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wicked1 18th October 2008 03:30 PM

TubelabSE power supply trouble
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I built the tubelab SE earlier this year. It's been working perfectly for the past 6 months.
Yesterday I turned it on and sparks flew around inside the rectifier. The 5ar4 rattles now, so I guess the filament is broken.

I had an old 5ar4 sitting around. I have no idea if it is good or not, but i plugged it in (after checking the circuit for obvious shorts or other problems) and powered it up.

This time r5 almost instantly turned bright red, and r6 shows signs of being hot.

So, I've checked again, and I do not see any visual signs of any other components having failed, and i don't see any shorts anywhere.

I'll attach the power supply schematic, and here's a link to the full schematic, if it helps anyone to help me. The components I've marked in red on the power supply schematic are the ones which have visibly failed.

Please let me know if you have any ideas as to what would cause these components to fail all of a sudden!
Thank you very much! I'm already in emotional pain from not having any music to listen to today!! :)

w5jag 18th October 2008 03:48 PM

I would look at having a shorted capacitor C6, this takes the full B+ surge at power up, and when it goes R5 is seconds behind.

I had this happen a couple of times, and changed C6 to a 450 volt part, no more problems.


Evenharmonics 18th October 2008 03:50 PM

Depending on the quality of 5AR4, it has been known to arc inside. I have a restored Dynaco ST-70 which had that happened twice and when it did, the fuse blew. Both were cheap 5AR4 (Ruby and Sovtek). Besides making sure your 5AR4 is in good condition or replacing it, you may also want to make sure the power caps are ok as well.

wicked1 18th October 2008 04:24 PM

Let me tell you a couple of other things I've tried, and then ask a stupid question...
1st, I tested the transformer, and it tested OK

Then, I replaced the resistors, re-connected the power transformer, and applied power w/ no tubes.

Now the transformer makes an audible hum! (Not when it's disconnected, but when it is in the circuit)

So, let me know if that gives you any new clues.
( have my suspicions about those freed diodes.. only because if the resistor directly in front of them had enough current to glow red, would all that power be going through those little silicon diodes, too? I don't know how much they can take... they don't smell like blue smoke, and they still look OK)

Then on to my stupid question.....
Is there a way to test a capacitor w/ a cheap multi-meter? (no cap. test setting)

I did check for an internal short, and the caps aren't. And, i did use higher voltage caps from the start... They didn't even fit on the PCB, so I had to use short jumper wires to connect them.

But, if there's another way to test a cap, please let me know!

This was a cheap 5ar4 that blew originally.. (sovtek) But.. now that the transformer hums w/ no tubes, either something else caused the problem, or the 5ar4 took out some other components when it died.

I wish I could do some more troubleshooting on my own, but unfortunately, I don't have any extra components. I guess I'll wait for a little more advice from you guys, then order some parts so I can start testing.
Thanks for getting back to me so soon!

wicked1 18th October 2008 04:52 PM

Yes, one of the FRED doiodes is shorted... 0 resistance either direction.

Do you think that would cause the transformer hum?

Urg, I should have learned the lesson years ago to buy 2x all the components I need, so I have them on hand when I need them for repairs.

w5jag 18th October 2008 05:06 PM

When R5 gets red hot, it is from excess current flowing through it. The only way I see this happening is a short somewhere down stream from it. The PT humming also points to a short somewhere.

The resistor will probably destroy itself before the diodes are damaged.

I have never tried to check a capacitor on a DVM. When I think one is suspect, I toss it in the trash, so I don't have any failed caps to try to measure. Sometimes they are fine at lower voltages, such as what a DVM might apply, but break down catastrophically at higher voltages.

There is almost always a lot of collateral damage around a failed electrolytic capacitor. When in doubt, I just replace them.


w5jag 18th October 2008 05:14 PM

You could get a couple of garden variety rectifier diodes from Radio Shack and sub them in for the FRED's to get you back up and running.


wicked1 18th October 2008 05:15 PM

I think I'll take the advice of your last post, and just wait and order some new caps, and get the diodes in that order.


Evenharmonics 18th October 2008 05:19 PM


Originally posted by wicked1
Yes, one of the FRED doiodes is shorted... 0 resistance either direction.

Do you think that would cause the transformer hum? .

I'm not sure about the cause of PT hum but FRED diode problems I've had recently. When mine went bad the fuse (250V 2A slo-blow) blew right away. I replaced the diodes with Fairchild "Stealth" mentioned in this thread. I'll see how it holds up.

As for 5AR4, I heard that once it arcs, likely it's done. You may want to get a new one just to make sure. I have JJ brand and they are not that expensive. Is it really good like RCA and Mullard? No, but I think they are better than Sovtek.

wicked1 22nd October 2008 10:07 PM

New diodes, new capacitors, new rectifier tube... Back in business.

I think the caps were ok, but better safe than sorry. My guess is that either the diode or the tube failed, and took the other one out w/ it.

I used the fairchild "stealth" diodes that were mentioned in the previous post.

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