Open frame trannies and AC magnetic coupling issues

[EWBrown]

Over the weekend, I did some experiments, using an open frame (unshrouded) Hammond 261M6 power trannie (215 V @ 269 mA; 6.3VAC @ 4 ma). With 123VDC line voltage, the outputs are more like 230 & 7VAC). The purpose was to determine just how far out the AC induced stray magnetic fields can reach out and couple into various OPTs.

Later on I built up (literally) on a bread board a DC supply using the same trannie, and it delivers 268VDC @ 184 mA (a somewhat overstressed Philips 230V 40W lightbulb). This DC voltage is near the "perfect" level for any 6EM7 design, as it allows for the best plate to cathode voltage and current combination to get the best sound out of these tubes (along with 13EM7, 10EM7, 6GF7, 6FD7, etc).

FWIW, the trannie runs only moderately warm with 50 watts of loading on the B+ (filament not loaded at the time of the test, that would add another 25.2 watts, for a total of 75 watts consumptu\ion).


I started out with a 230V / 40W light bulb load on the (230VAC) winding, for a 175mA current load that a possible PSE 6EM7 amp design.

Where did I manage to find 230V lightbulbs?

My sister and her hubby were over in Europe last fall, and I asked them to bring me back some "souvenirs" in the form of 230V / 40W lightbulbs.

For the test OPTs, I used a couple of Hammond 125ESEs with the 8 ohms output connected to a couple of small speakers, to measure the "safe" separation" between power and output trannies. Primaries were left "open".

With the above combinations, the 60 Hz AC was audible in the speakers all the way out to 18 inches separation :o :o :o Even laying the OPTs flat in order to create a 90 degress axis separation didn't help all that much. I didn't achieve acceptably low levels of hum until after two feet of separation. :o

Since these open-frame trannies and OPTs normally all orient the core in the same vertical axis, the mutual coupling really reaches out.

I tried the same power trannie / load combination with a pair of Transcendar 3K, 10 W OPTs, and the "kill range" was about six inches, with best oriantation.

Next test was a couple of Z565 clones, these weren't nearly as magnetically "sensitive" but they still needed careful orientation in order to keep the 60Hz AC magnetic fiend coupling inaudible.

I repeated all the tests with the DC supply, and had nearly identical "AC DX" results. :o

So, the lesson is: if you use open-frame type power trannies, test before committing to a chassis layout and design. Or use a separate PSU and amp chassis... This includes using "back to back" filament trannies, isolation / step-up trannies, etc. I suspect that some of my "incurable" low-level hum issues with the K502 amps is caused by the opn frame power trannie, and original open framce OPTs - the Hammond 1608 / 1609s are better, but still need more separation than I used....

FWIW, toroids (either shielded or open) are the best at self-containing their stray magnetic fields, and I would suspect that the use of the toroidal PS trannie in my 13EM7 PF SET amp is the reason that it is so unbelievably quiet, with zero detectable hum.

/ed B in NH

[Webelo]

interesting post.

I just built my first amp (k12-m) and am ready to put it in an enclosure. I picked up some hammond OPTs and plan on mounting them on top. I'd like to mount the Mains tranny on the underside (for aesthetic purposes). So, I have a couple questions... Do the magnetic fields of trannies still play with each other even when their mounted on a different plane? Any suggestions on mounting orrientation?

[EWBrown]

The K502 power trannie doesn't seem to couple into the Hammond OPTs, as they are enclosed reasonably well, and have interleaved EI type cores, though there is some coupling into the open-framed UTK OPTs supplied with the kit. I found this was still true with the new K-16 as well, though the PT is further away from the OPTs in my "breadboard" setup. The magnetic coupling is present, but at a very low level.

The best thing about mounting the power (mains) trannie under teh chassis, is that no holes to pass the wires through are needed. This also allows for "fine-tuning" the power trannie's orientation and position before committing to drilling the mounting holes.

It is still best to maintain a 90 degree axis separation between power and output trannies in any situation. If you're using a steel chassis, and have the power trannie below, then the steel will act as a shield, and help keep the "magnetic gremlins" at bay. If you use a PSU choke, position it so that its core is "orthogonally oriented" (mutually perpendicular) to the power and output trannies. For example: power trannie core axis oriented front to back; OPTs, side to side; and PSU choke, vertically. That's more or less how it's done on the ST-70.

/ed B in NH

[kheper]

A cautionary post by EWBrown.

I have an open frame NOS Stancor 6.3 @
10A transformer and a potted NOS UTC 6.3
@ 10A. Of course, the UTC got mashed by
the package-destruction-department at UPS.

Looks like I'll be getting out the pliers to fix,
as best I can, the UTC.

[crispycircuit]

I refuse to ship UPS... They have ruined several items shipped to me and then refuse to pay the insurance claim. Any thing heavy will be broken!! Beware! ... Now the US Postal Service has been excellent and have many new features on pickup and selfservice shipping....

[Slartibartfast]

I refuse to ship UPS... They have ruined several items shipped to me and then refuse to pay the insurance claim. Any thing heavy will be broken!! Beware! ... Now the US Postal Service has been excellent and have many new features on pickup and selfservice shipping....

My story is just the opposite. I have had two things lost by the USPS. I have shipped a lot of stuff, both personal and for companies I have worked for and have never lost anything or had it arrive damaged. I just sent a Scott 340 receiver to a guy and it arrived, without any breakage.

Robert

[crispycircuit]

I bet USPS paid the insurance claim.... UPS left me with a $300 damaged item, THAT HAD INSURANCE... Big difference in claim settlements... Out $300 for me on 1 item... I'm glad you've had good luck so far. All companys have lost/damaged packages, but when insurance is bought and then refusing to pay is unacceptable! I really don't know how they can get away with it?? Sorry for interrupting the thread but you struck a nerve!

[Slartibartfast]

I bet USPS paid the insurance claim.... UPS left me with a $300 damaged item, THAT HAD INSURANCE... Big difference in claim settlements... Out $300 for me on 1 item... I'm glad you've had good luck so far. All companys have lost/damaged packages, but when insurance is bought and then refusing to pay is unacceptable! I really don't know how they can get away with it?? Sorry for interrupting the thread but you struck a nerve!

I would not know. Those were items shipped to me and lost in transit. One company resent the item, the other refunded my money.