1 post / 0 new

Did new E34L fail during bias?


Doug Elick
Posted on Saturday, January 01, 2005 - 03:27 pm:   

I swapped out my old, worn out Jolidas for a quad of JJ E34Ls from the Tube Depot in my 12AX7 driven HD-130 Reverb.

I didn't measure the bias with the old tubes in the amp (didn't think about it), but when I took the amp out of standby with the new JJ's, the voltage as read across the 10 Ohm resistor attached to pin 8 (on socket 1 and 4) was about .34v (both readings were within .004v of eachother); seemed awfully cold, considering the good ol' .5 v specified in the tech. note should put the tubes @ 25ma and about 17.5 watts idle dissapation each.

I upped the voltage to about .43 and let things settle down. I then adjusted it up to .48 and let it settle down again, turned the lights out and observed no plate glow. Then I barely bumped the table the amp was on by accident and immediately heard a Bzzzzzzzzzzt come from the speakers. As quickly as I could, I shut the plates off (standby).

Now, even with the amp on low, if I take it out of standby, a specific tube has a strong blue glow, a loud hum is heard from the speaker and the bias voltage reads 2.2v regarless of what the trim pot's set at! The problem's with tube since it doesn't matter what socket it's in and with one of the old 6CA7's in it's place (not the best match, I know), the bias settles back close to where it read before the bzzzzzt.

Did I wind up with a weak tube that arced over? The I measure the bias wrong and misadjust it?

Any thoughts? I want to have a good idea of what happened before I talk to Tube Depot about this.


michael kaus
Posted on Saturday, January 01, 2005 - 06:16 pm:   

You just had a new tube take a dump. Get it back to the dealer and tell them the story. they should have no problem replacing it. It doesn't happen often but you got the lucky straw. I've been using Eurotubes and he burns his tubes in a while and rematches them himself at real voltages so they get a workout. So far, I've had no failures.
Douglas Elick
Posted on Saturday, January 01, 2005 - 06:56 pm:   

Ok. Thanks.

I was worried that somehow I had misinterpreted the instructions and was reading from the wrong test point or had the tubes too hot.

After thinking about it, it sounds like the grid somehow went non-functional. I'll report back how helpful "Tube Depot" is.