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Transformer alignment check

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Hi all
For some time, I've been working on a 300B SET amp from this schematic DIY 300B Single-Ended-Triode (SET) Hi-Fi Amplifier Project. Unfortunately, I never managed to get the hum out of it, which I think was due to limitations imposed by my unconventional chassis. I've therefore decided to pull it apart and rebuild it on a more normal chassis with a better layout. Given this experience, I'm somewhat paranoid about building in hum now though, so wondered if I could get people to check my transformer alignment before I bore any holes.

The attached photo is my proposed layout. The large Edcor box at the bottom left is the power transformer. The two smaller Edcor boxes above are the 5H power supply smoothing chokes, and the grey boxes at the right are the output trannys. The two gold boxes are the 200H chokes for the pre-amp supply. I'm intending to hang them in the pictured orientation upside down from the top plate under the the output trannys.

Is this a decent layout for avoiding inductive problems, and if not, what should I be doing differently?

Thanks in advance.

9EK3FmKHy8bnSG8p7
 
Use Aluminum, Copper, or non-magnetic version of Stainless Steel.

Do not use a Magnetic Steel Chassis.

Use DC on the 300B filament.

Contain the large transient ripple current in a local loop: B+ secondary, first filter cap return,
second filter cap return.
Do not bring out the B+ ground to the central ground until you have kept the large transient return currents entirely local.

Keep the input RCA return isolated from the chassis, and keep it local (connect to the input cathode circuit).
After that, bring the cathode circuit return to the central ground.

Relative orientation of the power transformer and the chokes; versus the output transformer is important.
But the distance from the power transformer and chokes to the output transformer is important.

There is no fooling Mother Nature.

Good luck, and shoot for 100uV hum or less at the 8 Ohm output.
 
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Thanks for your reply 6A3sUMMER. Some great tips that I'll watch while building. Does the transformer/inductor layout in the photo seem like a sensible one for minimising inductive hum, or are there changes I should make? The metal underneath is aluminium, not stainless steel, so have ticked off one point so far :).
 
DoktaDave,

The picture you posted only shows a Rectangle that has a Question mark in it.
There is no picture.

I love the look of potted transformers and chokes. But they are expensive.
And if I had any of them, I would have to use a scope to discover the orientation of the coil inside. Just put 60Hz in one, and measure what comes out of the other.
A scope lets you see a small signal, so you do not have to use full amplitude 60Hz.

I like your Moniker.
They used to call me 'Doctor Dave' at my work, before I retired.
 
Thanks for your tips 6A3sUMMER.

In my guitar amps I did something similar, but there's usually more available space to move transformers. What do you think about Having the PT lay-down, chokes N-S and OPTs E-W? This should help to keep the layout more compact.

One thing I would suggest, in case there will be some modifications, is to go with Aluminium (2 mm thick) instead of AISI 316L (1 mm thick), because drilling 316L is way more complicated.
 
1. Magnetic Steel causes the magnetic fields of the power transformer and the power supply choke to get all the way to the output transformer.

2. Chokes have Air Gaps, and Es on one side, and Is on the other side.
If you use a Choke Input filter, you better believe that there is a very large magnetic field transmitted through the air, and through magnetic steel.

3. Single Ended transformers use Air Gaps, and Es on one side, and Is on the other side.
They are much more sensitive to hum pickup than push pull transformers (Es and Is are interleaved, and no air gap).

4. There are 3 directions you can use to keep the fields from one magnetic device from transmitting to the other:
"North - South"
"East - West"
"Up - Down"

Yes, you can lay a power transformer, choke, or an output transformer down.
But again, you still have to keep them from "lining" up with the others.

5. Interstage Transformers, and Plate Chokes are very sensitive to magnetic fields.
There is more sensitivity, because it is on the output tube's grid, not on the output tubes plate.

6. All Generalizations have exceptions.
C-cores, or Double C cores are less sensitive than Air Gapped Es and Is.
 
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The point about choke input filters is a good one, as they have a ton of AC voltage across them.

The last time I used double C cores, I found them exceptionally picky to core orientation and I had to completely redesign my chassis to get them in the one spot where I found that they didn't pick up a ton of magnetic junk from my PT.
 
audiowise,

You are right about the double C cores (actually 4 C cores).
They are sensitive to magnetic hum fields.

Just for illustration, to those who do not know, pass DC through the coil.
The two C's at the top are North
The two C's at the bottom are South
The coils are wound over the inside gaps.
The gaps at the outside do not have coils on them.
The top Cs and bottom Cs are not interleaved.
That is similar to Stacked Es and Stacked Is (non-interleaved) of a traditional single ended output transformer, or of a filter choke.
Therefore, it picks up magnetic hum fields.

The C core (actually 2 C cores), like Lundahl uses, have coils wound over each gap.
But that means one C has North at one end, and South at the other end.
There is not any one core that is Only North, and there is not any one core that is Only South.
Therefore, the single C core transformers are less sensitive to magnetic hum fields.

I had a pair of Lundahl interstage transformers that actually worked well inside the [magnetic] steel chassis of a (extremely highly modified) Dyna Stereo 70.
I was pleasantly surprised at the lack of hum.
 
But why 6sn7 cascade in 2020? We have a dozen of capable high GM tube to drive 300B, few of them even can drive transmitting triodes/pentodes.

I think, hum management is also easier with single input/driver tube. Please consider, if not now in your future Vandoeuvre.
 
Generalization,

Some high GM tubes have much more 2nd harmonic distortion than the very linear 6SN7.

But, you may want the high GM tube's extra 2nd harmonic distortion to cancel the 300B's
2nd harmonic distortion.

You will change the amp that has perhaps 15 dB more 2nd harmonic than the 3rd harmonic distortion . . . to an amp that has perhaps equal 2nd and 3rd, or even more 3rd than 2nd harmonic distortion.

The choice is yours.

Tradeoffs, Tradeoffs, Tradeoffs.

I work very hard to make my amplifiers have 100uV or less hum.
But that requires a lot of attention to very many details.
I design amplifiers for Loudspeakers, but will not attempt amplifiers for Headsets.
 
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You will change the amp that has perhaps 15 dB more 2nd harmonic than the 3rd harmonic distortion . . . to an amp that has perhaps equal 2nd and 3rd, or even more 3rd than 2nd harmonic distortion.
Thanks, may I know what is your preference?

I work very hard to make my amplifiers have 100uV or less hum.
But that requires a lot of attention to very many details.
It would be great if you will write down some tricks to achieve so. At least what you feel confident in sharing.
 
zintolo,

Great question!
Which did I prefer?

I have built 2 stage single ended amplifiers with low u / low transconductance drivers; and with hi u / hi transconductance drivers.
I used type 45, 2A3, and 300B output tubes.
I used interstage transformers.
I used RC coupling.
I used Battery Bias on the output tube.
I used Self Bias on the output tube.
I used resistive driver plate loads.
I used SRPP drivers.
I used IXYS current source driver plate loads.
I used AC powered DHT.
I used DC powered DHT.

For the most part, I liked almost all of those various parts, and many combinations of them.
Sometimes I would build something and just say no, not this amp (can not think of a good example for the DHT amps).

All of these amplifiers did Not use negative feedback.

Usually, the driver and the output tube do not completely cancel the 2nd harmonic distortion, in fact usually there is more 2nd than 3rd harmonic distortion.
But whatever you measure with a resistive load, you have to remember that a Loudspeaker is anything but a resistive load (except at a very few frequencies). And even when it is resistive, it is usually different than the Rated Impedance of the loudspeaker.
Think of load lines that are different than the 8 Ohm resistor you used to test the 2nd and 3rd harmonic distortion of the amplifier.
And a different load line slope = a different distortion.
Then, there are all the reactive impedances of the loudspeaker, where it is not resistive. At the frequencies where it is a reactance, the load line is not a line, it is an ellipse. Even more complication.
But, just like the Bumble Bee that the engineers said could not fly, the amplifier and loudspeaker put out enjoyable music.

Still more single ended . . .

I built single ended Pentode/Beam Power output, tried both with and without negative feedback.
I did not like that.

I do like single ended Pentode/Beam Power output with Triode Wired mode, and with Ultra Linear mode.

Now, on to Push Pull amplifiers I built . . .
That is another story.
Perhaps on a later post.

But as a teaser, take a look at my low power push pull thread:
A Simple Low Power 7591 Push Pull Amplifier - in the Tubes / Valves part of this forum.
I think I posted that on July 11th.
I have so many things to do, I will have to add some performance details to that post on another day.

I have often posted on others threads, listing techniques for them to get hum down to a very low level.

One more thing about 2nd Harmonic distortion. Depending on the percent of distortion, and the phase, two devices may at least partially cancel the 2nd.
We ran an experiment with one single ended 2A3 Amplifier, and one Spica 50 Loudspeaker. They both have 2nd harmonic distortion.
A Double Pole Double Throw switch was connected to conveniently and immediately change the loudspeaker connections to the amplifier (change the phase). A 100 Hz pure sine wave was applied to the amplifier, and set to Moderate volume (not quiet, and not Loud).
One person would listen, and one person would switch the phase; multiple times. Then the persons would trade places.
There was a different tonality/timbre depending on the switch position. We did not try an say which was better, but we did notice a definite difference.
After all, that was a pure sine wave, it was not music.

I still enjoy listening to my single ended amplifiers, and I enjoy listening to my push pull amplifiers.
 
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Same old....

I didn't mentioned anything about 2nd or 3rd HD. I think 2 stage 300b se sounds better and simple to build. Also 6sn7 cascade give more gain than necessary.

''I have built 2 stage single ended amplifiers with low u / low transconductance drivers; and with hi u / hi transconductance drivers.
I used type 45, 2A3, and 300B output tubes.
I used interstage transformers.
I used RC coupling.
I used Battery Bias on the output tube.
I used Self Bias on the output tube.
I used resistive driver plate loads.
I used SRPP drivers.
I used IXYS current source driver plate loads.
I used AC powered DHT.
I used DC powered DHT."

Done all the above except inter-stage. All the config has its own strength and weaknesses. Then comes system synergy and personal liking. I have high regard for all the DIYers.

Regards
 
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