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JoeM
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a power surge?

Greetings. I am relatively new to the tube amp world, and I have a vexing problem. My brother gave me his ten year old Jolida 302A (he claims there was only five years of wear on it). I had it for a year and it played superbly.

Last Thanksgiving it blew a power tube, which I replaced with a new EL34. It played wonderfully until News Year Eve when it blew the new tube in the same socket. I took it in to the shop, and the repairman said he replaced a bias resistors and capacitors.

The machine played superbly until two nights ago when it blew another tube, right next to the socket that blew before. The repairman seems uncertain what could be the problem, other than a transformer which he has tested, since he said he has replaced everything that could go wrong.

I am at my wits end: three blown tubes in four months. Is it possible that I have some weird power surge in my house that is blowing them out? I use a surge protector. There are other machines (cd player and turntable) connected to the same socket but these are fine. In fact, I have a tube headphone amp on the same line and that is fine as well.

I would appreciate any advice on this. Thanks!

Joe

KBK
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Re: a power surge?

tubes should be matched, and from the same manufacturing lot. You need to be replacing all power tubes in the given channel at the same time, with a matched set of tubes. Otherwise, you get small to large levels of unequal power sharing (under load), which eventually leads to a tube failure. For example, if a tube channel utilized 4 tubes, and one blew, and it was found to be the single tube alone, then replace all four tubes, and keep the old three remaining working ones...as a spare emergency 'short term, low level use' set.

This is expressly done with SS amps on the output stage, as any other method does eventually result in failure of the SS amp's output section transistors. Besides sounding potentially 'bad'-the whole time it is working. For my power amp, for example, I keep an entire set of new transistors from the same lot, and matched, for the output section-for both channels. In case of a failure. Granted, I'm a tech kinda guy, but my output transistors are not available anymore, so it is wise of me to have done so.

A good analogy is imagine a rowing race, where the boat has four rowers. One has his heart burst, so you throw 'any old guy' in there to replace the one who's gone. Now, what's going to happen to the 'whole, overall' rowing effort, and in what ways?

This may not end up being your exact problem, but the indication is that it is most definitely a solid possibility.

JoeM
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Re: a power surge?

Thanks KBK! I appreciate it. I would mention that every time the tube did blow it was a matched EL 34 from Electro Harmonix (sp?), which as I understand it, is a pretty reliable tube. The repair guy said something about a transformer sending out a bad surge every so often, but he also said the transformer tested fine. He is looking at it again.

I really don't know what to do. I am tempted to say that this is just an old amp at the end of its day, and that I need to buy a new one. My worry is that if there is something wrong with the power in my house I could just repeat the same problem.

Thanks again for your advice!

Joe

Welshsox
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Re: a power surge?

Hi

Im not an expert on valve amps but im reasonably upto speed on power.

The surge protector you are using is pretty much useless to protect the equipment form subtle voltage issues, surge protectors are designed to trip at very high transient spike voltages. Its a personnal issue of mine the way that the audio industry treats power, the monster type surge protectors do absolutely nothing for you in terms of power quality, they just protect in the case of lightning level surges. If you truly want to improve your power you need what they call a power conditioner, this is a big heavy duty transformer based peice of equipment that will remove all forms of subtle power issues.

Can i ask a question about where you live ? are you in a normal surburban street with lots of houses or remote on a farm or similar ? if its the former its very unlikeley that you have power surge issues, the latter than you might have to think about things a bit more,

Alan

JoeM
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Re: a power surge?

Alan -

Thanks for responding! Yes, I live on a normal suburban street with many other houses around. Your comment on that a power surge here is probably not the cause makes me feel better.

On one hand, I think it cannot be a power surge. If it were a power surge, why would it only blow one tube instead of all four? And why would my tube headphone amp, as well as the cd player and turntable, not be affected?

On the other hand, if the questionable components have been replaced by new ones (capacitor, etc.), I cannot figure out what could be blowing the brand new tubes.

Just out of curiosity, how much would a power conditioner cost?

Thanks for your help.

Joe

Welshsox
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Re: a power surge?

Joe

A good power conditioner is an expensive item, at least $1500 for something fit for purpose.

Id ask your reapir shop to run an insulation test on the transformers. If they have already swaped out the capacitors then it might be that the insulation between the coil windings is breaking down under full load.

I dont think you need a power conditioner

Alan

JoeM
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Re: a power surge?

Alan -

Thanks. Will suggest that to the repair guy next week.

Joe

KBK
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Re: a power surge?

Voltages are rather high in tube amps, and this can sometimes stress even resistors to near breakdown levels in the best cases. I think the transformer winding test might be a good idea, or a really good look at the solder junctions on any power resistors. Also, re-stressing of all pin contacts on any tube sockets is a must. Loose tube socket = tube/circuit failure.

Regardless, this is all 'poo talk' as we are not really the technician 'on site' nor do we know the specific amp in question. Tube amp and tube gear problems can be difficult, as they can be like 'ghosts in the machine'. They are my least favorite device to attempt to find a hidden or difficult problem in..due to these high voltages requiring that testing, many times, take place with live devices,which means messing around with your hands inside of areas chock full of 350VDC to 500VDC.

As I get older, I begin to understand that shocking my heart and body..is something I should have aversion to. I do have aversion, yes, but I have been electrocuted 100's of times. The last time it happened, it was over 275VDC, and someone was looking right at me, and I was looking at them, and talking. I did not change the look on my face, or even stop talking. But I digress!

JoeM
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Re: a power surge?

First, I would like to thank those who gave me advice. I really appreciate it.

I got the Jolida back from the shop. The repairman replaced another resistor and said everything was fine. I asked him if he did the winding test suggested above, as well as check out the sockets as also suggested above. He said he did both.

The amp plays fine. I keep a journal, and I noted that the tubes blew almost exactly every six weeks. Given that, I figure I might be writing you fine fellows mid-May.

Thanks again!

Joe

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