Power pentodes just don't seem to last as long as triodes, in a nutshell. |
@Ralph -- any idea why ARC recommends that its power tubes be replaced every 2000 hours (6550s and KT-120s); 3000+ hours in case of KT-150s. I'm sure you know that KT-150s ain't cheap. My amp uses 8 KT-150s. Yikes!!
Not sure this is relevant, or that I am framing the point properly, but KT-150s are rated to have 70 watts of plate dissipation capability; ARC drives them to about 27 watts.
Does that permit the inference that ARC is not driving their power tubes all that hard and that tube life expectancy should be longer?
Sorry for not framing my question in a technically correct way. But I assume you know what I mean.
Could you put a little more gloss on why your power amp tubes last so long??
BIF |
Solid states usually don't have tube problems. The blown transistors happen far more rare than tubes and not too difficult to replace. |
@Bifwynn, we don't have a recommended set of hours on the tubes we use. FWIW power triodes are often good for about 10,000 hours owing to their simplicity. Octal base signal triodes can often do that as well.
We've been warranting our tubes for a year for some 3 decades now and getting away with it, so they must last a good long while. |
+1 Mapman.
Tubes are great when they work well, and a real PITA when they don't.
Luckily, they seem to work well most of the time. |
We all have problems. Tubes have their fair share. They must be good though or else nobody in their right mind would tolerate them. Same goes for records.
One of my problems is I wish I had fewer tubes and records. But I want things to sound really good and never know when I might want to play a record. That's another problem I have.
Problems, problems....
With my tubes I find they tend to get noisy over time. I have never had one just suddenly go up. Currently I have them in my ARC pre-amp only. No tube power amps. That's really asking for trouble. :^)
On the bright side (no pun) tubes tend to glow and look really cool, especially in the dark. Transistors can't do that. If they do you have real problems. Plus some vendors have even found ways to make the tubes look like they glow even more than they do. Now that's good marketing! |
Rodman99999, that's a hoot. LOL. |
If your system is bringing in al-jazeera: look for tube with a beard and towel-wrapped head. Eliminate it, with extreme prejudice, before it blows up your house! |
PHd, I actually was referring to Al jazeera...lol..although that might not be the station, it could have been anything for all I know!
Anyhow, I am going to get a handle on which is the offending tube and replace it. |
Daveyf, lets hope the bad tubes if any are in the preamp. I think you have some concerns but more importantly you are picking up a station I've never heard of. |
Actually the tubes that are making the noise are ( I think ) in my preamp. One of them lets me listen to the Al-Jareera radio station when the gain is turned up! Interestingly, they are not that old, probably have only a few hundred hours on them, if that. Nonetheless, I am happy that they are in the preamp vs. the ARC amp. My ARC D70mk2 amp is a bear to re-bias if it needs tube replacement. |
Bifwynne, of course, let us pray! |
Daveyf,
I wouldn't presume to offer a tech answer when the likes of Ralph (Atmasphere) provides such comprehensive advice. That said, being a fellow "tube-o-phile," I would only mention that many manufacturers suggest that tubes be replaced within a certain life span. I surmise that tubes are more susceptible to failure or performance degradation if use exceeds recommended replacement points (as measured in hours in service).
In my case, ARC recommends that 6H30 signal tubes be replaced at the 4000 hour point. I understand that the new KT-150 power tube has a 3000+ hours life span.
I'm sure every manufacturer has its own recommended re-tube specs. Ralph, what do you recommend for your amps?? |
Atmasphere, thank you for your in depth answer. I was hoping that you could explain arc-over and its cause. |
These days arc-over is a frequent symptom of power tube failure. They can also fade away. If you see the silver that is on the inside of the glass turn brown or transparent, it means that the tube is gassy and will run hotter and with less output. If the silver area has turned white it means the tube has lost its vacuum.
In signal tubes they can develop intermittents; crackling, noise, etc. Sometimes they respond well by thumping the tube hard but sooner or later the noise is back if this is a problem. Microphonics is not a source of failure but it is annoying and hand-picking is really the only way to get rid of it- dampers help but only slightly. Finally, signal tubes can just fade away, often replacing output level with some noise (hiss).
Cryo-ing a tube will not improve its performance, FWIW. |
I agree that microphonic tubes do not indicate failure but in fact I have used these tubes with great success, some of which sound pretty darn good. If they are too microphonic you can tame them a bit with tube dampers.
Does anyone know for sure what a tube exhibits before it dies. Maybe that can vary from one piece of gear to another. In my case the tubes in a preamp slowly lost gain and the gain would suddenly drop. I have heard that tubes in power amps can go out with a big bang and would require more frequent testing to ensure safety. |
Every time when I replaced microphonic tube by new one the microphony of amp dissapeared. |
Before I take out the many screws to get at my tubes I decided to buy a new rectifier before diving in. I'm hearing a very low thump, or thud, intermittently, and irregular when it shows. The two 6sn7 driver tubes are Shuguang Black Treasure which are only a few months old. Not sure which is the problem, but will switch out the rectifier first since I'm unsure of its age. (Mullard) |
Atmasphere, you're right, but I just went ahead and replaced all 6 tubes (which were Electro Harmonix and Tung Sol) with matched and cryo'd JJ's. |
The idea that a microphonic tube is one that is going to blow is incorrect. Microphonics is a construction issue but not an indication of imminent failure.
@ Joeylawn, its likely that only one of the tubes you replaced was actually the one giving you troubles. To find it, swap the tubes left for right one at a time rather than the whole bunch. |
I heard popping from my Jolida FX-10's tubes on my right speaker. To see if it was the tubes, I swapped all the tubes from left and right channel and visa versa. The popping then happened on the Left side.
This proved it was the tubes - I went ahead and replaced all of them, fixing the problem.
If you do the swapping as mentioned above, and the popping continues on the same channel, then it's Not the Tubes. |
Clean the top surface of all tube sockets just in case. It is often arcing from one pin to another via dirty sockets that produces the pop or crackle. |
Microphonic tubes are usually ones that soon will blow.
You can tap amp chassis and if you hear tapping via speakers, your tube(s) are about to blow and you'll need to determine which ones by swapping between channels.
I would use sewing needle or similar that is able to go outside of contacting flaps and inside the socket and work on each one. Slight bending of tube terminals may also increase contact strength.
Tube that can't reach desirable bias may also be bad as well. |
Thanks, Newbee. What is the procedure for retensioning the tube sockets? |
Often popping sounds are just the result of dirty pins or poor contact of the pins with the tube sockets. When this occurs before you fret too much just try resetting the tubes in place or by switching channels. If that doesn't work you can try to retension the tube sockets. |
