How to tell when power tubes need to be replaced?

[esteban]

Lately, I have been noticing some strange behavior on my Prima Luna DiaLogue One integrated amp. Factory tubes were EL-34's, but I quickly replaced those with Gold Lion KT-88's. This was not even a year ago. In fact, only 10 months ago, based on my receipts. Last couple of nights, I noticed that I had to increase the volume level to a higher-than-normal setting to get music to "come to life", so to speak. Additionally, from time to time I have noticed a slight change in the overall tone of the music. Sometimes, this is jarringly noticeable (just now I barely recognized the first few notes in Coltrane's "Blue Train"), sometimes less so (hard rock sounds about the same, but bass seems muddier). Sometimes, everything sounds just great. But today I could not bring the system to "breathe" as usual throughout different attempts. Music was just not as involving. This is my first tube amp, so my lack of experience forces me to ask... do I need new power tubes? After not even a year? What signs should I look for to determine this? Thanks for the feedback.

[jdandy]

Esteban.......I am not familiar with the Prima Luna Dialogue One amplifier, so I am only offering general advice. It may well be one or more of your small tubes, particularly the driver tubes that provide the voltage gain that feeds your power tubes. Most power tubes will last between 3000 and 5000 hours before any sonic degradation begins to become apparent, and typically the small tubes are double those hours, but you could be experiencing an issue with the driver tubes. You should find a dealer or tech shop that has a calibrated tube tester, and have all of the tubes tested.

There is a possibility that the Prima Luna's Adaptive AutoBias circuit is misbehaving. This circuit uses a reference signal from the input voltage to adjust the bias voltage on the power tubes. There are no manual bias adjustments. If this circuit is not operating properly it will cause issues with the sound as well as with tube life.

You did not say whether you leave your amplifier on all the time or just when listening. It is a good idea to have an hour meter that comes on with your amp. Then you can accurately track the hours of use. This helps to know when you may begin to expect tube failure.

[esteban]

Thanks for the reply, Dan. I would say I use the amp an average of 3/4 hours a day. Sometimes no longer than 1 hour, sometimes as much as 8 hours in a row. But I never leave it on unless music is playing. Same principle applies to all my electronics, including computers. I am very power-conscious and try to keep the electric bill down.

The change started to occur during the last couple of days only. The amp itself is about a year old, so hopefully that rules out the possibility of the small tubes needing replacement. Prima Lunas are, supposedly, rock solid, and have a great reputation for being hassle and maintenance free. I hope my unit is not the exception.

You might be unto something with your comments regarding the auto-bias, since the problem I described in my first post SEEMS to be happening, for lack of a better description, "on the fly". In other words, during a song I THINK I can hear the tone of the music changing slightly, and sometimes I can even detect very small alterations in volume. Again, very subtle, very brief. And then, all of a sudden, the stars align again and all is well. It is very difficult to describe. I wonder if there is an issue somewhere else in the chain (higher than normal demands on the power grid of the condo I live in, etc.) It's been unusually humid and rainy here in South FL. Perhaps that has something to do with it?

[jdandy]

Esteban.......If the Prima Luna is still under warranty, I would let the dealer inspect it on the test bench, and test the tubes while he's at it.

From your comment about the voltage in your condo, there may be an issue with supply voltage fluctuation. It might be time to consider using an AC regenerator like the PS Audio Power Plant Premier or the PurePower 2000. Both of these units are able to maintain a steady 120 volt output even when the supply voltage sags.

[esteban]

Dan, I bought the unit directly from Kevin Deal (US distributor of Prima Luna), so I would have to ship it back to him. Not an easy task since the amp is about 75 pounds! I will get in touch with him on Monday to describe the problem and see what he has to say. This is when I wish I had audiophile friends who lived in my area!

In in any case, and to go back to my original question, what are the symptoms one usually experiences when power tubes need replacing? Even if my amp was simply having a bad day or two (it seems to be sounding fine as I type this), I know that eventually I will need to replace the tubes. What tell-tale signs should I look for?

[jdandy]

Esteban.......My experience with aging tubes is you will begin to notice an increase in background noise.

[ronenash]

Originally Posted by jdandy

Esteban.......I am not familiar with the Prima Luna Dialogue One amplifier, so I am only offering general advice. It may well be one or more of your small tubes, particularly the driver tubes that provide the voltage gain that feeds your power tubes. Most power tubes will last between 3000 and 5000 hours before any sonic degradation begins to become apparent, and typically the small tubes are double those hours, but you could be experiencing an issue with the driver tubes. You should find a dealer or tech shop that has a calibrated tube tester, and have all of the tubes tested.

There is a possibility that the Prima Luna's Adaptive AutoBias circuit is misbehaving. This circuit uses a reference signal from the input voltage to adjust the bias voltage on the power tubes. There are no manual bias adjustments. If this circuit is not operating properly it will cause issues with the sound as well as with tube life.

You did not say whether you leave your amplifier on all the time or just when listening. It is a good idea to have an hour meter that comes on with your amp. Then you can accurately track the hours of use. This helps to know when you may begin to expect tube failure.

I agree with Dan. It sound like your input gain tubes are losing gain. I dough its the power tubes.

[esteban]

Well, I've been playing lots of diverse music for the last 4 hours, and now the system sounds wonderful (knock on wood). I am at a loss as to why that was not the case the previous two days, unless there was indeed some sort of power-related problem (not unheard of in a condo building). Ironically enough, last night I noticed the problem got really worse around 2AM, when you'd think there's barely any heavy stress on the electric grid (or perhaps I was simply dead tired?) Now, Sunday at 3PM, the stars are aligned once again. Strange.

I will keep an eye on things and report back with any findings. I'd be curious to hear more members stories regarding how often you change your tubes, which ones you had best results with, etc. (even though I know this can be very system-dependent), though. Thanks for your feedback.

[Whart]

There may be a couple of other things you could do as a reality check and the expense is not a waste- have a set of back up tubes and a reasonably accurate voltmeter. Try replacing all the tubes and see if that makes a difference. Maybe there is one funky tube that is interfering, although it could be something, as Dan said, in the autobias circuitry or other parts of the amp having nothing to do with the tubes. Tubes should last way longer than the use you've described, although their performance degrades over time, it is in my experience not usually a dramatic degradation (until you put new tubes in and realize, "OMG, i should have retubed a while ago.") Tubes do fail occasionally, though. (Not sure if they should fail quickly, once installed, or that could happen over time too).
the voltmeter- well, it doesn't sound like you can do anything to check the bias settings yourself, I don't know the amp either, but you could- without lighting yourself up- periodically check the voltage at your wall sockets.
And while i normally don't like AC conditioning, if your building does have electrical issues, maybe one of those regenerators is the way to go.
And yes, see what Kevin says- short of shipping the amp to him, he could probably supply you with a new tube complement without breaking the bank... (assuming tube warranty is probably very limited duration).
Good luck and let us know what happens.

PS, one other thought, you are right, you'd think that the optimal time would be in the middle of the night, not in the middle of the day, i guess it's hard to determine what else is on that line. Even though I have dedicated lines for the audio, i can still 'hear' stuff from the electrical system elsewhere in the house- the convection oven, and a set of low voltage lamps in the kitchen, as well as a room humidifier plugged into a bedroom socket. Sorting out these gremlins- other things on the electrical system that can interfere with the music system, can be a little maddening.
Is there anything in your unit that is 'on' at 2am that is not at 3pm?

[esteban]

Originally Posted by Whart

There may be a couple of other things you could do as a reality check and the expense is not a waste- have a set of back up tubes and a reasonably accurate voltmeter. Try replacing all the tubes and see if that makes a difference. Maybe there is one funky tube that is interfering, although it could be something, as Dan said, in the autobias circuitry or other parts of the amp having nothing to do with the tubes. Tubes should last way longer than the use you've described, although their performance degrades over time, it is in my experience not usually a dramatic degradation (until you put new tubes in and realize, "OMG, i should have retubed a while ago.") Tubes do fail occasionally, though. (Not sure if they should fail quickly, once installed, or that could happen over time too).
the voltmeter- well, it doesn't sound like you can do anything to check the bias settings yourself, I don't know the amp either, but you could- without lighting yourself up- periodically check the voltage at your wall sockets.
And while i normally don't like AC conditioning, if your building does have electrical issues, maybe one of those regenerators is the way to go.
And yes, see what Kevin says- short of shipping the amp to him, he could probably supply you with a new tube complement without breaking the bank... (assuming tube warranty is probably very limited duration).
Good luck and let us know what happens.

PS, one other thought, you are right, you'd think that the optimal time would be in the middle of the night, not in the middle of the day, i guess it's hard to determine what else is on that line. Even though I have dedicated lines for the audio, i can still 'hear' stuff from the electrical system elsewhere in the house- the convection oven, and a set of low voltage lamps in the kitchen, as well as a room humidifier plugged into a bedroom socket. Sorting out these gremlins- other things on the electrical system that can interfere with the music system, can be a little maddening.
Is there anything in your unit that is 'on' at 2am that is not at 3pm?

Unbelievable! I think you are on to something. I mentioned previously that the amplifier's odd behavior (an overall lack of "life" and energy to the music, plus a loss of detail and transparency) started a couple of nights ago. After reading your post, I just realized that these symptoms coincided exactly with the time when my wife installed two separate new floor-lamps in the living room, and both of them actually run on a dimmer (a well-known contaminant of the electrical line). Last night, while I was in my room listening to some classical SACD's around 2AM or so, she was reading a book in the living room, and in fact both lamps, plus a third lamp (this one has been there for a few years, though) were on, casting a lovely orange glow all over the room, "for ambiance", as my wife said. Funnily enough, here I was thinking that the system had been sounding a lot better during the day, and I could not figure out why!

I do have a PS Audio Duet conditioner, but the amp is connected straight to the wall. Thanks for your input, Whart. It is truly incredible how these every-day occurrences that we usually overlook can affect our equipment's performance and thus our enjoyment of music.