Eros burning tubes?

[arveedub]

I am having some serious trouble with my new Eros. At first the amp was fine. Sounded extra great. Within a couple days, however, one of the tubes went noisy. Real noisy. With shorted inputs I measured 20mv on the output. 7mv on the other. The noise moved when I swapped the sides of the tubes. Grumbling to myself about how I was sent a bad pair of tubes with the kit I plugged in a second set. Everything was great till a couple days later. One of the tubes was suddenly noisy. 20mv on the newly noisy tube, 7mv on the other. Noise switches sides with the tube.

I wasn't paying attention the first time around, but the left tube went bad the second time around. I seem to recall the left went bad the first time, but I may be incorrect.

I already posted my measurements on another thread & was told they were OK. How does an amp that measures fine break tubes? Are the tubes ruined? Is there anything that I can test to track this down further?

Any help?

[Paul Birkeland]

I would first measure the voltage between the + and - holes on the big green PCB that say 6.3v.  That voltage should be 6.2-6.5v.  If that voltage is high for some reason, then it would make sense that you would be ravaging tubes pretty quickly. 

Next I would look at your Kreg to A3/B3 voltages, and the OB voltages.  Measure these, write them down, then swap the tubes between channels and measure again to be sure those track properly. 

After that, very, very carefully measure the cathode voltage of each triode on the 6922, which would be pins 3 and 8.  Be sure not to short any pins together when you do this. 

Knowing all of these numbers, we should be able to get an idea of any issues that would cause malfunctioning EF86's. 

[arveedub]

Thanks for your reply.

The heaters measure 6.3V

With the bad tube in the right:
OB left 101V
OB right 101V
Kreg -> A3 0V
Kreg -> B3 0V
C4 100V
C8 99.8V

With the bad tube in the left:

OB left 99.4V
OB right 100.3V
Kreg -> A3 0V
Kreg -> B3 0V
C4 100.8V
C8 101.6V

Thanks for any help!

[Paul Birkeland]

Sorry, I mean the K-reg DC voltage for the EF86's.  There should be a volt or so at that terminal.

[arveedub]

Seemed odd to me, seeing as how there's a wire connecting them. "What do I know", I thought 8-)

Here's a link to the post about my measurements. Everything measured fine & there is roughly 1V on the terminal on both sides.

http://www.bottlehead.com/smf/index.php/topic,367.0.html

[Doc B.]

Unfortunately we don't have the ability to long term audition each tube that we send out with the kits, but we do have a policy of replacing any tube that might be an issue. It doesn't sound like there is anything going on in the preamp that would damage a tube in two days of use, and it's not impossible that you just happened to get a couple of flaky tubes in a row. It might be worth letting them cook in the preamp for a couple more days to see if the problem goes away as the cathode forms. And we are always happy to send another tube if the problem persists.

[arveedub]

OK, I'll give them a couple days.

Leaving the amp on should be sufficient, right? Or do I need a signal running thru the amp?

[arveedub]

BTW: with shorted inputs and stock tubes, what would you expect to read at the output? I was getting 7mv before all of this started.

[Paul Joppa]

No signal needed for the cathodes.

The EF86 has a bunch of very low frequency noise (i.e. below 20Hz), making it hard to assess noise by simple measurements. We were getting a fraction of a millivolt using a 50Hz highpass filter (a "C" weighting network), but on the 'scope the VLF stuff overwhelms the rest of the noise. When we find a good method, we'll post it.

[arveedub]

OK, I've got the two bad tubes in & they've been hissing away since yesterday. I'll give it a day or so more & see what's what. There's been no change & I take that as a good sign. If the amp were in fact damaging the tubes it would seem reasonable to expect more damage the longer they were in place.

If there's no change then I will try the two good tubes before asking for a replacement. If they don't get noisy then I'll readily accept that I got doubly unlucky.

Thanks!

[arveedub]

After testing out the tubes for several days, I contacted the good folks at Bottlehead. I had a new tube within a week & everything has been fine since.

I just wanted to say thanks for all the help

[Doc B.]

You are most welcome. Thanks for the followup and the kind words!