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Merlin Blencowe / Valve Wizard Grounding - questions
Merlin Blencowe / Valve Wizard Grounding - questions
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Old 20th August 2018, 07:25 PM   #1
overtone85 is offline overtone85  United Kingdom
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Question Merlin Blencowe / Valve Wizard Grounding - questions

Hi All

I am planning to build a 12000 series Marshall Superlead 100w clone where I try to implement as much as possible the scheme suggested in Merlin Blencowe's article about grounding, taken from his book.
you can read it here:
http://www.valvewizard.co.uk/Grounding.pdf

I am redesigning the whole marshall layout and I have a few questions about some sections of the circuit.
  • Bias supply (paragraph 15.2 of the article), shall I connect the bias-0 side from the PT directly to the positive side of the first filter cap rather than to a solder lug on the chassis like traditional plexis?
  • Heaters center tap, where would you ground it? I tried the so called Larry Grounding in a previous build where this was connected near the input jacks, but from what I understand the heater noisy currents should return to a point that is closer to the PT. Merlin suggests to experiment but I would like to plan this before I build so I dont have to test and keep the wiring as neat as possible without corrections.
  • Bias sense resistors and test points. In my previous build ( a ceriatone Kit) I have 1Ohm sense resistors with external test points for each tube and a ground test socket for the multimeter. I want to include these on this new build as well and I will ground them at the power amp star ground as described in paragraph 15.2. Is it important to connect the test ground (for the negative probe of the multimeter) to the star ground for accurate reading or I can just ground it to the chassis at the most convenient point?
  • 1M resistors at the input jacks. According to the article these should be part of the valve stage grid leak circuit and therefore they should be grounded at the cathode of the tube rather than the jack. Is this reasoning correct?
Thanks in advance for any suggestion.
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Old 20th August 2018, 09:16 PM   #2
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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Bias supply; it will make no difference but stay with the original, they possibly have more knowledge than you.
Heater centre tap; as practically no detectable current is involved, it makes sense to go to the nearest convenient point.
Bias current sense; I always use a 1R resistor rated at 1Watt. 10R would require a diode across it to sink any shorted valve load.
The 1M resistor can be anywhere but I always include a grid stopper resistor of around 10k to stop any parasitic noise, directly on any high gain stage.
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Old 20th August 2018, 09:32 PM   #3
Malcolm Irving is offline Malcolm Irving  United Kingdom
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Hopefully, Merlin might chip in here, but in the meantime:

The bias supply is part of the grid-leak circuit for the power tubes – so you should ground it at the same point that you ground the power tube cathodes.

The main purpose of grounding the heater centre tap is have 3.15Vac on each side, rather than 6.3V and zero, also to prevent the heater circuit from floating to an indeterminate DC level. In a perfect world it is just a voltage reference and there would be no current going into the heater ground. In the real world, there will be stray current going into the heater wiring and out through the ground. This stray current can come from heater-cathode leakage (resistive or capacitive) and/or inter-winding capacitance in the power transformer, and/or some other stray capacitance or mutual inductance. This stray current is in a loop comprising: stray current source – heater wiring – heater centre tap ground – ground connection – back to source. This stray current is likely to be very small anyway, but the best place to ground the heater centre tap would be at the ground nearest to its source. Possibly, the worst source (for noise) could be mains noise coming through the power transformer. If so, I think the best place for that ground would be at the reservoir cap ground. To be honest though, I’ve tried various places for that ground and I haven’t found any noticeable difference.

For an accurate bias sensing reading you can attach the meter lead to any convenient ground point.

For the second and subsequent stages, it is good to ground the grid leak resistor to the same point as the cathode. This is to avoid any unwanted ‘buzz’ voltage drop in the ground connection between the two, caused by power supply return current. In the first stage, there is no power supply return current passing through the ground (prior to that stage) and so it is OK to have the 1M at the jack.
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Old 20th August 2018, 09:45 PM   #4
overtone85 is offline overtone85  United Kingdom
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Quote:
Originally Posted by Malcolm Irving View Post

The bias supply is part of the grid-leak circuit for the power tubes so you should ground it at the same point that you ground the power tube cathodes.
Thanks for the reply Malcolm. So if I get this right the Bias 100v part of the PT secondary goes to the first resistor+diode as per the marshall bias circuit, then I can connect the other wire of the bias secondary from the PT to the positive lead on the first bias filter cap. From this particular connection I will start the bias supply own little ground bus where all the bias caps, dropping resistors and pots are grounded, and this bus will then connect to the same star ground as the power tube cathodes. Which according to the book it is also where the screen filter caps are grounded.

Correct?
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Old 20th August 2018, 10:04 PM   #5
Malcolm Irving is offline Malcolm Irving  United Kingdom
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Yes, I think that would work well.
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Old 21st August 2018, 03:30 AM   #6
Malcolm Irving is offline Malcolm Irving  United Kingdom
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Second thoughts about where to connect the heater centre tap ground: the main reservoir ground is very 'unquiet' from a voltage point of view. Probably better to connect to a quiet ground nearer (or at) the preamp.

On the other hand, if the issue really is mains-noise coming via PT inter-winding capacitance, that would need to return to the mains earth. This could be done by grounding the heater CT to the chassis close to the safety ground. (Do not use the safety ground itself - as that should always be a dedicated connection.)

Last edited by Malcolm Irving; 21st August 2018 at 03:38 AM.
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Old 21st August 2018, 04:56 AM   #7
PRR is offline PRR  United States
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Grounding by committee does NOT work.

I don't know why you should change Marshal's grounding.

Merlin's plan will work, but understand the WHOLE plan.

Be prepared to experiment!! The best grounding is done with wire-snips and a hot iron.
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Old 21st August 2018, 09:18 AM   #8
overtone85 is offline overtone85  United Kingdom
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Quote:
Originally Posted by Malcolm Irving View Post
Second thoughts about where to connect the heater centre tap ground: the main reservoir ground is very 'unquiet' from a voltage point of view. Probably better to connect to a quiet ground nearer (or at) the preamp.

On the other hand, if the issue really is mains-noise coming via PT inter-winding capacitance, that would need to return to the mains earth. This could be done by grounding the heater CT to the chassis close to the safety ground. (Do not use the safety ground itself - as that should always be a dedicated connection.)
I appreciate the help Malcolm. In my previous build (first build ever actually) I never tried the traditional Marshall grounding and went straight for the so called Larry star grounding that became popular on the Metroamp forum. The amp was very quiet and I only had to shield the signal wire going from the mixer resistors into V2a grid because it was picking hum when close to the chassis (or close to the heater wire looping around the socket). In that instance the heaters CT was grounded at the first star point in the chassis together with the input jacks and the V1 cathodes. After the v2a grid shielding the amp had basically no hum.

All the other star connections were divided in the same way as Merlin's but of course they used the whole chassis as a buss.
I guess at this point I can test the heater CT grounding either at the chassis where the preamp bus ends or near the safety earth. Maybe I can leave the wire running under pots along the front panel and keep an isolated solder lug closer to the PT so I can compare the two without taking everything apart.
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Old 21st August 2018, 12:14 PM   #9
Malcolm Irving is offline Malcolm Irving  United Kingdom
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Quote:
Originally Posted by overtone85 View Post
I appreciate the help ....
You're very welcome.

I might be wrong, but I seem to remember the old Marshalls used to buzz quite a lot if the volume was turned up with nothing plugged in!
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Old 14th January 2019, 10:33 AM   #10
overtone85 is offline overtone85  United Kingdom
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Hi all, I just wanted to give an update.

I finished the build and I’ve been testing for the last few days.

The PT is a little bit noisy, and the whole chassis resonates slightly with it. You can hear it if you put your head close enough.
When turning on the mains switch there is a slight coupling between the transformers and if you put you head against the speaker cone you can barely hear the hum, it is very low. The transformer on the chassis is louder in the room, and still i think it would be difficult to hear when put into a cabinet.

When putting my hand on the PT and really pay attention there might be a slight vibration. This is a transformer made by Chris Merren.

Now I got my first scope and used it for the first time on this build so I hope I got the measurements right. I made a probe with a 600v 0.1uF coupling cap I had at hand.

With the mains switch ON and the amp in standby I can read 10mVpp of 50hz buzz anywhere in the amp that is not the heaters and the bias supply.

It’s on the chassis, the ground buss, the preamp grids etc.

When I fire up the amp this kind of buzz is also on the B+ in various degrees:
5mVpp on the OT CT
10mV on the plates
5 or 10 mVpp on the V1 plates.

It looks like this:
IMG_5699.jpg

So it is not a ripple.

As I wrote above the circuit is grounded only near the input jacks.

Now in this case I am connecting the heater CT to this ground spot. I left myself an extra ground terminal lug near the safety earth. I can try connecting the heaters CT there and see what happens.

Do you think the PT it self is causing this buzz?
I can also measure 15mVpp 50hz buzz on the transformer end bell itself.

I forgot to mention almost all the signal wires are shielded (grounded only on one end, to the nearest chassis spot, using standoff bolts or between pots and chassis.)
Wires to preamp gross are shielded except V3.
Even the negative feedback wire is shielded.

The amp looks like this.

IMG_5681.jpg IMG_5683.jpg IMG_5682.jpg IMG_5655.jpg
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