Tubes, transformers and proximity

[pjd3]

Heyo all, hope you are fine as can be. This is an extention of an earlier pondering Ihad concerning tube-to-tube proximity.

Now, my latest pondering is Tube-to-transformer proximity concerns.

As I sit with a blank twin reverb chassis and a couple of 11.8" x 3" pre-holed turret boards, I'm playing around with various layouts, primarily for the output transformers and tubes. This is to be a 6V6 stereo plexi amp for the intention of a good sounding stereo tube amp that is both a quick set up and will utilize the great sounding stereo time based pedals that are our there now. Some pedals will be front of amp, time-based pedal will utilize a Zero-loss high voltage series effects loop. I figured micing each speaker and panning them left and right through the PA wouldn't fail in rendering a very nice stereo presentation to the audience. Really, this won't be a stereo amp, it will be two completely independant and soveriegn amps in a cabinet, each even having their own power transformer on each far back corner of the chassis. A benefit of this would be that there would always be a spare if one amp were to develop an issue during a gig. Thats my story, and being the beligerent petulant 66 year old weekend classic rock war rior I am, I'm sticking to it.

I find that there are a couple of different methods of layouts that could occur. One with the PT's at the ends, OT"s side by side in the middle, with the 6V6 pairs sitting in between. Seen (or imagined) benefits might be that the PT and OT's have max distance between them and power tubes can be near the power section shortening connective paths to them while also staying away from the entering signal paths in the middle front of the amp. Of course, the OTS are now sitting about 7 inches away from the sensitive input section in the middle-front.

Another layout is to have have the OT's placed a bit separated from the middle of the amp with a power tube on each side, allowing the OT's to be further away from the middl-front input area but a little more close to the PT. I would likey do the "headphone" trick on the OT secondary to see about minimizing radiated hum from PT to OT (that worked well on my builds).

So, what am I worried about? That would be I don't have any direct experience in how tubes interact with transfromers with regards to proximity. I imagine that while tubes have metal elements in them that they would be facing similar issues as PT's and OT's, perhaps not as much because they are not indictive coils. But, I don't know anything about that for certain.

Do you folks have any guidlines concerning how you orient tubes and transformers? It there are any known electrical or sonic hazards that are prevalaent by which you avoild certain proximities? Angles to each other?

Thanks. Any input on this would really be helpful in nailing down a final layout for the tubes and transformers. It should be mentioned that this amp is for the most part a pedal platform while having a good clean plexi tone. Its really going to be two Sluckey 6V6 plexi's in one amp. My second build was a Sluckey 6V6 plexi that is my gigging amp now. I use it out gigging every week and get alot of compliments on my tone. I like it as a base sound.

So. Thats whats keeping me awake at night this week. Tube-to-transformer proximity concerns.

Thats everyone for hanging in on this long winded post. But, I'm really exited about this project and want to get it right.

Best,
PJD3

[R.G.]

Magnetic fields leaking from transformers deflect the electrons inside the tubes. Exactly how much this changes things is a math problem of magnetic field intensity and the exact orientation of the field direction and the tube's rotation to the field in x , y, and z directions.
Field intensity goes down with the square of the distance from the transformer. Double the distance, 1/4 the field intensity. Distance is your friend with magnetic interference.

You can also insert a 0.1" to 0.125" (2.5-3mm) thick plate of aluminum vertically between the transformer and the tubes. This significantly reduces the field getting to the tubes because of eddy currents in the aluminum plate.

[pjd3]

Thank you RG for your input.

I had wondered if electromagnetic radiating fields followed the inverse square relationship as many radiating forms of energy seem to, so sure, looking for creating distance is something I will look to do where it can be applied. I probably won't develop all the math necessary to severely minimize radiated interference in the x, y and z directions but, perhaps I could pay good attention to how and where the radiated fields form around the power transformer is emmiting and to see if there is any rotation of the power tubes that is optimal in respect fo the field it would be seeing.

But, its already enough to know that proximity could be an issue to look out for. That in itself will begin guidance on layout possibiliies and where or how to begin looking for potential trouble.

Thanks RG, this is a start for me but one I will keep vigilant about until I know a bit more.

Best,
Phil D.

[R.G.]

One thing that many commercial amps do to minimize magnetic pickup from power transformer and inductor to output transformer is to orient each one in a different direction of x, y, and z. That way both the power trans and inductor have minimized transmission to the OT just on direction. The PT and are ideally at opposite ends of the chassis for distance shielding, and the inductor is in the middle because it has less power flowing through it and less AC excitation to make an alternating field.

I've seen power tubes very close to the PT, without apparent ill effect. Any magnetic effects on the tubes will be amplified by all the gain stages after that tube, so you want the input tubes furthest from the PT, and can stand succeeding higher level stages to be closer, just because there is less gain after them. I suspect that it won't matter with the output tubes and phase inverter, getting more significant as you go back toward the inputs.

[pjd3]

Oh RG,

Regarding the presence of aluminum plates... These are fine bolted to an earth grounded chassis? I ask because I don't know it the grounding is an essential part of the plates working to minimize interference or it its just the presence of the plates is enough.

A curiousity question really,

thank you!
Best,

Phil D.

[R.G.]

They're fine bolted to the chassis.

The way magnetic shielding works is that a field trying to get through a conductive plate induces many tiny circular electrical currents inside the plate, each flowing at right angles to the incoming magnetic field. The eddy current does two things - it both wastes the magnetic field's energy as heat in the plate's resistance and also the current flows in a direction that creates an opposing magnetic field to the original. The opposing fields try to keep the incoming magnetic field from entering the plate.

This happens very locally, only where the field is hitting the plate. Bolting the plate down to the chassis doesn't get into the process or interfere with the little circular regions of eddy currents.

Long, rambling technical discussion follows; you can just skip this if you just need to know about the aluminum plate.

The better the conductor, the better the production of opposing fields. Silver, copper, and aluminum, in that order, are best for this because of their low resistivity. Silver's out because of cost, and copper has become a semi-precious metal, so lower cost aluminum gets used. The copper bands on some transformers work this way, with the low-resistance copper band opposing magnetic field leaking out of the core. This is also the reason that transformers are laminated, made of silicon-iron, and have insulating oxide coatings. For transformer cores, you want the opposite of a shield. You want the magnetic field to flow through the iron, not escape into the air, and not heat the iron with eddy currents. So the iron is as continuous as possible where the magnetic field should be flowing, but split into thin sheets. The thin sheets keep eddy current loops tiny The thin sheets are made of silicon iron allow to make them higher resistance to keep eddy currents lower and lower the I-squared-R heating.

The higher the AC frequency of the field, the more eddy current heat losses happen. Above mains frequencies, the eddy current effect gets more intense. This is why high frequency switching supplies use ferrite cores instead of iron laminations. The magnetic particles are tiny, dust-like, so the eddy currents are suppressed, and the magnetic materials have much higher resistance, so lower eddy currents flow and less heating results.

[bepone]

In my last build, power transformer is almost touching output tube (very small chassis for the purpose0 .. there is no any humm or problems.

[xtian]

I have also, in my ignorance, placed power tubes very close to OT in the past, with no trouble.

[pjd3]

Thank you all for the replies.

Although theoretically I'm sure there is some magnetic interaction happening. Its probably a matter of degree and there is more breathing room than I may be anticipating.

I certainly can keep power tubes at least an inch or more from each other and anything else so, I may be good. if there are "good layout practices" to follow however, I will keep an eye out for them and apply whenever feasible, such as knowing the radiating fields of the transformers an placing tubes on the less magnetically intrusive sides of them, rotational orientation and such.

I'm assuming that as long as the row of 12ax7's are at minimum a few inches away from anything else that all will be good there.

I suppose my greatest fear is creating a situation, say, as RG mentioned, causing electron defection in a power tube that would even marginally compromise the tone quality of the amp, to the point where the sound is acceptable but, not what it could have been. I'd rather have a complete failure of the amp! Then at least I'd have something substantial to investigate. I do know that tubes getting too close to the back of speakers can and will cause problems with the final tone result as I restructured a cabinet after finding a very will described issue someone had with that situation. They were able to slide the chassis in and out while seeing (scope) and hearing dramatic effects on tone quality.

OK. So while being vigilant with tube/transformer proximities its down to decide layout - 6V6 pairs side by side Or, on each side of the associated output transformer. Decisions, decisions.

Theres still time!

Thank you again, all.
Best,
Phil D

[martin manning]

See here for an experiment involving magnetic effects on tube characteristics: https://ampgarage.com/forum/viewtopic.p ... 90#p440790

[pjd3]

OK,

I'd really be interested in feedback on this:

Would you generally expect there to be less of any issue with power tubes in proximity to output transformers in comparison to power transformers?

I'm inclined to ask this because although there are obvious strong 60Hhz fields emanating from power transformers, output transformers are supplying a good amount of current to speakers, as well as pretty high voltages swinging on their primary.

Perhaps if output transformers supply a less innocuous magnetic field to power tubes, that could be a guide to a final layout. I can make it so that the power tubes are pretty far away from any transformer at all but, that would just entail having the output transformers closer to their respective power transformers, a few inches away maybe.

Thank you!
Best,
Phil D

[martin manning]

I would expect that if the output tubes were too close to the PT you might get a 60 Hz hum. If they are close to the OT, you could have a feedback loop. Might be positive or negative, depending on the details of the orientation.

[R.G.]

Spot on, as usual, Martin.

@pjd3: this is one of those situations where exact details matter. I've heard of amps that would produce 60Hz hum WITH ALL THE TUBES REMOVED! The PT and OT were in an unfortunate arrangement where the field from the PT could transmit directly into the OT's core, get picked up by the OT windings, and make hum in a speaker.

This kind of thing is why the standard advice is PT on one end of the chassis, the OT on the other end with it's E laminations at right angles to the PT's, and inductor in the middle, with its E laminations at right angles to both PT and OT. There is the competing effect of tube locations. Ideally, the power tubes could live closer to the PT because they are the least sensitive (lower gain to the speakers direction) and the preamp tubes as far from the PT as possible. That flies in the face of wanting the preamp tubes as far from the PT as possible on hum grounds, and as far away from the OT as possible on feedback/oscillation grounds.

One trick we used to use in the power supply design group - you can use an air-cored coil of a few turns or even a single turn of copper wire as a "probe" to place near and in different orientations to the transformers to pick up stray magnetic field emissions on an o'scope.

[didit]

Hello --

Material improvement :: https://en.wikipedia.org/wiki/Mu-metal.

Not cheap, though quite effective in my limited experience.

Best ..