Just bought an amp and I tested all the tubes discarded the bad ones and now have a full complement of tubes that test good. I know from the previous owner that they have a good number of hours on them but dont know exactly. They are quiet no noise. How do I know if sonically they are tired? The 6550 should be replaced after 3000 hrs and the tubes date codes are 2001. Not sure I want to drop $500 only to find out I dont need them.
What was your method of testing?
3000 hours is a minimum service life. If they work in the circuit, they work.
When they are wearing out, they will usually be current limited - your maximum clean volume will be lower.
Remember KT88 will also plug in and work in place of 6550 - this give you more choice when you need new ones.
3000 hours is a minimum service life. If they work in the circuit, they work.
When they are wearing out, they will usually be current limited - your maximum clean volume will be lower.
Remember KT88 will also plug in and work in place of 6550 - this give you more choice when you need new ones.
They have a graininess to them. Also on loud passages they sound congested and seem overwhelmed.
I dont know how many hours are on them but these are the tubes the previous owner got with the unit so possible I am the 3rd owner of the tubes. I would assume they easily have 3000 hours. I also assume they lose clarity with age. I dont have a basis for comparison since this is my first tube power amp.
I dont know how many hours are on them but these are the tubes the previous owner got with the unit so possible I am the 3rd owner of the tubes. I would assume they easily have 3000 hours. I also assume they lose clarity with age. I dont have a basis for comparison since this is my first tube power amp.
None of those descriptive words mean anything in any technical sense, but they do describe the behaviour of a five year old in a church choir 
But if I had to guess, they are running out of current to give. If you replace them and the music can play louder while sounding clean and clear, the old tubes are weak.
In my experience with tubes, I find the average set of power tubes will work for a couple of years 24/7 (17500+ hours) before they are noticeably weak. This is while running at 100 - 150% of design centre.
The easiest way to test if the tubes are making clean power is with a DDS and a scope. Sometimes it's easier to see the beginning of clipping than hear it.
I have signal tubes with decades of use that still work fine though.
But if I had to guess, they are running out of current to give. If you replace them and the music can play louder while sounding clean and clear, the old tubes are weak.
In my experience with tubes, I find the average set of power tubes will work for a couple of years 24/7 (17500+ hours) before they are noticeably weak. This is while running at 100 - 150% of design centre.
The easiest way to test if the tubes are making clean power is with a DDS and a scope. Sometimes it's easier to see the beginning of clipping than hear it.
I have signal tubes with decades of use that still work fine though.
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He likes to listen groupings of sound frequencies tinged on electro-acoustics instruments formerly in the past millennium called song or music now and then when hard work and family allows.
Its called musical perception with the music and equipment he knows so well.
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This is a good advice.
In my experience KT88/6550 will show a soft bass when senile,
but how much time life yet remains is a more useful info.
It's simple, after 20 years of use just replace them, they do wear out. I have no idea what amplifiers these may be since you didn't say, but I serviced a limited range of ARC amps in the 1990s and invariably they needed new output tubes.
I assume that since you are asking here you do not have the means to measure amplifier performance without a substantial investment in new (to you) test gear, likely to cost more than a set of new output tubes.
The most reliable 6550/KT88 family I have found in recent years are the Genalex Gold Lion KT88 reissue, and I recommend Jim McShane as a source as they are tested and matched under real life operating conditions.
I assume that since you are asking here you do not have the means to measure amplifier performance without a substantial investment in new (to you) test gear, likely to cost more than a set of new output tubes.
The most reliable 6550/KT88 family I have found in recent years are the Genalex Gold Lion KT88 reissue, and I recommend Jim McShane as a source as they are tested and matched under real life operating conditions.
Just buy a replacement tube set and try it. It you hear an improvement, keep the new tubes in the amplifier; otherwise put the original tubes back in the amplifier and keep the new ones as spare. Check again every few months. This way, you will also have a replacement tube ready if one of the tubes suddently fails (due to wear on the filament, etc).