ST-70 Restoration first Power-Up: Blowing fuses........any advice please??

Hi Guys -

Well I finished my St-70 (VTA PCB) project.....at last.

I went to power it up tonight and I have a problem. Can anyone advise, please?

I put a 1amp fast-blow fuse in, and have been following the instructions that came with my VTA Board. I put in JUST the driver tubes first, and they glowed. I powered off, removed the driver tubes and put in the rectifier and Right hand power tubes. The fuse blew. I put in another 1amp and that blew too. I changed the power tubes, tried once more. Blew.

The rectifier is a NOS and tested good on my borrowed tube tester.

Can anyone help?? I went over and over the wiring before powering up, and I don't think I've made a mistake. EVERYTHING except output trannys is brand new. THANKS!!

The ST70 should have a 3 amp slow blow fuse. 1 amp is too small.

Tim L.

Thanks Tim!

I replaced the 1a with a 3a slow-blow and I just successfully set the LH bias. Gotta go out now but with set RH later. Super-excited!!

I read somewhere to do first power up with a 1a: I didn't know when to replace that with a 3a.

Doctor Hugocat wrote:Thanks Tim!  

I replaced the 1a with a 3a slow-blow and I just successfully set the LH bias. Gotta go out now but with set RH later. Super-excited!!

I read somewhere to do first power up with a 1a: I didn't know when to replace that with a 3a.

If one uses a metered variac and has a light touch, a 1A fuse is *just* feasible to start a 70. But, not for actually running it. And I use a very light fuse to test radios after restoration and/or repair - but without the tubes in it. This tests the most basic wiring, especially the transformer. But once the tubes are in, or the rectifier starts to pass B+, that is when the real load begins.