Hum in Marshall 18 Hoffman build

[Oddvar]

Hi guys, a little problem coming up:  I have built a Marshall 18w stout from the Hoffman Stout layout.  It's working, but I get this hum that is very loud, but it varies a bit with the volume, loud hum at zero, the decrease audibly and the increases from a certain point.  I have checked most of the parts, but can't figure out the cause.  It sounds a bit like grounding hum, I have all the transformer grounds at one place, the others attached to on foot of the mains transformer.  Around 390 volt at A/B/C,  Any help?

[Oddvar]

I am thinking maybe the heater power from the transformer is somehow not working.  It measure -0.4 and Wonder if there is a way to verify this?  And will the amp work if the heaters are faulty?

[Leevi]

The amp will not work if the heaters are not properly working.
Check that the tubes are glowing. Did you measure the heater voltage with AC?


/Leevi

[Oddvar]

Thanks for answering.  The heater ac is 6.48v.  So I guess the hum isn't from there?

[92Volts]

I’ve experienced hum with volume at zero and I think it was a bad pot. Noise in the ground line would cause hum at zero volume, but I’m not sure why raising volume to 1% (for example) would abruptly shut out ground noise.

Heaters carry AC and radiate noise at that frequency (60hz). Twisting the pair of heater wires together is usually “good enough” but tweaking the location of heater wires can help. Hum from the heaters would usually increase with volume. Note that an instrument or no instrument depending on the jack (if it shorts/grounds without a cable plugged in) will also pick up noise from the environment and no changes in the amp or amp design will change that.

[Leevi]

Localize first if the hum is coming from the preamp or power amp.
Shortcut the PI-input to ground and if the hum disappears it is coming from the preamp.
If not it's caused by the power amp or heaters.


/Leevi

[Oddvar]

I suspect that it's coming from the power amp.  I have disconnected all the connections between the pots and the board/tubes, and the noise is still there, also when the volume is down.  I have swapped tubes and I have tried to move the leads in different ways. no reaction on the sound.  It might sound like I have a ground loop of some sort, but I am stuck at the moment.

[Oddvar]

Pardon my ignorance, but what is a PI?

[SILVERGUN]

PI = Phase Inverter

Does your transformer have a center tap on the heater winding or are you using two 100ohm resistors as an artificial center tap?

[Leevi]

I have disconnected all the connections between the pots and the board/tubes,

The disconnection is not always enough since an open wire can absorb hum from the circuit.
By connecting the inputs of different stages to ground is a better way.


/Leevi

[Oddvar]

PI = Phase Inverter

Does your transformer have a center tap on the heater winding or are you using two 100ohm resistors as an artificial center tap?

Yes it has a CT, it's a Hammond PT 290PAZ

[Oddvar]

I have disconnected all the connections between the pots and the board/tubes,

The disconnection is not always enough since an open wire can absorb hum from the circuit.
By connecting the inputs of different stages to ground is a better way.


/Leevi

OK, I will try that.

[Oddvar]

Shortcut the PI-input to ground and if the hum disappears it is coming from the preamp.
If not it's caused by the power amp or heaters.
/Leevi

I don't know how to ground the Pi, any advice?

[Leevi]

I'm not sure if you are using the following layout?


The PI input is the pin #2 on the second (from left) 12AX7.

http://el34world.com/projects/images/18WATT_VOLTAGES.gif


/Leevi

[Oddvar]

I am using the layout, with volume an tone.

[Leevi]

You can connect it ground by soldering temporarily a wire across the 470K resistor
which is connected to pin #2.


Another way is to use screwdriver but there is a risk that you touch a high voltage on pin#1.


/Leevi

[Oddvar]

I did that, still hum.

[Leevi]

Ok, put focus now on power amp or heaters. Do you have a multimeter with Hz measurement? Just to check If the hum is 50Hz or 100Hz.


/Leevi

[Oddvar]

Yes i have.

[92Volts]

I did that, still hum.

If the power and output transformers are too close, you can get magnetic coupling of hum straight into the output. You can check for this-- it's some of the only noise present before the tubes warm up, and rotating or repositioning the OT will affect it. But using a known layout, I doubt this is your problem.

Push-pull amps with matched tubes should cancel power supply noise because the tubes see/respond to the same noise, but work in opposition to each other.

This benefit decreases with non-matched tubes and in extreme cases (one tube isn't working) you rely on a silent power supply to eliminate noise...

It doesn't look like that circuit has separate test points for each EL84 so it's hard to tell. Can you check if voltage across the cathode bias resistor (130 ohm) is at least in the right ballpark?

[Oddvar]

From my phone.

[shooter]

cool app, appears to be 120hz noise, PS.  need to be carful though to make sure your "test" isn't aliasing, thinking you're measuring one thing but it's 1/2 or double what is really there.

measure the PS tap for your power tubes, but measure VAC, NOT VDC, what do you get?

[SILVERGUN]

cool app, appears to be 120hz noise

He's in Norway, so 100hz is the proper exchange rate

[Leevi]

I suppose your wall socket is grounded?


I have built several 18W Marshall clones and always used a single bus that I have grounded to the input jacks (metal) and pots.


/Leevi

[Oddvar]

Voltage: 240 v inlet, 6.4 v heater, 600 v rectifier. 

[PRR]

When looking for 50/60/100/120Hz hum/buzz, it is good to raise your "FFT size" from 2048 to 4096 or 81?? to get better resolution at the low freqs. (This may reduce your top-end to less than 20KHz but that is OK for hum measurement. Depending on the software, you may have to reduce sample-rate down from 44K. Depending on soft- and hard-ware limits, maybe it won't do that.)

You expect "sharp peaks" at the hum/buzz frequencies. Your FFT rate is slow enough that I too was unsure if you were in 50 or 60 Hz land. I could only guess that "193Hz" is more likely 200Hz, the fourth harmonic of 50Hz, not the 180 or 240Hz of 60Hz.

[Leevi]

A picture of the chassis (inside) might help in analysis.
/Leevi

[Oddvar]

This is the state of the amp at the moment.  I have put in a new set of heater wire, changed the groundings around, no reaction to the sound/noise, 

Could it be that the filter caps are making the noise?  Faulty or bad quality?  (I have ordered a new set)

[Leevi]

Couple of actions to start with:

You have grounded power cord and preamp and power amp to the same point
Ground the power cord to an own bolt of the PT.

You have 2 grounding points for preamp, input jack and PT bolt.
Disconnect the preamp bus from the PT bolt

http://www.el34world.com/charts/images/grounds.gif

/Leevi

[sluckey]

Pull V2. Hum go away?

[Oddvar]

V2 gone, the noise falls to much lower.

[sluckey]

Put V2 back in and remove V1. Hum go away?

[Oddvar]

V1 out, hum still there...

[Oddvar]

Hi again.  I am a bit puzzled about the grounding.  Is the wiring on this layout correct?  Is there supposed to be a wire from the pot to ground on both sides to speak? And is there a wire going from the speaker jack to ground too? It seems like it's not there at the picture from the build here: http://el34world.com/projects/18WattStout_TMB.htm

Anyone?

[shooter]

And is there a wire going from the speaker jack to ground too?

IF, you have NFB, your speaker jack needs to be grounded.

as far as your preamp, your question is VERY confusing, what pot?
here's what I would verify;
you have it grounded like Doug shows, see attachment.  The BIG take-away; don't have preamp grounds tied to the same point as poweramp grounds

[Oddvar]

Will this be a proper way to ground the amp?

[shooter]

Yup

[labb]

That grounding system works for most..Just to check separate your preamp and main board grounds from the power tubes and transformer grounds and ground them right at the input jack.

[Oddvar]

So, in this ongoing quest in troubleshooting, I have changed most of the parts on the board, and now I have no sound, except a tiny buzz in the speaker.  When I measure the V2 I find that that the voltage going from "C" is from 340 to 4mv across the 100k resistor, meaning that the V2 doesn't get the power i needs,  What could this be?  I have changed the resistors and measured them to be 100k.

[Leevi]

voltage going from "C" is from 340 to 4mv across the 100k resistor

I didn't get that really. How much is the voltage at the point "C" and how much across the 100K resistor?


/Leevi

[Oddvar]

According to the schematics, I believe, from C filter cap, 350v connection, it's going through a 100k resistor to the V2-1 on the tube, there it should be approx 237v, but it's close to zero?

[Leevi]

OK, V2 pin#1 voltage is close to zero.
If the voltage on the other end ("C") of the 100K resistor is still 350V then
either the resistor is broken or there is something wrong with the V2.


/Leevi

[Oddvar]

There are two places with 100k resistors going to V2 point 1 and 6, plates I believe.  When I measure the 100k resistorsr on both sides I get 340 volt, when I measure ground and tube side of the resistor I get close to zero volts, I have changed the resistors three times, same thing happens. 

I am wondering if there is another place that influences this?  A bit beyond me to be honest.

[Leevi]

If you you pull V2 what are the voltages then on pin#1 and pin#6?
/Leevi

[Oddvar]

Then it's 360v.

[Leevi]

Then there is something wrong with the V2 or with its wiring.
Put V1 to V2's socket and measure if you don't have an extra 12AX7.


/Leevi

[Oddvar]

I have tried different tubes, but I'll go over it. Thanks.

[Oddvar]

You were right, resoldered all the points an now shows all the right voltages.  No sound though, it starts and then the noise fades out.  I dont' quite get the grounding though, if 1&8 on the power tubes should be grounded. I am not sure where the preampgrounding and the power tubes are seperated?

[Leevi]

1, 8 and 6 of the EL84s are empty and they can be used for other purposes i.e. You don't need to ground them.


Measure the voltage on power tube pin#3 or on the cathode resistor (130 Ohm).


/Leevi

[sluckey]

Follow Hoffman's layout exactly!