vox ac30 cc2 blowing fuses and rectifiers sparking????

[sdre]

sovtek

[trevorus]

sovtek

That's the one that failed on me. I'd go with something different. Maybe this amp and that tube just don't get along.

[Robert M. Martinelli]

sdre,
"+B" ( or + HT ) referred to tube ( or valve ) amps refers to the high DC voltage rail supplying the tubes' ( or valves ) plates ( or anodes ). In your amp the +B is the positive terminal of C4-C5.

In Push Pull amps it goes from the filter caps to the OT primary winding's center tap, with the two ends of the primary winding "feeding" the power tubes' plates.

Push Pull designs need little filtering because whatever hum/ripple tends to be canceled by phase opposition on the OT; the preamp stages need better/additional filtering to keep the hum/ripple from being amplified together with the "useful" signal.

As to measuring caps with a DMM, you could try with the DMM set to measure high resistance. Discharge the caps first, and MAKE YOURSELF SURE they' re discharged by measuring the DC voltage across them.

Disconnect one terminal from the cap under test to keep the measurement from being spoiled by other components.

Set the DMM to measure high resistances ( MOhms ) and put the leads on the cap you're measuring.
You should see the resistance start from zero and then slowly rising to infinite, if this does not happen and the reading stops showing a certain resistance, then the cap is leaky.

Obviously, the best thing to do would be to get a capacitance meter, because the DMM method tells you nothing about the caps' real capacitance and ESR, but in this case, since we're looking for a leaky cap, that's not of our concern so the DMM method should probably suffice.

Though unlikely, it's always possible you got yourself two bad rectifier tubes, as unfortunately tubes' quality is not as good as it used to be in the "old days". If all the filter caps test OK you either have a bad rectifier or a short to GND somewhere else. As already said, check for shorts, bent lugs, solder drops etc.

Hope this helps

Best regards

Bob

[sdre]

You guys thank you so much for your time! I went ahead an tested the caps..... they where fine. so i bit my tongue and bought a new tube of a different brand. Works perfect! Now for future reference are their any accurate tube testers somewhat affordable so I don't run into this again???

I appreciate your time and patience.

[sdre]

One more thing. For testing caps in tube amplifiers, are there any recommendations for a capacitance meter?

[Robert M. Martinelli]

Hi sdre,
glad you fixed it, glad we've been able to help and glad you've probably learned something new, which is always a good thing....

As to choosing a good/reliable tube tester I can't help you, as I use a self-built one, but I'm sure you'll get a lot of good suggestions from our fellows solder fume breathers ( thanks Enzo! )

As to the capacitance meter, i use a Lutron 9073 ( LCR Meter ) and it's good value for the money....I use it also to measure pickups with pretty good results; anyway, if you only need to state if a cap is leaky or not the DMM method I have explained you in one of my previous posts will suffice.
Best regards
Bob

[Enzo]

I don't use a tube tester. One can tell me a tube is definitely bad, but it cannot tell me a tube is definitely good, and that is what I need to know. If I want to know if a 12AX7 is bad, then I stick a new one in the socket and find out. That is a ton faster than testing the old tube. If the new tube makes the difference, then thre you are. if the new tube makes no difference, then stick the old one back in.

And not so be a stick in the mud, but I don;t use a cap tester either. In my repair experience, I don't find caps off value much. If they test off value, it is most likely because they are real leaky or dried out. ANd leaks often don't show up at hand held tester voltages. A cap might be fine at 2v, or 10v, or even 100v. But leak like a sieve at 400v. I gotta know what it will do under the conditions of a real tube amp circuit. So unless the tester puts 400v across the cap to do a leakage test, it doesn;t help me.

I find it simple enough to find a cap is leaky by using my voltmeter in circuit. And just like the tube tester, swapping parts is a lot faster. If I think a cap is bad, I replace it. At a dollar a minute labor, how long should I spend removing a cap from a circuit and setting up the test gear all to see if a dollar part is bad, then returning it?

[Truter]

Similar problem

I've got a similar problem wth my AC30-CC. No lights, no power, no nothing. I've replaced the rectifer tube(it had a sovtek, so I've replaced it with the same) thinking that might be problem. But it has done nothing, the fuses seem ok, and all connections seen fine too. the only thing I've noticed is that one of the caps(22uF on the board the retifier tube is connected to) has a darker connection than the others. could it be a falty new valve or more likely the cap, or perhps somthing else.

Thanks alot in advance

[Truter]

Fixed

I fixed the problem, I mistook the HT fuse for the main fuse which was hiding in the input connector, wierd place to put it I thought, but once I changed that, she was all good.

So in the end, just the rectifier tube and main fuse, so stoked...

Ps. I know this thread is from ages ago, but it may help someone else.

[Alex R]

Yep that little black drawer under the mains input socket is only easy to spot after you've opened for the first time and found the fuse in there. Generally they have room for a spare too, which is handy, though I know from experience that it is possible to fit a fuse in the space for the spare and find the amp still don't work, and then take it to a repairer - and then to be lucky enough to find that the repairer (me) was having a good month and didn't charge the bench fee for moving the fuse.

Rectifier tubes do fail all on their own often enough, and you can see sparks in there sometimes.