Tubes dying prematurely

[mcandmar]

I have a recurring issue in that tubes start making strange noises and eventually will get so noisy that i have to retire them.  Each set of tubes seems to last me about a month, two at most. They get approx 8 hours of use every day so i estimate ~3-400 hours is when they start to give trouble.

What is happening now is one tube is making a faint bump, bump, bump sound. The interval can vary but its consistent in that the sound is always there, and it only started doing this since friday evening. I have switched the tubes around and the sound has moved channel so that confirms it is the tube and not the amplifier.

What is worrying me is this is the third set of tubes in a row that have failed like this, in fact some of the others were worse in that they would either make strange whistling sounds or randomly make a very loud POP sound which is never pleasant while wearing headphones.  It has become so frequent now i am convinced something is wrong and killing tubes prematurely.

I ran though the resistance and voltage checks in the manual and nothing looks out of spec, if anything most of the voltages are slightly below the manual specs. The only one that reads in any way high was the C4S specs,

OA/OB read 72.4/77.5   (77 being the noisy tube)
10/20 read 413/413     (C4S manual lists 405v, whereas the main S.E.X. manual lists 420v ?)

I'm at a loss now, and slightly frustrated.  I have another set of NOS tubes but i am loathed to install them until i have confirmed there is nothing causing this issue.  Any thoughts, or anything else i should check?

Thanks,

Mark

[physicsmajor]

I've experienced the very loud bass-heavy POP. Got rid of that tube ASAP (one of the originals), replaced with a set of NOS RCA, and haven't had an issue yet (probably close to 1k hours).

[Grainger49]

Check the voltage on your heaters.  I don't have the manual handy but I expect that the heater wires are twisted and are shared by the two tubes.  Just go to the last tube in the line and measure the voltage.  High voltage will burn up tubes much faster than normal.

I don't expect this is the cause but it is the first thing to check when you are having short tube life. 

[Paul Birkeland]

You might consider yanking out the four 220 Ohm carbon composition grid stoppers and replacing them with carbon film resistors.

In my experience, sometimes those carbon composition resistors can fail physically, but not visibly, and become intermittently connected. 

It would be very abnormal for a SEX amp to need tubes more than about every 10 years. 

[mcandmar]

Check the voltage on your heaters.  I don't have the manual handy but I expect that the heater wires are twisted and are shared by the two tubes.  Just go to the last tube in the line and measure the voltage.  High voltage will burn up tubes much faster than normal.

I don't expect this is the cause but it is the first thing to check when you are having short tube life. 

Getting +2.89 and -3.1 at the tube, so ~5.99v.   I do have the two 100ohm resistors in place to center the supply, without them it used to be -2.48 and 3.61 = ~5.95v.  I have been tempted to remove the .1 resistor from the + rail and see does it center better and raise a little bit closer to 6.3v.  As discussed in another thread those 100ohms made a huge difference to background noise.

Another point on the back of my mind is the wiring, as you say the wiring runs from the filter cap to the right channel, and then from there to the left channel. In the S.E.X. 2.0 there was two separate wires running from the filter cap to each channel, which in my mind seems like it would be better balanced.  I would be curious to know why the change was made.

You might consider yanking out the four 220 Ohm carbon composition grid stoppers and replacing them with carbon film resistors.

In my experience, sometimes those carbon composition resistors can fail physically, but not visibly, and become intermittently connected. 

It would be very abnormal for a SEX amp to need tubes more than about every 10 years.

I will indeed try that, as they are in the signal path i will use that as a good excuse to upgrade.  Though at no point have i ever experienced any cut out while using the amp, pops and noises aside the music has been constant.

[Grainger49]

As long as you have about 6V on the heaters you will not harm the tubes, or their longevity.  The "nominal" 6.3V is well in range at 6V.  Don't give that a second thought.

The 100 Ohm resistors put an additional load (didn't say bad load) on the supply, but that wouldn't affect the tubes at all.

[mcandmar]

Do the grid stoppers for the 2nd stage see any real voltage?  i.e. will a 250v rated 1/4watt do the job.

I would be interest to hear if anybody has any experience with Takman carbon films?   The only other alternative i can find are Kiwami but the smallest they do are 2watt.  The only other recommendation i found on the forum was for Riken metal films but i cant source those anywhere locally.

http://www.hificollective.co.uk/components/takman_resistors.html

Cheers,

Mark

P.S. That noisy tube gave me the loud pop of fright about an hour ago, want to order these tonight and sort it asap.

[Paul Birkeland]

The resistors see nearly 0V.

Don't bother with the botique parts.  Get some 1/4 watt generic carbon films just so you can rule them out.

If you have to go boutique, the little PRP's should work OK.

-PB

[mcandmar]

Righto.  I have to mail order them anyway so i may as well buy the boutique ones as they are only £2 for the set, costs more to post them

[mcandmar]

I ended up ordering a couple of Ohmite resistors with my last Mouser shipment and they look identical to what was installed, was that a lucky guess?

Stupidly i only ordered two of them when i needed four so i swapped out the two before the 2nd stage which didnt seem to make any difference.  At first i thought it had but i recently switched from using the Grados to the HE-500's so the intermittent pop is just a mild background pop now, where as with the Grados is was like somebody sneaking up behind you and popping a brown paper bag beside your ear.  I'll order another few and switch out the two for the 1st stage and see if it makes any difference.

[Loquah]

Good luck, Mark. I'll be really interested to read updates and the cause of your problems