Blown Fuses

I have recently built a st 70 kit from Triode USA and am having a problem getting it to work. After I built it, I went to test it and ran into some difficulties. I turned it on and I heard a loud hum and then the rectifier tube went bright and I blew a fuse and some smoke came up from under the rectifier tube. I got a new rectifier replaced the fuse. no humming but after about 15 seconds the rectifier got bright and the fuse blew...no smoke or noise. Tried again and another fuse blown. Talking with triodes tech I discovered I installed the 1000v diode backwards. Chalk it up to me being a newbie to kit building. I reversed the diode and still blown fuses. I dont know what to do...did I destroy the capacitors? One of the two 100uF 100v caps is bulged. is this my problem? Did I break something? Please any help would be greatly appreciated.
Thanks,
The new guy to the dynaco world

It's the little things that'll getcha ...

Would it be safe to assume the other 100uf cap is also bad? Or at least stressed and suspect?

Wow! I feel for you as you experienced a kit builder's worst nightmare. With the protection diodes installed backward essentially created a large negative voltage at the rectifier tube input. It seems this negative voltage caused a cascading breakdown of the rectifier tube and severely reverse biased the electrolytic capacitors. The bulging of the capacitor is caused by steam from the electrolyte boiling. If you had tubes (other than the rectifier) installed at the time the problems could effect them as well but generally are more rugged that the caps.

I think I would consider replacing all the electrolytic caps. They do not like negative voltages and could be a time bomb waiting to blow down the road if reused. And definitely do not use that rectifier tube. First, get the high voltage rail squared away without any preamp or power tubes installed. Be mindful though that without tubes the high voltage supply can drift upward and exceed the electrolytic caps maximum voltage ratings. Perhaps someone with ST-70 experience can advise on that.

Good luck to you and we are here if you need any assistance. But is definitely fixable.

Thank you all so much for the advice. I am going to replace my caps and rectifier tube...hopefully thats the problem and nothing else was damaged. I guess its part of the learning process that I make a small, silly mistake like that. You live and learn. Thanks for the help, I will keep you posted on how everything turns out once I get my new caps and tube installed

I had a customer do this recently with a tube preamp build.
It's unfortunate that "newbies" don't seem to have available the general electronics and kit building background that used to be so common.
For example, most know that some components like capacitors and diodes are "polarized" meaning they have a positive and negative lead.
But on a circuit board, the common standard practice is to notate these polarities with a square hole for positive and round hole for negative,
but on a circuit board, for a diode, it's the opposite!! The cathode end of the diode with the stripe on it goes to the square hole.
This may be what happened in your case if you were putting diodes onto a circuit board.
I wish someone had the time to either find or write a new general electronics guide for newbies that had all these things written down and illustrated like Heathkit and Dynaco did 60 years ago! Somebody find and post some illustrations from one of these old manuals!
Need a new topic - "basic electronics and kit building"
Currently AudioNote has the best kit building manuals I've ever seen. Part of why you pay $2800 for a kit that is the equivilent of
our VTA70 amp that sells for $800, so your'e paying an extra $1000 for the name and an extra $1000 for the manual.
If I could sell hundreds of manuals for $1000 each, I'd do it!!!
Here are two examples . . . http://audionotekits.com/PDF/Manuals/L4_EL34_35_PP_LowRes.pdf
http://audionotekits.com/PDF/Manuals/M2Preamplifier_LowRes.pdf

I find Bob's kit manual equally good. The instructions are detailed and fool proof (speaking for myself ). A few additional photos are all that one would need to add to jazz it up a bit.

Michael

The Audio Note kit manuals are very professional looking. There is one thing I would do differently: I would have used a woman's hand instead of a man's to help better illustrate the steps in the construction process. And some sexy orange nail polish!

I peaked at the Triode assembly manual... Is the reversed diode D1? If so then it is far downstream of where I was thinking (modern two diode mod on the rectifier tube input). I didn't see a schematic but D1 appears the be the negative bias voltage rectifier. So should be no sweat to fix this up in no time. Couple of caps and reversing the diode should make it right. My first rectifier blew in the ST-120 on first try and I had everything wired correctly!

Yes it was the D1 diode. What a silly mistake...I think I was just so excited to build the amp that I missed that. Do you think I might have damaged the amp other than the fuse, rectifier tube, and capacitors on the power board? I ordered replacements and am looking forward to getting the problem fixed. I am just a little nervous about something else being messed up as well.

Yes, these are hard but valuable lessons and we all have similar stories that we would like to forget. I have soiled myself on few first time power ups! Get a good night's sleep and recheck the wiring connections when fresh. Your rectifier may have blown from the classic AC power interruption issue. Fuse blows (or power failure), rectifier cathode gets cold but the high voltage caps are charged, and you get rectifier arc over back through the transformer windings. There's a two diode mod, that I thought originally you had and connected backward, that will prevent this by blocking the reverse current. Or the VTA relay board is another option that opens the circuit when power is removed. Or use a solid state rectifier sub like the Weber.

arledgsc wrote:Your rectifier may have blown from the classic AC power interruption issue. Fuse blows (or power failure), rectifier cathode gets cold but the high voltage caps are charged, and you get rectifier arc over back through the transformer windings.

Probably a bit off topic, but it's all about protection, which is why I invested in one of these ...



TRC 26020L-4-001 Shockshield Inline GFCI with 14-Inch Cord

It's an inline GFCI adapter geared more towards the construction trade. Unlike most, it's NON-LATCHING, which means that if there is a power interruption or fault, you need to manually reset it, after of course waiting the obligatory minute or two for the caps to discharge to safe levels. Fairly sensitive, so even a short blip will trigger it and kill the connection. Seems like a reasonable investment at $35 ...

We now return control of your television set ...

Its alive!!!! Shes up and running. I replaced all the power capacitors, well basically rebuilt my supply board and got a new rectifier and she works. Thank you all for the help and advice you have given me. All I can say is thank you a million times over!!!!

Good to hear!

Note to self ... learn from other people's misteaks. I should be getting into mine shortly, so I made an extra couple notes on that page of the assembly instructions. With arrows. In red. Circled even.