Built this but it has a loud hum

[rw6250]

First off, I'm a noob. I built this Matsumin valvecaster (modified with no tone knob) tonight using a 12v power supply and a tungsol 12au7.



I currently have it on the breadboard. It sound absolutely killer going into a partially distorted channel on my amp. The problem is there is a really loud hum. It goes away as soon as I play, but the second I stop, it's there. The hum is louder than the sound of the guitar. What can I do about this? Any advice would be greatly appreciated.

[amptramp]

What sort of power supply are you using?  Is it regulated or even filtered?  If it is a 12-volt wall-wart, be aware that some of them have marginal filtering with diodes feeding the output and having a series R-C internally for filtering.  (The R is to limit the inrush current.).  There is no filtering inside the unit, which could do with a capacitor between pins 5 and 4 with the positive on pin 5 may be necessary.  Note also that the inrush current of the device may exceed 600 mA due to the low resistance of the heater when the tube is cold.  If your supply is near its limits, this could be a problem.

It would help to add the component values.

[LucifersTrip]

you could prob rule out a power supply prob by trying it with a battery

[rw6250]

I did try a 9v battery. There was no noise, but the tone was thin and not very distorted.

I would like to use the 18v out of my pedal power distribution unit. Does anyone have a layout of what that shoot look like as part of this circuit? This is new territory for me and some of the explanations that I've come across are difficult to decipher.

EDIT: After doing some more reading and looking at a few schematics, I think I understand. My confusion at this point is, some show a 100uf cap before AND after the 7812, while others show one just before. Which is the way to go? What's the difference?

[boogietone]

Is it a high pitched whine or 60 cycle hum? That it goes away when you play (i.e., touch the the strings) indicates that it may be a ground loop or your strings are picking up interference from a light possibly a CFL.

[amptramp]

I did try a 9v battery. There was no noise, but the tone was thin and not very distorted.

I would like to use the 18v out of my pedal power distribution unit. Does anyone have a layout of what that shoot look like as part of this circuit? This is new territory for me and some of the explanations that I've come across are difficult to decipher.

Since you were operating the 12.6 volt heater from 9 volts (or whatever the battery sagged to under load), I am surprised you got much of a signal at all.  If you use 18 volts, you will need to drop the heater voltage at pin 5 of the tube with a 36 ohm resistor, but you can maintain 18 volts on the plate supply by taking the R2 and R3 connection off pin 5 and moving it to the power input jack.  You would still need to add a filter capacitor at the power input jack.

If you want to run at 18 volts, there is a possibility that would avoid the heater resistor, but requires a change to the tube pinout.  Use a 20EZ7 which is basically a 12AX7 with a 20 volt / 0.100 amp heater rather than 12.6 volt / 0.15 amps.  It would give different results from a 12AU7 because of its higher gain.  But the 20EZ7 is a rather rare tube and you can get 12AU7's anywhere they sell tubes.

[rw6250]

Is it a high pitched whine or 60 cycle hum? That it goes away when you play (i.e., touch the the strings) indicates that it may be a ground loop or your strings are picking up interference from a light possibly a CFL.

It is the 60 cycle hum that dies as soon as I play.

[rw6250]

I did try a 9v battery. There was no noise, but the tone was thin and not very distorted.

I would like to use the 18v out of my pedal power distribution unit. Does anyone have a layout of what that shoot look like as part of this circuit? This is new territory for me and some of the explanations that I've come across are difficult to decipher.

Since you were operating the 12.6 volt heater from 9 volts (or whatever the battery sagged to under load), I am surprised you got much of a signal at all.  If you use 18 volts, you will need to drop the heater voltage at pin 5 of the tube with a 36 ohm resistor, but you can maintain 18 volts on the plate supply by taking the R2 and R3 connection off pin 5 and moving it to the power input jack.  You would still need to add a filter capacitor at the power input jack.

If you want to run at 18 volts, there is a possibility that would avoid the heater resistor, but requires a change to the tube pinout.  Use a 20EZ7 which is basically a 12AX7 with a 20 volt / 0.100 amp heater rather than 12.6 volt / 0.15 amps.  It would give different results from a 12AU7 because of its higher gain.  But the 20EZ7 is a rather rare tube and you can get 12AU7's anywhere they sell tubes.

I want to use my MXR DC Brick with the 18v out, but step it down to 12v.

[rw6250]

bump

[kurtlives]

Use whatever power supply you want but filter it. Make sure the heaters (pin 4 and 5 for you) run on 12V DC regulated (again make sure there is filtering).

[PRR]

If the hum goes-away when you play, there's a drifty DC voltage.

Meter the plate pins (1 & 6) idle and playing. Report voltages and sound.

[kurtlives]

If the hum goes-away when you play, there's a drifty DC voltage.

Meter the plate pins (1 & 6) idle and playing. Report voltages and sound.

Often though you just don't hear the hum while your playing though. Gets drowned out.
Think of 60 Hz hum with single coils, I know I never hear it when I play.

[PRR]

According to OP, drown-out is not the reason: "The hum is louder than the sound of the guitar."

My theory is that something is way off-bias (wrong DC) at idle, and getting temporarily "set" by signal.

[rw6250]

Thank you for the replies so far. They have certainly helped.

Well, I picked up a 7812 and wired it in. It completely eliminated the hum. I was skeptical, but it worked like a charm. (On a side note, the 7812 gets pretty hot). Anyway, I tried  various combinations of electrolytic caps before and after with no noticeable difference, so I left them off. I confirmed that I have slightly over 12 volts at pins 4 and 5.

The only issue now is a constant hiss, but I'm assuming that it is a characteristic of the circuit. Any thoughts?

[amptramp]

The only issue now is a constant hiss, but I'm assuming that it is a characteristic of the circuit. Any thoughts?

Tell us what circuit values you have used.  The gain will be over 100 for the circuit, so resistive noise is going to be there, but this can be mitigated by circuit changes, if necessary.

[petemoore]

  Try a regulator that is a little bit more than 2v less than the measured power supply voltage, the 78__ voltage regulators 'use' 2 volts to work, if you measure say 19.2vdc out of your supply then a 7818 would work, but a 7816 might give a little more 'elbow room' in case voltage somehow 'spikes' when the power upply is plugged in or something.
   Regulators turn 'excess voltage' into heat, the more volts you drop the hotter they get, as with anything more current = increased heating potential also.
  You can use heat sinks, carry a spare regulator...and use a heat sink on it if the first regulator fries...might take a short time to get up to melting temperature or it might take a long time on a hot day...
  After burning my thumb twice, I figured out these chips can get burnin' hot really quick, the heat-ramp-up-time generally tells the experienced logic processer to unplug the chip before the heat ramps up to magic blue smoke degassing temperature [burnt chip].

[rw6250]

The only issue now is a constant hiss, but I'm assuming that it is a characteristic of the circuit. Any thoughts?

Tell us what circuit values you have used.  The gain will be over 100 for the circuit, so resistive noise is going to be there, but this can be mitigated by circuit changes, if necessary.

I used the same values as illustrated in the Matsumin layout, but modified to the layout at the top of my post. I sounds really good.

[rw6250]

 Try a regulator that is a little bit more than 2v less than the measured power supply voltage, the 78__ voltage regulators 'use' 2 volts to work, if you measure say 19.2vdc out of your supply then a 7818 would work, but a 7816 might give a little more 'elbow room' in case voltage somehow 'spikes' when the power upply is plugged in or something.
  Regulators turn 'excess voltage' into heat, the more volts you drop the hotter they get, as with anything more current = increased heating potential also.
 You can use heat sinks, carry a spare regulator...and use a heat sink on it if the first regulator fries...might take a short time to get up to melting temperature or it might take a long time on a hot day...
 After burning my thumb twice, I figured out these chips can get burnin' hot really quick, the heat-ramp-up-time generally tells the experienced logic processer to unplug the chip before the heat ramps up to magic blue smoke degassing temperature [burnt chip].

I'm thinking about mounting it on a hit sink.

Edit: I added a heat sink and an LED to the bread board and burned out the 7812. What's going on?