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Old 29th March 2005, 01:42 AM   #1
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Default Life of tubes - what to expect?

I have finished restoring my first tube amp - a Harman Kardon A50K/A500 that uses four7355 output tubes, push pull arrangement.

These tubes are no longer in production, so I am curious how much life one can get from a set of four in the unit. Do you estimate life on hours of use, or does number of times you poer up and off affect life?

What can one expect?
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Old 29th March 2005, 02:22 AM   #2
PRR is offline PRR  United States
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> Harman Kardon A50K/A500 that uses four 7355 output tubes... These tubes are no longer in production, so I am curious how much life one can get

If the idle current is less than around 3/4 of rated dissipation, and you are not using it as a guitar amp, possibly the rest of your life. Yes, guitar amps can soften good tubes in 100 hours (and bad new-made tubes quicker), but in sane uses tubes give 1,000, 5,000, even many tens of thousands of hours of life.

I once had a set of audio amps that had been in 24/7 duty driving film projector motors for over 5 years. They used 8417, which is not the most reliable tube ever made. When retired, 23 of the 24 tubes were factory original, the amps met factory specs, and after some hi-fi tweaks I did some very excellent music reinforcement with them.

> does number of times you poer up and off affect life?

No, or shouldn't. Certainly the plates don't care, and oxide-coated cathodes are pretty tough about power-up cold-stripping. Again, there were exceptions: series-string TV sets could be very hard on the smaller tube heaters, and failed heaters were routine and a pain to localize. But in parallel-power heaters, in all my decades of tube-abuse, I have only ever seen one failed heater.

You can get NOS 7355 from reputable vendors for $20 a pop. Get a set for spare and enjoy.

If you really get stuck, 7591 or good-old 6L6 (and kin) will "work", though you will have to check pin-out and re-bias them, and space may be an issue.
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Old 29th March 2005, 03:36 PM   #3
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Default Nothing like experience...

Thank you so much for sharing your expertise!
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Old 29th March 2005, 04:45 PM   #4
Giaime is offline Giaime  Italy
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Hi there! It seems that the good ol'6L6 would work in that case... You might find goos NOS ones at cheap prices because people has finally understood that they're different from 6L6GC for their Fender!

I see cheap NOS here at www.tubedepot.com

Hope this helps!
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Old 29th March 2005, 08:37 PM   #5
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Quote:
guitar amps can soften good tubes in 100 hours (and bad new-made tubes quicker), but in sane uses tubes give 1,000, 5,000, even many tens of thousands of hours of life.
Ahh....My record for a reputed good tube.. shortest tube life is 15 hrs for a Brand new Shugang KT88 til kaputt.. used regulary in fixed bias overdrived guitar amp. Cause of failure.....dirty assembly ....contamination of g2 grid wires......eventually flared.
Replaced in situ with NOS 5881 (us orig) still going strong after 5 years.



richj
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Old 30th March 2005, 10:30 AM   #6
Giaime is offline Giaime  Italy
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...have you really replaced a KT88 for a 5881? 42W pa for 25W pa? How did you do that?
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Old 30th March 2005, 03:06 PM   #7
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Quote:
Originally posted by Giaime
...have you really replaced a KT88 for a 5881? 42W pa for 25W pa? How did you do that?

Yes....an emergency on stage for a 35W amp...an NOS 5881 can accept 400V on plate....at this B+, the KT88 was relatively underrun.....However this tube failed in UL mode whereas the 5881 needed diff - bias voltage. Later modified with sep g2 supply. This sounded far better than UL mode.

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Old 30th March 2005, 05:32 PM   #8
Giaime is offline Giaime  Italy
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Yes I agree. A proper screen supply many times make many amps better.

But there's a thing that I haven't understand: up to now, I've changed tubes in amps within their name (ex. a EL34 for an EL34), but I haven't tried to change type of tube. What is the difference between a 5881 and a 6L6? The 5881 has 25W Pd, the 6L6GC has 30, other than this? And between EL34 and KT66?
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Old 31st March 2005, 01:24 AM   #9
anatech is offline anatech  Canada
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Hi Giaime,
KT-66 is a bigger 6L6GC. You can swap a 6CA7 for an EL34 (tetrode vs pentode). The 5881 / 6L6 / 7581A = KT-66 family have similar plate curves. As long as you don't exceed the ratings of a tube you can change them. Watch the plate current.
I don't know if that helps you at all Giaime, check out some tube manuals out for the truth.

-Chris
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Old 31st March 2005, 02:23 AM   #10
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tubes will last much longer than you think. The tubes that typically wear out the most are teh output tubes. These should last you several good years. I have several amplifiers, all with the original EL84's or 7591 (EL84 spec'd for 400 volts on the plate, basically the same thing, or a slightly tougher version, with a different label on it. ) in them. These were church organ amplifiers, used every day at hgih output level. One has 300 volts on the plates, another 400 volts, one was ultralinear with 450 on the screens/platea t idle, 400 under load.

Tubes typically do not "burn out" in normal use, it's more of a wearing out. They still work, but new tubes wil sound better and louder. Badly worn tubes have a dull sound. keep your old tubes as spares incase one suddenly fails on you. The most common failure (besides breaking the glass) that kills a tube would be a internal short or arc,and these are typically caused by a cheap (Chinese)/defective tube or abuse. If you see flashes inside the tube, shut the amp down, there is arcing. A blue glow might indicate that the vaccum is leaving (but there is a different kind of blue glow that is perfectly normal in some brands of tubes). If you see the plate of the tube glowing red, shut the amp down and check the bias supply. The tube is being asked to dissipate more power than it can and the metal plate is glowing red hot! This usually does not damage the tube if it does not happen for a prolonged period of time. If you see the internal structure glowing red, check for excessive grid or screen current draw. These should not be issues in a peroperly designed amplifer that is functioning reasonably well.

The reason guitarists change tubes so often is three fold. The first reason is that these amplifiers are pushed HARD, since the cllipping of a tube output stage is what gives the best Rock 'n Roll sound. Nothing compares to a wall of dimed Marshalls ala AC-DC. The second reason is that guitarists are typically told (by music stores selling tubes) to change their tubes once a year or so for the bes tone. The third reason is that guitar amplifiers (even when not overdriven) are very harsh to their tubes. Marshall's have problems with EL34 screen grids. Fender Deluxes run 6V6's at 450 volts! VOX AC30s are bias pretty hot and have bad ventilation with a crampt chassis.

Rectifier tubes do wear out somewhat, but they do not have as much of an impact on the sound as the output tubes. They last a long time, and are cheap to replace. Preamp tubes seem to last a very, very long time. The originals should be able to last the life of the amp, or at least for several output tube sets.

I recommend you get spare tubes for your amp (and experiment to see which kinds you like the best. Tubes for amplifiers are like strings for a guitar; you can get slightly different sounds from different ones, but you can't turn a Les paul into a Strat with different strings!), and keep them in a safe place. Having an extra set of output tubes and possiblt rectifiers, and a couple of spares for the preamp should supply your amp almost forever.
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