humming in new build...grrrrr!

[phsyconoodler]

I just finished a 4-6CA7 Overdrive Rocket and it has a hum problem.It has a small hum even before the standby is switch on.It's the transformer(s).It's livable clean but gets crazy when I engage the OD channel.I tried all the usual fixes but I'm thinking it's just amplifying the initial hum I have.
  I have built quite a few of the same amps except this one has older Traynor Iron.Nothing is different and I even used more shielded cable in this build.
  Do you guys think it's just amplifying the transformer hum?The amp has tons of gain on tap,that's for sure.I'm thinking I should have moved the OT around before mounting it.It's quite a ways away from the PT and the cores are perpendicular.Maybe I could rig up a Faraday shield?I'm grasping........

[Dave]

Did you double check that artificial CT on the heaters. I have seen at least two examples where somebody either forgot to do it, or used 100K resistors and couldn't figure out where all the humm was coming from.

Dave

[JayB]

I bet it is something way to obvious that you over looked. With it in standby, would mean something from your main filter caps, OT and the plates, rectifier, heaters or the PT itself.

[eleventeen]

Hum...in standby...?  

If you can actually HEAR hum with a non-functioning output stage, eg; in stdby, then it would seem you either have a heater to cathode leak/short somewhere, or the filament winding on the PT has an odd short to ground. You might install the synthetic heater CT *and* measure the voltage drop across each resistor. If the drops are drastically different, there could be a heater short to ground at an odd point in the winding. I understand heater to cathode shorts can cause hum and I surely believe it, but I don't recall ever having personally encountered it in an amp I was working on.

Pull the output tubes. Do you know if your B+ would jump to a destructive level if you operated the amp with no output tube load? Even if so, you should be able to run the amp in stdby with no output tubeskis.

Other than that, I'd be pulling preamp tubes one at a time to see if I could locate the evil stage.

[phsyconoodler]

A guy over at the amp garage suggested it might be the relays humming.I'm going to disconnect them and see.If it is,then I may have to use a separate transformer for the relay supply.
  I'm using the filament taps with Hoffman's schematic for the relays and the filaments.It works but may be what's happening here.

[HotBluePlates]

With the amp in standby, nothing should be coming through the speaker, because the output tubes are effectively off. In standby, you could safely disconnect the speaker. If you do that, do you still hear hum? If so, that source is probably the transformers vibrating. In good transformers, that happens most often in an over-current situation, but could also be a lack of potting.

Anyway, the idea is that with no plate voltage, there couldn't be a signal output from the output tubes, so hum couldn't come from the circuit. It would have to be radiated into the OT (maybe from the PT) and then to the speaker (if the hum is coming out of the speaker). That then implies positioning of the OT and PT is the problem.

Dramatic increase in hum when switching in the OD channel suggests a second source of hum, and very well could be relays.

Anyway, I think you have 2 or more problems happening.

[JayB]

A guy over at the amp garage suggested it might be the relays humming.I'm going to disconnect them and see.If it is,then I may have to use a separate transformer for the relay supply.
  I'm using the filament taps with Hoffman's schematic for the relays and the filaments.It works but may be what's happening here.

When running the relays off the heater winding, you really need to use a heater center tap, if it has one, and not the 100ohm resister artificial center tap. The artificial center tap with relays creates hum. Another way is to full wave bridge the heater supply after the tubes.Then use the +/- sides of the bridge for the relay grounds and power as that would avoid any ground loops.

[phsyconoodler]

This transformer doesn't have a heater centre tap so I'm stuck with a pair of 100 ohm resistors.Doing it AFTER the heaters is an interesting idea,but I don't have the room to get too elaborate.
  I'm thinking that a separate relay transformer will solve all my issues.
 The hum while in standby can still be from the relay supply because it gets power right away.I'll disconnect it and see if it changes.Then I'll know for sure.
  Thanks for the suggestions guys!

[JayB]

This transformer doesn't have a heater centre tap so I'm stuck with a pair of 100 ohm resistors.Doing it AFTER the heaters is an interesting idea,but I don't have the room to get too elaborate.
  I'm thinking that a separate relay transformer will solve all my issues.
 The hum while in standby can still be from the relay supply because it gets power right away.I'll disconnect it and see if it changes.Then I'll know for sure.
  Thanks for the suggestions guys!

The minute the relays are grounded to complete the circuit using the 100ohm resistors on the heater string, it creates a ground loop. I never had any luck doing it that way. I always end up using a separate small transformer tucked away for relays if the PT doesn't have a 5V rectifier winding that I'm not using.

[phsyconoodler]

I'm getting a filament transformer today.I'm giving up on this design.Not because it doesn't work,but because I'm not smart enough to make it noise-free.
  I concede defeat.

[JayB]

I'm getting a filament transformer today.I'm giving up on this design.Not because it doesn't work,but because I'm not smart enough to make it noise-free.
  I concede defeat.

That's what I would do. The only way I could ever get it noise free was using a bridge rectifier and floating the ground. But that is a hassle and not really user friendly because if some how that negative rail grounded out, it could take the heater winding with it.

[phsyconoodler]

i have access to a 5v center tapped transformer.It should give about 7.1v rectified.Will that be sufficient with a 5v regulator?

[sluckey]

7.1 volts into a 5 volt regulator is fine. However, I don't think you need a regulator. You're powering a 5 volt relay that has an LED connected in series. The LED probably has about a 1.7 to 2 volt forward drop.  Sounds about right without the regulator. Just use a 1000uF to 4700uF filter cap.

[phsyconoodler]

Thanks Sluckey.
  I'm just wondering why the other system is so noisy?I did it exactly the way it was drawn.
It starts humming as soon as the power switch gets turned on.Then it gets nasty when the OD channel is engaged and on clean it's livable until the master gets turned up high.

[JayB]

I don't even bother with a regulator unless I'm dealing with a higher voltage. 2 extra volts isn't going to kill anything plus your 7v will sag a bit. Their rated voltage is just that, a rated voltage. Most will turn on with little over 3v and can take 12 volts no problem. The higher voltage will shorten their life span of course. However, I would only go half of that value for a cap though.

[eleventeen]

You seem to be leaning away from it, but 2 volts is probably NOT enough headroom to run a 3-terminal regulator. In my humble experience. Oh, it might run it with zero load, but I've never seen anyone get away with less than 3 solid volts, and 5+ is better.

[phsyconoodler]

Well,sorry to burst your bubble,but it works.And works very well I might add.It has 7.7 unloaded volts and it's working flawlessly.
  And the hum is gone too.
Thanks guys for all your help!

I have a 5-0-5 transformer,2 watt with a diode rectifier and a 470uf/50v cap.Works perfectly.

[DummyLoad]

but 2 volts is probably NOT enough headroom

more than adequate with an LDO reg.  e.g. LT108x series.