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Old 28th August 2004, 06:14 PM   #1
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Default 7591EH failure

About a year ago, I installed a quad set of Electro-Harmonix 7591EH tubes in my old and fully restored HH Scott model 299C amplifier. The tubes have performed beautifully ... sonically they have been better than the original 7591A tubes.
A couple of days ago, I noticed a slight 120 Hz hum coming from the right channel speaker so, using the instructions provided by HH Scott back in 1962, I removed the 6U8 driver and adjusted the bias balance pot for minimum hum. I was able to reduce the hum somewhat when the pot was at the end of its travel.
I replaced one of the output tubes with an original 7591A and was able to null out the hum.

I'm not sure what has happened to the 7591EH but I will build a test jig so that I can compare the bad tube with a good one.

Has anyone else in the Forum experienced a similar failure with the 7591EH tubes?
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Old 28th August 2004, 06:25 PM   #2
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I don't have any experience with Electro-Harmonix 7591 tubes, but this looks like heater-cathode leakage or short. The tube could suffer from it due to less quality of the materials used inside your tube.

Also make sure you don't exceed the maximum heater-cathode voltage by connecting the heaters to a reference point.

BTW, I love Florida, was there 3 weeks ago.
Dick
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Old 28th August 2004, 06:37 PM   #3
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Dick,

Thanks for your quick reply. Florida is hot and humid today.

The cathodes in the 299C are at ground potential and the filaments are floating with a balance pot to ground.
The tubes are operating well within their specs.

It appears that the 7591EH was continuing to operate ... but with far different specs. I really didn't notice an increase in the distortion nor did I notice a reduction in output power.

Tests will confirm the failure mode.
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Old 28th August 2004, 08:22 PM   #4
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I had one of a quad of fairly new EH 7591 exhibit the same problem - loud hum from the speaker but the tube carried on working. It did test weak, but also seemed to go up and down a bit. Couldn't find any reason why that particular tube gave out - it was not driven hard at all. Edicron in the UK, who supplied me, replaced it immediately with one day turnaround - I couldn't ask for better service. Don't know if this is a coincidence or whether there's some potential weakness.
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Old 28th August 2004, 08:38 PM   #5
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andyjevans,

Thanks for your input.
One thing I just remembered. The amplifier is in my office and operates about 8 hours per day.
Last week, I noticed a noise when I turned the amp off.
I had never noticed the noise before.
I heard a "tink, tink, tink" as tubes cooled down.
Perhaps the noise was coming from the bad tube.
A broken internal support?
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Old 28th August 2004, 10:06 PM   #6
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Hi,

Quote:
I heard a "tink, tink, tink" as tubes cooled down.
I doubt it is anything to worry about. i hear that everytime I turn of my amps. Perhaps you only just noticed because you were paying extra attention to the amp?

Unless the tube was glowing cherry red, I'd say that pretty normal behaviour.

Cheers,
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Old 28th August 2004, 11:25 PM   #7
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Thanks for your input, Frank.

I hear the same thing on my power amplifier at home.
I just never noticed it from the one at work.

New tubes are on the way. Until then, I'll use the old 7591A tubes and enjoy the music.
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Old 29th August 2004, 04:23 PM   #8
anatech is offline anatech  Canada
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Hi Frank,
I've used many Electroharmonix tubes in several amps. I have been very happy with them. I have run into a bad tube also, but that is to be expected.
When I'm finished rebuilding my Eico ST-70, I will use Electroharmonix again.
-Chris
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Old 29th August 2004, 06:00 PM   #9
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Chris,

I trust them too. I've ordered a replacement pair.
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Old 29th August 2004, 06:02 PM   #10
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I should add that the EH 7591s themselves sound great - very detailed and punchy. Very useful valve, because it's so easy to drive - I'm driving with a 6C5G DC coupled into a 12B4 concertina. Using it UL without feedback into 6.6K
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