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Old 2nd August 2004, 07:59 PM   #1
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Default Tube Life

Hi Tubeland

I was wondering what the popular consensus is on preserving tube life.
Is it harder on the tubes to keep shutting the set off or is it better to
leave it on?
I ask because I had a Audio Harmonix 7591 go bad on me after only 6 months of use.
Is it something I am doing or are these just crappy tubes?
The amp is biased correctly a little on the colder side at - 20 Vdc.
Help.
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Old 2nd August 2004, 08:03 PM   #2
mcs is offline mcs  Denmark
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Switch your amp off when you're not using it - unless you are going to use it 15 min. later or something like that.

The 7591 that went bad could just have been bad from the beginning. And BTW, the -20V doesn't mean anything - you have to measure the current, not the bias voltage.

Best regards,

Mikkel C. Simonsen
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Old 2nd August 2004, 09:07 PM   #3
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And some tubes just die... it's a probability thing.

I've got seven 12AU7s that run 16 hours a day pretty hot (300 8mA(?) per) and it's the same set I put in two or three years ago.

Tim
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Old 5th October 2004, 01:01 AM   #4
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Default Tube Life 2

Hi Tube fans

I just had the second Audio Harmonix 7591A tube go bad in about 8
months from purchase.
I am pretty frustrated as I expected that these tubes would last at least
4-5 years if not more.
The two that have gone bad have been in different positions and
I can't see how I could be exceeding their capacity as all the operating voltages
are within spec. 455 Vdc on the Plate -20 on Grid.
In both cases the Plate starts to glow red after warm up and then the fuse blows.
When I replace the tube with my spare old GE everything works fine.
Has anyone else experienced problems like this using Audio Harmonix?
Can anyone recomend a different source for 7591A tubes?
I think I have had K-Mart light bulbs last longer than this!
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Old 5th October 2004, 01:18 AM   #5
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Hi,

Quote:
In both cases the Plate starts to glow red after warm up and then the fuse blows.
Looks as if the tubes lose bias somehow.

Quote:
I think I have had K-Mart light bulbs last longer than this!
I really don't think they sound as good...

Quote:
Can anyone recomend a different source for 7591A tubes?
Other than the very pricey NOS ones, I think the EHs are the only
ones available for the moment.

Cheers,
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Old 5th October 2004, 01:47 AM   #6
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Default Re: Tube Life

Quote:
Originally posted by Glowin Plates
Hi Tubeland

I was wondering what the popular consensus is on preserving tube life.
Is it harder on the tubes to keep shutting the set off or is it better to
leave it on?
I ask because I had a Audio Harmonix 7591 go bad on me after only 6 months of use.
Is it something I am doing or are these just crappy tubes?
The amp is biased correctly a little on the colder side at - 20 Vdc.
Help.
The reason is because the tubes are crap. I had a pair out of four Sovtek 6L6WXT's do the same thing but in my case one went stone cold first. It turned out to be a poor solder connection in one of the socket filament pins on the bottom of the tube base. I resoldered the tube pin and it came back for a week only to start hot plating.

If your amp has self bias (a large cathode power resistor of about 250 ohms) you might be able to reduce the tendency of these crappy tubes to run away by putting a lower value resistor from Grid One to ground. If there is a 470 K in there now drop it to a 250 K. This will make it harder for the gas bombardment which heats the grid to emission status to go as positive as it otherwise can and draw runaway anode current. It is because these tubes are not made clean enough inside and not properly baked and outgassed.

Oops, I just re-read your post and it seems your amp runs fixed (but adjustable) bias. In this case the grid resistors run to the bias supply controls, not ground. They can still be reduced in value and this may help.

The best solution is to buy good tubes to start with. If it were me I'd be rewiring the sockets to let the amp use tubes that are much, much easier to obtain in the good old stock, like 6L6's.

Also, like another caller suggested, you have to actually measure the tube's plate or cathode current while adjusting the bias to set it properly. Many amps use a 10 ohm resistor from the cathode to ground which allows a voltmeter to effectively measure the cathode current while the circuit is in operation by a simple ohm's law calculation.
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Old 5th October 2004, 08:44 AM   #7
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I also lost two Electro-Harmonix 7591A tubes prematurely.
They have quality control problems. At this point, I'm using the JJ Electronics 7591 tubes. They seem okay. I purchased mine from www.dougstubes.com at $35 per matched pair.
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Old 5th October 2004, 08:50 AM   #8
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Does anyone have a table of expected life of different tube types? I know this is highly dependent on how they are used, but some general guidelines would be fine. How long should eg. a good quality 300B tube last? 1000 hours, 500 hours...?
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Old 5th October 2004, 09:10 AM   #9
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There's a relevant EH discussion on news://rec.audio.tubes - it appears EH has put out an entire run of lemons lately. Try Shino.

As for tube life, the only spec sheets I have seen lifetime rated on are Russian (2,000 hours for preamp tubes like 6N1, 2 and 3P) and NOS JAN tubes (10,000 hours for an 807 for example)
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Old 6th October 2004, 12:32 AM   #10
anatech is offline anatech  Canada
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Wow, I've had good luck with every Electro Harmonix tube I bought (1 noisy 12AX7 out of 20, shipping damage?). I was just about to buy some 7591's for my Eico ST-70. I guess I'll hold off till things settle down. The 6L6's are especially good.
Note: Modifying a unit for 6L6's from 7591's will change the sound.

-Chris
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