Hvps

[tommyp]

I have an EI transformer that outputs a bit too much voltage. The transformer came out of a 3 phase vibration table shaker amplifier using six 5881 tubes (shaker table to simulate environments). I load tested it to 1.5 amps on the output side and it did not hum or burn up, but the load resistor lit up like a toaster oven! The voltage I need is 480 volts DC to drive four 6550 tubes w/predrivers (I assumed about 700-800 mA for worst case load). The transformer output voltage is as follows:

red - 760 vac
blue - 273 vac
red/yellow - 0
blue - 273 vac
red - 760 vac

The resistance across the reds is 50 ohms, across the blues is about 18 ohms. It's all center tapped.

I tried all regulator configurations to find the closest to 480 VDC, but the closest I can get is by using the blues with a full wave bridge. I get about 500 vdc (unfiltered). Once I add a filter cap, the voltage jumps too high. A while back, I used this transformer in an amp with a choke input CLC filter. The chokes got a bit warm for my tastes. Perhaps the chokes used were too small, I don't know. Space doesn't permit me to use a larger choke. I'd like to remove the chokes to free up space for other things.

So, I'm in process of retrofitting the amp now. I suppose there are two options for to find a remedy for the too high voltage issue. I prefer to use solid state (diodes).

Option a - Is it possible to leave the EI laminations intact, remove the outer paper, and remove a portion of the secondary winding? Once that's done, I can put new kraft paper back on and seal it up with some high temperature conformal coating/sealant. FWIW, the outermost winding appears to be the "blue" leads.

Option b - Can I use something like Welborne Labs PS3 regulator without using an the PS filter cap [C1] right after the diode regulator? Without the cap, it looks like it should work, but I seem to recollect that the pass regulator is a capacitance mulitiplier and I can't say what will happen if that part is omitted.

Any help appreciated.
Thanks in advance for your help

[h_a]

Honestly, I'm a bit surprised you play with high voltage gear given your modest basic knowledge. I don't want to offend you, but hey, that's 760 V and stock multimeter leads are not even rated for that voltage. Take care!!

If you rectify 273V_RMS (what you call AC), you get sqrt(2)=1.41 times that value in DC, so about 380VDC*. That is using only one blue lead and the center tap. You put a cap across both blues (760VDC!), so you just had luck your 500V cap didn't blow.

I don't see any way around rewinding the transformer.

You cannot reasonably drop the next larger voltage (2x 380VDC=760VDC) down to 480VDC.

Dissipated power is power=voltage x current, so the drop of 280VDC would cause (760-480)x 1 amp=280 watts power dissipation!

Please use these simple relations next time to know before trying that your load resistor will glow like the sun.

Have fun and take care! Hannes

* that is if your multimeter shows RMS-readings and not straight nonsense.

EDIT: oh and it's not a good idea to use a regulator without any cap on secondary as that simply won't work at all.

[tommyp]

Thanks for the reply. I've been asking others about rewinding vs PS filtering. The jist I've been getting, including your reply, is that rewinding the transformer is the way to go. I just wanted to be reaffirm things before I start the dissection.

[h_a]

No problem at all!

Enjoy!