I bought 6L6G Tung-Sol from New Sensor to use in an amp designed for 6L6 (in otherwords they we're running far below the limits) and they lasted 3 months in an amp that was on 24/7.
I now use Soviet 6P3S (not -EV) because I figured at the time they we're $4CAD. I thought "why not try them instead of the $50CAD 6L6G?" and so far they've plugged away for over a year running at 23W Pd.
Since KT66 and 6L6 are usually interchangable you might look into whether or not the 6P3S is suitable. They are far less money, too.
I now use Soviet 6P3S (not -EV) because I figured at the time they we're $4CAD. I thought "why not try them instead of the $50CAD 6L6G?" and so far they've plugged away for over a year running at 23W Pd.
Since KT66 and 6L6 are usually interchangable you might look into whether or not the 6P3S is suitable. They are far less money, too.
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I had a similar experience with the same tube. I bought a pair, and one went gassy in about 3 months, and the other followed pretty quickly. They were running in a single ended amp and dissipating about 20W at idle. Sovtek EL34s lasted for a couple of years in the same amp.
Some of these issues may have been resolved by now, this was about 8 years ago.
The KT66 and 6L6 are not interchangeable.
Not even all 6L6 types are interchangeable.
Yes, they have the same pinout.
Yes, you can re-bias the tubes.
That is where it ends.
KT66
1.3A filament
Plate Watts 25 design
Plate Watts 30 absolute max; 32 triode mode absolute max
Plate Volts 500 design
Plate Volts 550 absolute max
Screen Volts 500; 3.5 Watts design
Screen Volts 550; 4.5 Watts absolute max
Transconductance 7000 uMhos
6L6/6L6-G/6L6-GA/6L6-GB;
0.9A filament
Plate Watts 19; 19 triode mode
Plate Volts 360; 275 triode mode
Screen Volts 270; 2.5 Watts
Transconductance 6000 uMhos
6L6-GC
0.9A filament
Plate Watts 30; 30 Watts triode mode
Plate Volts 500; 450 triode mode
Screen Volts 450; 5 Watts
Transconductance 6000 uMhos
And have pity on the poor power transformer:
3.6 A filament versus 5.2 A filament for 4 output tubes.
Sometimes an amplifier is modified for more B+ filtering:
Double the B+ input capacitor and the transient cap charging current will double.
You will get 4 times hotter B+ secondary winding, P = (I)Squared * R. (2 Squared = 4)
Not even all 6L6 types are interchangeable.
Yes, they have the same pinout.
Yes, you can re-bias the tubes.
That is where it ends.
KT66
1.3A filament
Plate Watts 25 design
Plate Watts 30 absolute max; 32 triode mode absolute max
Plate Volts 500 design
Plate Volts 550 absolute max
Screen Volts 500; 3.5 Watts design
Screen Volts 550; 4.5 Watts absolute max
Transconductance 7000 uMhos
6L6/6L6-G/6L6-GA/6L6-GB;
0.9A filament
Plate Watts 19; 19 triode mode
Plate Volts 360; 275 triode mode
Screen Volts 270; 2.5 Watts
Transconductance 6000 uMhos
6L6-GC
0.9A filament
Plate Watts 30; 30 Watts triode mode
Plate Volts 500; 450 triode mode
Screen Volts 450; 5 Watts
Transconductance 6000 uMhos
And have pity on the poor power transformer:
3.6 A filament versus 5.2 A filament for 4 output tubes.
Sometimes an amplifier is modified for more B+ filtering:
Double the B+ input capacitor and the transient cap charging current will double.
You will get 4 times hotter B+ secondary winding, P = (I)Squared * R. (2 Squared = 4)
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My JJ KT66 tubes ran very well for many many hours.
I got my JJ tubes at EuroTubes, they do a good job of retesting and matching.
I always run my tubes lower than the maximum ratings.
A good rule is that if you want to run the tube at a maximum rating, do so for only one of the maximum ratings.
i.e. don't run at the max voltage and the maximum power dissipation at the same time.
Or don't run at the max current and the maximum power dissipation at the same time.
Also, watch that your filament voltage is near the center rating. Don't run it low, and don't run it high.
A moderately priced tube that lasts a long time, is a far better value than an inexpensive tube that has a short life.
I got my JJ tubes at EuroTubes, they do a good job of retesting and matching.
I always run my tubes lower than the maximum ratings.
A good rule is that if you want to run the tube at a maximum rating, do so for only one of the maximum ratings.
i.e. don't run at the max voltage and the maximum power dissipation at the same time.
Or don't run at the max current and the maximum power dissipation at the same time.
Also, watch that your filament voltage is near the center rating. Don't run it low, and don't run it high.
A moderately priced tube that lasts a long time, is a far better value than an inexpensive tube that has a short life.
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AMEN!
Quite a few multi-grid O/P types, say the 6Y6, can have the max. anode voltage limit exceeded by a fair amount, provided that g2 B+ is regulated within the documented limit and plate dissipation held within the documented limit.
Within reason, mere volts don't damage tubes. OTOH, excessive dissipation is "the kiss of death".
Some commercial Guitar amps are an example of the fact that tubes can be run quite a bit over their ratings. Perhaps some of those amps need new tubes more often.
I remember some commercial Hi Fi / Stereo amps that are also designed to run the tubes over their ratings.
Designers of commercial equipment sometimes choose to ignore the tube ratings.
Sometimes tubes may sound better in a Hi Fi or Stereo, if they are not run way over their ratings. Yes, they will not have as much power, but finesse and power are not always obtainable at the same time (depends on the tubes, the topology, etc.).
I remember some commercial Hi Fi / Stereo amps that are also designed to run the tubes over their ratings.
Designers of commercial equipment sometimes choose to ignore the tube ratings.
Sometimes tubes may sound better in a Hi Fi or Stereo, if they are not run way over their ratings. Yes, they will not have as much power, but finesse and power are not always obtainable at the same time (depends on the tubes, the topology, etc.).
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