Merlin Blencowe / Valve Wizard Grounding - questions

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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.
 
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.
 
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.
 
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?
 
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.)
 
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PRR

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Joined 2003
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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.
 
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.
 
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
 
Guitar amp PTs often have a certain amount of acoustic buzz due to magneto-striction. Fortunately it is usually much less noticable once you have the chassis mounted inside a box on rubber feet.

When you measure 10mVpp of 50Hz on the chassis or the ground buss, how do you have the scope probe connected? If you have the ground side of the probe also connected to the chassis or ground buss in the amp there should not be any voltage.

The amp chassis is connected to safety earth and so will be the chassis (and probably the signal ground) of the scope, which would result in a ground loop (which is itself a source of buzz).

Try connecting the signal and ground sides of the probe directly together (but not connected to anything else) – should be a flat line (zero) on the scope.
Now connect the signal and ground sides of the probe very close together on the chassis of the amp – you should still get zero on the scope.

If the audible buzz from the speaker is less that the sound from the PT then there is probably nothing to worry about, but you need to also check this when you set standby to ‘play’ with all volume controls (and tone controls if you like) set to zero.
 
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Hey Malcolm I hope I’m doing this right. If I keep the probe over the PT I see the 50hz wave and when I connect the probe and it’s ground together I get a flat line.

If I connect both the ground and probe very close together I still get about 7 or 5mV of buzz. I also tried to connect the probe ground on the star ground where the circuit buss is connected to chassis and then I touch the alligator clip directly with the probe and I still get the buzz. I also tried to connect the ground at the end of the grid stoppers on V1, far from the socket. And then touch the socket end with the probe. Same buzz.

Yes the scope and the amp share the same ground at the socket.

With all controls to zero the hum stops and I can only hear a very quiet hum from the speaker, but the hum from the PT itself is louder than that.

I haven’t tried to move the heater CT ground yet.

I have a different question. Is it wrong to twist together the OT CT with the OT primaries? I get a kind of blocking distortion on the trail of guitar chords when the amp is on 10 but I probed the circuit with a 0.1uF cap blocking the DC into a solid state power amp and discovered that in a superlead most of the distortion is happening in v2a and the cathode follower. It was eye opening. If I cut the bass before going into the amp it goes away, or if I roll the volume back and I let the bright cap do the work. So I think this is normal but I still wonder if twisting the OT wires can be an issue in terms of oscillations or other things.

Thanks
 
overtone83 said:
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.
Using the chassis as a signal ground is a bad idea, although quite popular in the past when people did not understand grounding. Shielding the feedback wire can lead to HF peaks or instability (if the feedback is high impedance) or is unnecessary (if the feedback is low impedance).

The scope trace you show looks like charging pulses, so look in detail at the grounding of your charging pulse loop in the PSU section. The charging pulse loop (secondary-rectifier-reservoir capacitor) should not include the chassis or anything else, although it may have a single wire connection from the reservoir negative to the chassis - but only then if you have decided that this is the appropriate place for the one and only connection from signal ground to chassis and hence safety ground.
 
Using the chassis as a signal ground is a bad idea, although quite popular in the past when people did not understand grounding. Shielding the feedback wire can lead to HF peaks or instability (if the feedback is high impedance) or is unnecessary (if the feedback is low impedance).



I did not use the chassis as signal ground. I said “shielding”. Only the shields are connected to chassis.

The signal ground is a buss that starts at the negative side of the rectifier, running along the board and connecting to the chassis at only one point near the input jacks.

Do you know if the marshall superlead has a low impedance or high impedance feedback?
 
The scope trace you show looks like charging pulses, so look in detail at the grounding of your charging pulse loop in the PSU section. The charging pulse loop (secondary-rectifier-reservoir capacitor) should not include the chassis or anything else, although it may have a single wire connection from the reservoir negative to the chassis - but only then if you have decided that this is the appropriate place for the one and only connection from signal ground to chassis and hence safety ground.


No. The trace I’ve shown is with the amp in STANDBY. Only the mains and the heaters are on. And also the trace has both a negative peak and a positive peak in one cycle. Charging pulses don’t have that because of rectification.

As I said above I followed Merlin’s scheme to the letter. One connection to the chassis for the audio ground.
 
overtone85 said:
I did not use the chassis as signal ground. I said “shielding”. Only the shields are connected to chassis.
Any difference in potential between the chassis and signal ground will be coupled into your signal wiring via the capacitance of the shielding. Not a good idea.

The signal ground is a buss that starts at the negative side of the rectifier, running along the board and connecting to the chassis at only one point near the input jacks.
OK, that is fine if everything connects to it in the right order.

No. The trace I’ve shown is with the amp in STANDBY. Only the mains and the heaters are on. And also the trace has both a negative peak and a positive peak in one cycle. Charging pulses don’t have that because of rectification.
AC heaters? High voltage rectifiers disconnected, so charging pulse loop open circuit? Whether you see positive or negative peaks, or just one type of peak, depends on exactly where you are looking and how you are coupling to the charging pulse loop. You are seeing pulses; AC heaters don't produce pulses, so something else is doing it.
 
Any difference in potential between the chassis and signal ground will be coupled into your signal wiring via the capacitance of the shielding. Not a good idea.


Do you suggest to connect the shield to the local signal ground?

I think Merlin said at the beginning f the grounding chapter of his book that a shield could be connected to the signal ground but connected to a separate point would be better, so I chose to connect to the nearest most convenient chassis point as long as it’s away from destination of the signal (at the opposite end of grids)
On my input jacks thought the shielding is connected at the ground wire that connects the high and low input of the amp and then this goes to the same ground buss connection point I talked about above.
The only thing is there are 3 things connected to the same ground connection:
-The end of the signal ground buss
-The input jacks grounds.
-the heater center tap
I haven’t tried to move the heater CT closer to safety hearth though.

AC heaters? High voltage rectifiers disconnected, so charging pulse loop open circuit? Whether you see positive or negative peaks, or just one type of peak, depends on exactly where you are looking and how you are coupling to the charging pulse loop. You are seeing pulses; AC heaters don't produce pulses, so something else is doing it.


Yes I am using AC heaters. When I flip the Standby switch ON they actually get elevated by 40vdc but in standby mode it is regular 6.3vac.

I detect this buzz in every point of the circuit, chassis, signal ground buss, grids. As I said in reply to Malcolm even if the scope probe ground is close to the tip of the probe the 5mV buzz is still there.

And again, as I said above the PT is buzzing a bit itself and you can hear the chassis resonates with it slightly.

Bear in mind this doesn’t disturb the guitar playing at all. I just want to perfect things as much as possible. The hum is noticeable only if the amp is cranked and the presence and treble are at 0. When treble and presence are up the hiss drowning the hum.
I can crank the amp with all controls on 10, I also added a variable negative feedback control and I can set it to minimum feedback and don’t get squeals or oscillations. Even if I pull 2 power tubes out of 4 and lower the impedance switch by 1 step (4 ohm selected, 8 ohm load) and all this time the amp was out of the box without any shield on the bottom of the chassis.

I managed to make it oscillate in only 2 occasions:
When I was probing the signal path with the amp live I touched the output of the tonestack with the alligator clip and the squeal made it red plate.
And last night when I tried a home made 20 meters speaker lead which added too much resistance and increased the load mismatch with 2 power tubes instead of 4.
Other than that, the amp seems solid, but it’s only the second I build and I haven’t played a real superlead in a little while so I never remember what is the normal sound and what is not.
 
The heater secondary can pick up rectifier noise and inject it into the signal ground. I'm not sure how a heater CT can be both grounded and elevated by 40V DC, but maybe you mean that it is AC grounded to that common point? I once had buzz in an amp and the only way to cure it was to ground the heater supply and scrap the DC elevation I had planned to use.


Are you taking about the hum I hear behind the hiss when the amp is on? It is possible. But I thought the heater elevation circuit would filter that ripple as well. The reservoir cap node shows 25v of ripple that goes into the OT CT. after the choke there is almost zero ripple.
The heater elevation
 
The heater elevation has been done by connecting a 1M-100k voltage divider with a 47uF electrolytic on the 100k leg.

The heater CT is connected at the junction of the 2 resistors at the positive end of the cap.
I did try to remove the elevation and see if the hum would go away. No luck.

One of the things I’ll try is also a himdinger. How much out of balance do the heaters have to be in order to induce hum?
 
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