Actually I believe the most important benefit of potting is preventing moisture from getting in the windings as well as physically sealing it against vibration. Yup, it also looks nice too. Unless there is a mu-metal shield inside, there's often no better shielding then a regular end bell. And call me cynical if you want, but it also helps deter getting at it to reverse engineer it.
Victor
Victor
dsavitsk said:
You can be a bit theoretical. Think of the mains transformer as having the coil that is the primary of a transformer and the output transformer as having the coil that is the secondary of that same transformer. You now want to make the coupling of the notional transformer as poor as possible. Orient the coils at 90 degrees to one another and get as much distance between them as possible. Fine tweaking when you've made it may help.
Potting has its pro and cons
When you pot it looks nice
it prevents high tension build up and prevent corona discharge
No moisture can build up within the coils.
Helps to slighlty reduced vibrations as the epoxy acts as a dampener
Cons
The trans runs hotter as there is no air flow
For audio trans the potting materials will act as a dielectric
most potting material like james is epoxy which is bad for sound. The good sounding potting material which wax on the other hand melts at 60-90 C depending on wax types
When you pot it looks nice
it prevents high tension build up and prevent corona discharge
No moisture can build up within the coils.
Helps to slighlty reduced vibrations as the epoxy acts as a dampener
Cons
The trans runs hotter as there is no air flow
For audio trans the potting materials will act as a dielectric
most potting material like james is epoxy which is bad for sound. The good sounding potting material which wax on the other hand melts at 60-90 C depending on wax types
According to my very limited understanding, all potting compounds, polyurethane or epoxy based, shrink when they set (ca. 3 vol %). The shrinkage inevitably put the core/winding under stress (deoends on construction, elastic modulus, .... ). Stress in laminated cores reduce their saturation level, by as much as 30% if I remember correctly. Epoxy is worst than varnish in that respect.
But of course you can design for this, by using a larger core.
Patrick
But of course you can design for this, by using a larger core.
Patrick
Here's my method. Keep in mind I've now built a grand total of about 8 amps so take this with a grain (or kilo) of salt.
1- Don't drill the holes in the chassis to mount the trafos
2- If the OPTs are on either side of a PTX I generally orient them 90 degrees from the PTX and 180 degrees from each other. If they are on the same side of the PTX then orient the one closest to the PTX at 90 degrees from it and the other one at 90 degrees to the first.
3- Listen. If there is a problem then rotate the OPT on the channel with the problem until it (hopefully) goes away.
Note that doing #3 can fix the problem channel and introduce problems in the other channel.
Then I just rotate each OPT one at a time until the problem is gone or minimized.
4- Now drill the holes and mount the trafos.
To be honest I've only had one instance where there was any audible problem with trafo orientation. In that case the problem was always in the channel closest to the PTX. I was able to move the PTX about 1/2" farther away and then move the OPTs about 1/2" the other way. Problem solved.
1- Don't drill the holes in the chassis to mount the trafos
2- If the OPTs are on either side of a PTX I generally orient them 90 degrees from the PTX and 180 degrees from each other. If they are on the same side of the PTX then orient the one closest to the PTX at 90 degrees from it and the other one at 90 degrees to the first.
3- Listen. If there is a problem then rotate the OPT on the channel with the problem until it (hopefully) goes away.
Note that doing #3 can fix the problem channel and introduce problems in the other channel.
Then I just rotate each OPT one at a time until the problem is gone or minimized.4- Now drill the holes and mount the trafos.
To be honest I've only had one instance where there was any audible problem with trafo orientation. In that case the problem was always in the channel closest to the PTX. I was able to move the PTX about 1/2" farther away and then move the OPTs about 1/2" the other way. Problem solved.
One trick from the old days was to hook up a set of headphones to the secondary on the Output Transformer...
The output transformer would not be wired to the circuit yet...and to turn ON the power transformer... By rotating the output transformer while listening with the headphones you can find the correct orientation from a null in the hum...
I believe i read this in a old copy of "Wireless World" many years ago.....
I also read somewhere to take a old phono cartridge and wire it to an amp and that can be used to detect as well....
Chris
The output transformer would not be wired to the circuit yet...and to turn ON the power transformer... By rotating the output transformer while listening with the headphones you can find the correct orientation from a null in the hum...
I believe i read this in a old copy of "Wireless World" many years ago.....
I also read somewhere to take a old phono cartridge and wire it to an amp and that can be used to detect as well....
Chris
No B+ needed.....
The B+ is DC for the valves....
We are dealing with AC 60Hz coupled noise....
Just hook up the headphones to the low impedance side of the Output transformer...Then move the transformer with headphones on...You will hear the hum in the headphones when you have the transformer positioned outside of 90 degrees...
The only thing powered up would be the power transformer...
Chris
The B+ is DC for the valves....
We are dealing with AC 60Hz coupled noise....
Just hook up the headphones to the low impedance side of the Output transformer...Then move the transformer with headphones on...You will hear the hum in the headphones when you have the transformer positioned outside of 90 degrees...
The only thing powered up would be the power transformer...
Chris
Chris is correct in his method but, as with many things, with the passage of time there has been a bit of an improvement on the method. Note, this also works great with chokes, too. Take your headphone amplifier (you have one of those, right? If not go to headwise.com and search for "cmoy". This is basically an opamp driving a headphone and is about as simple as you can get.) and hook the primary of the output transformer or the leads of the choke to the input of the headphone amp. You should have wired at least the filament circuits and have the tubes in place but no B+ connected. This gets the power transformer to draw some current. I don't know if this really matters but it definately doesn't hurt. The output transformer is not connected to anything but the headphone amp. Adjust the volume so you get a reasonably loud hum. Then just turn the output transformer or choke around until you get minimum hum. This is basically the same as what Chris is saying except the headphone amp makes the procedure more sensitive and you can easily find the null. I do this as a matter of course ever since I read about it a year or two ago and have not had a problem with hum from coupling since. Hope this helps.
Ken
Ken
In my first TubelabSE, I mounted all 3 transformers in a row, with the cases almost touching each other. I had a small amount of hum audible in the left speaker (center transformer) when the amp was first switched on. The hum vanished as the amp warmed up. I used the amp this way for almost a year with my 87db speakers that have a 7 inch woofer. It sounded great.
I took the amp over to a friends house. He has 95 db speakers with a 12 inch woofer. Hummmmmmmm. The hum was loud when the amp was first switched on, and became softer as the amp warmed up. It was still too loud. Rotating the power transformer 90 degrees killed all of the hum.
It seems that SE transformers are more sensitive to picking up stray magnetic fields when there is no current flowing through it. I have verified this effect with several OPT's.
I think the headphones idea is a good one. It also seems that you would not want B+ applied.
I took the amp over to a friends house. He has 95 db speakers with a 12 inch woofer. Hummmmmmmm. The hum was loud when the amp was first switched on, and became softer as the amp warmed up. It was still too loud. Rotating the power transformer 90 degrees killed all of the hum.
It seems that SE transformers are more sensitive to picking up stray magnetic fields when there is no current flowing through it. I have verified this effect with several OPT's.
I think the headphones idea is a good one. It also seems that you would not want B+ applied.
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