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Benefits of Potted Output Transformers

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#25
#26
Unless there is a mu-metal shield inside, there's often no better shielding then a regular end bell.
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I don't think mu-metal shielding is necessary for large signal transformers such as ITs and OPTs. It is only needed for low-level magnetic field shielding (the earth's magnetic field, eg.) in small-signal transformers such as microphone input or MC step-up. Ordinary iron works well enough for shielding OPTs.

John
 
#27
jarthel,

No, I really did mean the input of the headphone amp. The output transformer or choke is not connected to the circuit you are planning on using it in. For this test, it is the source component for the headphone amp; it is a magnetic pickup coil. What you are trying to do is find the orientation where it picks up the least magnetic field from the power transformer. All the amp does is make the procedure Chris described more sensitive; it allows you to hear the null easier as you move the transformer or choke around the chassis. Hope that helps clear it up.

Ken
 
#30
Or, contrariwise, the epoxy can keep bits from moving around with vibration and heat.

In any event, a specific claim was made about a specific set of transformers from a specific manufacturer and I'm looking for evidence to support it, not vague speculation.
 
#32
Every time I see shiny pots in Sears I think of potted transformers on a polished oak. 😉

By the way, what if to fill the pot by a grease with low thermal resistance, as used for heatsinks, and seal by a silicone compound? 😉
However, it would be better if it is black with ribs for better convection, not shiny like a mirror. 🙂
 
#34
By the way, what if to fill the pot by a grease with low thermal resistance, as used for heatsinks, and seal by a silicone compound?
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In that case the best solution is silicon oil mixed with glass beads. I have disassembled military modulation transformers that were (thankfully) potted in this manner.

John
 
#35
As Chris said, any kind of potting will increase the capacitances and that pretty much always hurts. I have never experienced a case where impregnating the coil with any kind of dielectric improved the sound, though I have no doubt that some dielectrics are less harmful than others in terms of the way they affect the sound. And I'm not just talking about the dielectric constant, but something more subtle and complex than that. I don't need to read it in a book or perform double-blind tests to believe it.

Here's something that will really raise some fur: I've never encountered a case where bolting on iron endbells, or even worse stuffing a transformer tightly into a metal can improved the sound. The contrary is more often the case. The one exception is when the tx is handling VERY low level signals and the shielding is really necessary. Phono stepups for example. Then, it seems, mumetal is the only way to go. Again, just subjective anecdotes, but they're mine. 🙂

-- Dave
 
#38
Hi sorenj07,
Absolutely yes!!

I guess that's why Wavebourn uses it.

They do make a thermally conductive potting material. As conductive as that stuff can get of course. I agree that unless low signal, transformers are best in open air. End bells provide mechanical protection.

-Chris
 
#39
jlsem said:


Most potting compounds are special epoxy mixes that have low shrinkage rates, anyway.
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Proper ones do, yes. Additionally, there are usually materials added to make the potting more flexible (tougher).

For thermal conductivity, the additive of choice is powdered diamonds. As much as some people are charging for amps, I would expect no less. More common is beryllium oxide (not the same as metallic beryllium!). Adding the filler has the nice side effect of also reducing shrinkage.
 
#40
Let me guess again... No feedback un your SE amps?
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The TubelabSE uses no feedback, and the SimpleSE has switchable local feedback from the secondary of the OPT to the output tube cathode. I find that I turn the feedback off in triode mode (most of the time) and turn it on in UL mode especially with rock music on my big speakers. I also have a P-P amp that doesn't seem to need (or want) any feedback. The sound is very dynamic with no loss of control on heavy bass. I attribute some of this to the low plate resistance of the 300B's and my higher than usual 6600 ohm load.

The issue of heat should not be a concern in an OPT. Even the worst (high DC resistance, lossy magnetics) OPT's should have to dissipate only a few watts, unless it operates into saturation (guitar amp). If the OPT gets warm it is usually because it is absorbing heat that was radiated from nearby output tubes. A shiny metal can may help an OPT. There shouldn't be too much thermal stress in an OPT, and potting may help keep everything at the same temperature. Corrosion and moisture resistance should be improved. Corrosion and rust are a common problem here in South Florida. I have seen rust develop on the laminations of some popular OPT's in two years. These transformers have never been out of my air conditioned lab.

The power transformer is a different matter. It must dissipate a lot of heat. Many modern transformers run hot due to a design that is optimized for cost. Modern materials allow transformers to run hotter without insulation meltdown, however this heat may lead to long term degradation of the magnetic properties of the core material. I don't think that these transformers will still be working in 50 years.

I worked in a TV repair shop from 1968 through 1970. At that time about half of the TV's that I worked on came from non air conditioned houses. Power transformer failures were not uncommon. The Zenith brand TV's had power transformers that were filled with some epoxy like substance (you couldn't take them apart even with a hammer). They also had heat radiating fins and were painted flat black. I never saw one fail. Those of us that built amps using old TV parts, collected all of the Zenith power transformers that we could get. I still have one in my collection.
 
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