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Old 13th June 2010, 04:24 AM   #1
diblet is offline diblet  United States
diyAudio Member
Join Date: May 2010
Default help looking for a space saving isolation transformer for a portable tube amp

i have a tube amplified portable reel to reel which has a design that connects directly to ac which i would like to add an isolation transformer to - however the one i currently have is a tight fit.

so i was wondering if anyone might have any ideas of a smaller isolation transformer that they might recommend?


the unit i am adding isolation to wants 115 AC, 60VA 60C/S (according to the plate on the outside of the unit)

the isolation transformer i have on hand is 115 AC, 80VA dimensions 2.5"h x 3" x 2.75" - it is a tight fit and i would have to modify the case a bit to get it to fit (to gain about 1/4" or so on the height)

i mainly need one that is shorter than 2.5" h - i found this toroid unit, but it is 117V ...

it measures 1.52"H3.73"O.D - so that one might fit... i'd have to build a dummy to know for sure if the diameter is okay


just curious if anyone has any suggestions for a space saving isolating transformer for my application...


here is a schematic of the unit i am adding isolation to:

Click the image to open in full size.

Last edited by diblet; 13th June 2010 at 04:39 AM.
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Old 13th June 2010, 06:08 AM   #2
diyAudio Member
Join Date: Mar 2010
My suggestion is not to isolate the motor. That should reduce the load sufficiently to use a smaller transformer. The heaters draw 150 mA and the plates maybe half that, so you have a load less than 1/4 A, and a 30 VA transformer might do it.

The next step in reducing the load would be to allow the heaters to remain on the line and just isolate the plate of the rectifier. I don't like that, as it may create excessive heater-cathode potentials that could cause hum or even tube failures.
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Old 13th June 2010, 05:08 PM   #3
diblet is offline diblet  United States
diyAudio Member
Join Date: May 2010
oh that sounds great - i was actually planning on not isolating the motor anyway (as it is a pole motor and has its own transformer already) to keep the heat down on the isolation transformer

i think there is a cheap 50VA transformer i know of that will fit nicely in the unit - i just didn't know how to determine the VA i needed other than what the case said...

thanks so much - i'll give it a shot!
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