Filament "0" Volt Center Tap?

[Gene Ballzz]

More of a "theoretical" question than anything else. Without electrically "elevating" the filament center tap, where is the best place to connect this "center tap" of what seems to be the "dirtiest" unfiltered AC circuit to the rest of the "0" volt system?

At first glance, it may seem appropriate to simply tie it to the chassis, but that makes that circuit dependent on the chassis as an operational conductor, potentially putting voltage and current on the chassis.

Conventional wisdom suggests connecting this "center tap" directly to the same point as the HT "center tap." It seems there must be a better method? Is "elevation" the only real solution here? The downside I see there is that we are simply "re-introducing" that dirty AC connection to the later stages of the amplification process, paticularly in a cathode bias configuration?

Just Askin'?
Gene

 

[2L man]

Using chassis as an operative circuit is always bad idea!

If you don’t elevate filaments connect the CT to Zero Volt wire/bus close the main capacitor negative terminal where filtering is strongest. Just where PT CT vacuums all return current.

I have made combined electrolyt bleed circuit and filament elevate circuit using two series resistors and use their values to define filament elevate voltage. For example 220k and 47k and about 10uF 100V electrolyt parallel with 47k to smooth elevated voltage.

 

[South Park]

The center tap for the heaters is thar if you want use it for a DC circuit . You hook it to the HT center tap so the magnetic field in the transformer has some where to go . The filtering on the caps is on the positive side not the negative side . It is all about respect to ground that keeps things separated

 

[Guitar-Rocker]

More of a "theoretical" question than anything else. Without electrically "elevating" the filament center tap, where is the best place to connect this "center tap" of what seems to be the "dirtiest" unfiltered AC circuit to the rest of the "0" volt system?

At first glance, it may seem appropriate to simply tie it to the chassis, but that makes that circuit dependent on the chassis as an operational conductor, potentially putting voltage and current on the chassis.

Conventional wisdom suggests connecting this "center tap" directly to the same point as the HT "center tap." It seems there must be a better method? Is "elevation" the only real solution here? The downside I see there is that we are simply "re-introducing" that dirty AC connection to the later stages of the amplification process, paticularly in a cathode bias configuration?

Just Askin'?
Gene

The AC volts HT CT is just as dirty as the Heater CT AC volts. In fact they ride on the same iron core frame. The introduction of the mains filter reservoir is what starts the DC flat line process (allowing for the diode clipping humps).

 

[Pete Farrington]

The heater winding CT is totally clean, unless a valve heater shorts internally to an electrode, there’s 0 current flow there.
It’s purely there as a reference, to balance the circuit about AC 0V, thereby helping to minimise the radiated fields from the heater wiring.

Whereas the HT CT carries the dirtiest current in the amp, rectified spikes, that’s why good practice is to connect it directly to the reservoir cap negative terminal.

And hence connecting those 2 CTs together, eg at a star point, would probably not be ideal, perhaps the worst place.

If the DC reference is also to be 0V, the input stage’s 0V point may theoretically be a good option for the heater reference point (CT or balancing resistors).
But DC elevation potentially has many benefits.

 

[Gene Ballzz]

My sincerest and humble thanks to all for these fabulous responses!

@Pete Farrington
So my takeaway here is that connecting both Center Taps to the main filter cap "0" volt (point "A" as on the layout below) is not ideal and that the elevation is most likely the best option?

Thanks Again,
Gene

 

[Pete Farrington]

Yikes, I thought I’d been foist on my own petard then, as you cited a layout I recommend.
But thankfully, no.
Black is the HT CT, with a direct connection to the reservoir negative terminal.
Brown is the heater CT, that’s connected to the g2 & LTP decoupling cap -ve / main power amp 0V point. ie it’s outside the dirty rectifier current circuit, and also outside the heavy currents of the output valves anode circuit.
So all is in accordance with good practice.
Heater elevation would still be a multi faceted improvement.

 

[Gene Ballzz]

Thanks @Pete Farrington for correcting my inaccurate assumption that the short piece of wire connecting the "0" volt lugs of those two capacitors was inconsequentiol and generally making those two lugs act as the same connection! ICY now and clearly smell what yer steppin' in! In the build I currently need to finish (ahem, get working on, ahem) that third node is physically farther away and it's cap is on the board, instead of being another multi-cap can. I'll share a DIYLC diagram, including the filament CT elevation and ground scheme updates for critique and hopeful approval. FWIW, I can almost see the surface of my workbench!

Thanks Again, So Much!
Gene

 

[william vogel]

I connect the 6.3 volt filament center tap to the ground lug where I ground the input jack, about as far away from the HT center tap as possible. The reason is that both of these (heater CT and input) have the lowest current. The heater CT has no current (theoretically).
The HT center tap has the highest current and needs to be close but not connected to the chassis earth/mains input ground lug.

 

[Gene Ballzz]

Hey Folks,
It has dawned on me that this thread is heading towards a discussion of my TWI-LUXE build. So rather than having two "parallel" (pun inteneded) threads, watch for my TWI-LUXE thread to resurface from the land of necro! That's where further diagrams for perusal and discussion of this topic will appear.
Just Thinkin'
Gene

 

[neikeel]

I'm with William vogel on this one. Tried and tested in my hands - many times and very unobtrusive in vintage amp too.

 

[tschrama]

Same here.. Safety ground goes very near the power socket with a dedicated lug to chassis. The only other chassis connection is on the other side of the chassis from the input stage cathode resistor to a chassis lug. The filament center tab is goes to that first stage cathode. Although in practice it is not that important were the filament center tab is grounded. For lowest hum levels, it is much more important that the power supply leads that have high current spikes, e.g. from the power transformer, diodes, up to the choke, are bundled with their return leads. NOT the way is it depicted in that Modulus layout.

 

[Mcentee2]

Same here.. Safety ground goes very near the power socket with a dedicated lug to chassis. The only other chassis connection is on the other side of the chassis from the input stage cathode resistor to a chassis lug. The filament center tab is goes to that first stage cathode. Although in practice it is not that important were the filament center tab is grounded. For lowest hum levels, it is much more important that the power supply leads that have high current spikes, e.g. from the power transformer, diodes, up to the choke, are bundled with their return leads. NOT the way is it depicted in that Modulus layout.

I don't want to get off topic too much, but am interested in which wires are going the wrong way in that layout ?

On topic - from all I have read, there is little going on re the CT of the heater, but that putting it on a least noisy point is best practice anyway. The issues with noise is not so much that the heater circuit has it or inherits it or it bleeds form somewhere to it, but rather that that noise once on the heater circuit can bleed into the audio circuit via the Cathode/Heater adjacency in the tubes themselves, that elevation hopes to cure, or via bad lead dress.

Obviously minimising all those aspects helps, but the placement of the CT itself is poss one of the least important maybe ?

 

[Pete Farrington]

which wires are going the wrong way in that layout ?

Something I move is the bias supply 0V, ideally I think it should probably connect to the output valve cathodes (well the wire linking the g3 pin1s).
But the current there is pretty negligible, so we shouldn’t let the ideal be the enemy of good practice.

 

[tschrama]

For lowest humm, not noise (that is something entirely different), the HT centre tab (black) should be bundled with the both HT wires (red), all the way, via the standby switch, underneath the HT diodes, to the fuse, to the first resevoir capacitor.

 

[Mcentee2]

View attachment 100532

For lowest humm, not noise (that is something entirely different), the HT centre tab (black) should be bundled with the both HT wires (red), all the way, via the standby switch, underneath the HT diodes, to the fuse, to the first reservoir capacitor.

Wow - ok, with you now, and got me thinking more than is probably good for me! I knew about routing the AC power stuff etc, but didn't include those return/0v paths in that. I've only ever thought of those 0v paths on their own in terms of sequence of what joins what and where along the path back to 0v point, not necessarily the actual routing of them.

TBH - this probably answers another of my back burner puzzles as to why we would bundle the mains AC, the rectifier tube AC and the DC B+ HT all together in the first place (never mind where the B+ 0v return lies!) I always thought that surely those ACs would be right noisy against the B+.

 

[Mcentee2]

@tschrama if you don't mind could I ask your advice on my jtm50 layout, re the above comments about ground routing next to the feeds.

I am specifically Looking at the PI feed from the cap can on the side of the chassis above the PT, as I think I have the mains and reservoir routing sorted.

The PI feed and return circles round from the cap can positive side to the board components and PI tube then to the presence cap and back to the cap can -ve.

This looks like a loop to me - should this also follow your guide of the Fred wire and return being as close as possible ?

 

[Pete Farrington]

The PI feed and return circles round from the cap can positive side to the board components and PI tube then to the presence cap and back to the cap can -ve.

The AC current in that circuit will be negligible, so my take is that fields resulting from it will be too.
It’s the fields from the high / dirty current circuits that we need to bear in mind.
eg heaters, mains, HT AC, rectifier-reservoir, anode and screen grid.

 

[tschrama]

True, bit it might capture the stray ac magnetic field from the power transformer, not?

 

[Pete Farrington]

I suppose that anything is possible but it seems unlikely to me that would cause contamination of the signal path
By the same mechanism, the input grid circuit loop is also exposed to the PT field. A vastly lower field strength due to being further away, but then there’s a vast amount of gain applied to it.