Getting only 400 hours on Class A2 PP tube

[qguy]

I have PP amp running on Class A2 (Cayin A-70T) its autobias and so far it has gone through one quad of new production Tung-sol 6550 and 6l6GC, both tubes clocked 400 hours (estimated based on daily use) On both occasions one tube goes south, coincidence or these is just the way the amp works ? Is 400 hours normal for a Class A2 amp ?.

[SY]

How hard are the tubes run? Plate and screen voltages, idle current, max current? I've run 6L6-types in AB2 push pull for years with no premature failures.

[richwalters]

I'm running 6550 TungSols new edit at 100mA Iquies at 450VB+ and are quite consistent. I have another AB2 thread running which is currently hammering the earlier 6550WA versions. So far nothing going wrong.
We need more conditions info !

richj

[coresta]

A2 for 6550 , 400 hours liftime is a very good value Are you sure for A ? not AB1 to 2 ?

[qguy]

Not sure I can get those info as I am not a qualified technician and might fry myself, the amp, house, wife and kids

Yes its Class A2, Autobias, Push Pull. the amp also works as a heater I get 30 wpc on 6550 and 28 wpc on EL34.

I have heard that AB amps gets a 1000 hours to a tube so is 400 hours on Class A2 good ??? Should I switch to a stronger more powerful tube like EH KT 90 ?

Originally posted by coresta
A2 for 6550 , 400 hours liftime is a very good value Are you sure for A ? not AB1 to 2 ?

[coresta]

Originally posted by qguy
Autobias, Push Pull.

A2 : Impossible A2 mode ... Have a look at your coupling capacitors , leaking maybe ? Cheers, Pierre

[qguy]

Capacitors are ok as these were checked by the technician. I was advised to get a SERVO type voltage regulator, can voltage swings be the cause for the early demise of the tubes ?

[alexg]

Originally posted by qguy
Capacitors are ok as these were checked by the technician. I was advised to get a SERVO type voltage regulator, can voltage swings be the cause for the early demise of the tubes ?

I doubt that it is the voltage swing, if it is, it should be all the power tubes getting bad, not just one tube per quad.

[kevinkr]

Have the technician determine the actual operating point of your output tubes. A pair of 6550/KT88 should easily be capable of 60W or more in PP even with cathode (autobias) in pentode or UL connection and over 30W in triode connection.

It sounds to me like the output tubes are being run well beyond realistic operating points.

Conservatively run good 6550 can run for as much as 5K hours or more before replacement, although I generally replaced them in amplifier service at 2K hours.. I have a former client running a PPP amplifier I produced commercially and his output tubes are approaching 5K hours and are still fine.

I have never designed a 6550 based amplifier where grid current deliberately flows during normal operation.. (clipping maybe) IMO The 6550 should not be operated with significant grid current, just my point of view. Likely your amplifier is class A or more likely AB.

Is it always the same socket position that fails? If so suspect the coupling cap, and cathode bias components if not shared with the other tube on that channel.

[alexmoose]

This sounds Like my kind of amplifier! A2, KT88, Autobias could we see a schematic?