maybe something is throwing the filaments outta whack in the relay recto?

I would also check main filter caps....you may just have a weak one in there raising it's ugly little head there.

[bcook]

Humm....from the relays! That's possible!I'll give it a shot.
Wonder how to cure that?I'm using the filament supply with a bridge rectifier and 4700uf caps with a 180 ohm decoupling resistor.

I guess I might have to get a separate transformer for the relays.

Thanks for the tip heisthl.

Don't ground the rectified dc off of the filament supply, run a "ground" wire to the relays.

[Structo]

Just another wild assed guess, double check your quenching diode on the relay for proper polarity.

[phsyconoodler]

Quote:"Don't ground the rectified dc off of the filament supply, run a "ground" wire to the relays."

So you mean take the 100 ohm resistors and ground them at the relays?Where exactly?

[bcook]

Quote:"Don't ground the rectified dc off of the filament supply, run a "ground" wire to the relays."

So you mean take the 100 ohm resistors and ground them at the relays?Where exactly?

No, your dc+ wires to the relay+, but don't use the chassis as a ground for the dc-, run wire instead. The dc- needs to be isolated from the amp's ground.

[phsyconoodler]

Still not making sense to me.What other ground is there than the chassis?