how sensitive are tubes to faults with a bloo la2a or in general?

hey everyone 

i just finished building my bloo la2a, all voltages in my unit against the bloo manual test ok and are not far off from the specs written on the bloo manual. i found an oscillating pulsing clicking thumping humming noise when i turn it on. it would change speed of oscillation and pitch as the gain knob was turned.

now the volume of the noise would always be the same volume. it would be on maximum on the meters on my protools and if i have audio feeding the la2a, the noise would stay the same level and the audio i fed would change volume as i turn the gain knob. the noise would also change bass depth when the +10 +4 gr switch were changed.

this would only occur after a few seconds once the unit is turned on, i presume that is because the tubes are warming up. sometimes it will not make the noise until i start passing audio thru it or making the audio clip using the gain knob but it always makes the noise eventually. 

my bloo unit has r38 with a 22k resistor as instructed in the bloo manual. i also have the combination of r34/22k and r36/1K. ive read on numerous posts that r38 must not be a resistor and must be a straight wire if you use the 22k/r34 and 1k/r36 combo but it made no difference in solving the issue so i left r38/22k in.

i started changing the tubes and noticed i was getting different results and eventually changed the 12bh7a and the noise issue stopped. i had a low hum, but its in the boundaries of my other equipment hum. problem solved!!!.

stupid me,  i didnt stop there and started fiddling with the first and the second 12bh7a tube i was using and swapping back and forth without letting the tube or unit cool. i would take the tube out and install the other tube in once i turn the unit off!  :

now the oscillation has come back with the second tube i used. this time the noise is more deeper, more bass unlike the first 12bh7a where the noise was more mid and high pitch.

i am thinking to myself, maybe the tube is not faulty because of my poor handling of the tube but because i wired the unit up wrong? could leaving r38 resistor in, cause the 12bha tubes to fault or are tubes generally sensitve and because of my poor handling of the tubes, it then became faulty? the reason being is that the first 12bh7a tube i installed was new, never used, even though its 6 years old. the second tube i installed was nos and an expensive tube.

what do you guys think?

does anyone know where i can get the tube shield for the 12bh7a of the la2a? i bought one from a store and found that its too short and i couldn't shut the shield in place. it fits the 12ax tube though  :-\

i need a taller shield.

thanks

go to guitar center, the fender amp section...

no, jus kiddin,

antique electronic in la mesa aridzona has them.

lol, thanks cj  :-*

i did some more tinkling with my box and discovered that my wiring inside where the 12bh7a were a litle cramped. with some chopsticks and a little pull and tug here and there, i was able to use the second 12bh7a with no pulsing oscillation noise. the first 12bh7a was crook though, i guess not enough tlc for the past 6 years. poor tubey wooby. 

i was still getting some hum though. so i decided to look into my box and noticed that moving wires around gave more or less hum and noise. it seems those heater wires and vu lamp wires are really sensitive. so with a little repositioning, my hum is as low and acceptable as my other equipment.

hooray, woo hoo!  ;D

XOXOX


debby

Yeah, I've noticed the wires around the 12bh7 to be EXTREMELY picky about their location in regards to oscillation.  That's why I use solid core wire so that it stays where I put it!  It's also important that all the wires that need to be twisted together are twisted as tight as possible.

hey john : )

is your bloo dead quiet or do you still hear noise? i have a lil bit on mine but i can live with it. the space inside is just so tiny, i can't believe how i managed to close the unit.

I've never built a bloo.  I source my own parts and build em from scratch.  They're dead quiet until I get the gain about 3/4 of the way up or more, which is WAY past where I'd ever actually need it to be.  Since they're point to point, no two are exactly the same.  I do try to get the oscillations as minimal as possible, though.

what are the problem areas that you find cuases oscillation in your la2a john? i was only able to find the 12bh7a socket wiring too close to each other and also the heater and lamp wires too close to the power transformer area. i have grounded the front chassis well.

i wouldn't mind being able to get it dead quiet up to 3/4 turn on the gain like yours john. my small amount of low hum pulsing noise, is always on regardless. its like pulsing slowly 1cm high on the protools vu meters but you can't really hear it unless you are on maximum volume on your monitor amp.

i have been trying to figure out areas on the wiring inside that can reduce this noise. i think the other cause is tubes that is causing the noise but i am wondering if there is more that i should consider. what area would cause very low frequency (bass) hum on the unit?

thanks john 

You need to make sure that all of your wires running AC power are twisted extremely tight and run in the corners of the chassis.  Any cable that comes near the AC wires needs to cross it at a 90 degree angle, and if that's not possible it needs to be run somewhere else.  You might also have a ground loop somewhere that's causing the pulsing.  Also, make sure that anywhere that you tied a ground to the chassis, you scraped paint off so that there's good contact being made.  
I always use cayacosta's layout and match his ground spots exactly: http://www.recproaudio.com/diy_pro_audio/teletronix_la2a.htm  Also the pair of wires running from the output XLR to the meter circuit needs to be twisted tightly and run in the top corner of the chassis.
Tha main area for high frequency oscillations for me has always been around the 12bh7, but it's been the area between the tube and the turret board.  I try to keep them parallel and anything that crosses them at a 90 degree angle.  
Here's a pic of the first one I built several years ago before I started using internal panels.  This one was the most sensitive to oscillations because the wire runs from the tubes to the turret board were so long - it was also a bit messy as it was the first one I built.  I tried to keep everything as close to either parallel or a 90 degree angle:

Here's a later one with the internal panel.  I've had almost no oscillation issues since I've used them:

But I would check first that you don't have any ground loops and that all your ground points are making good chassis contact.

CJ said:

go to guitar center, the fender amp section...

no, jus kiddin,

antique electronic in la mesa aridzona has them.

Click to expand...

cj thanks for the link but the tube shield needs to be taller than 2 inches, 50mm. it seems like 55mm to 60mm would be the right tube shield size. are anyone making those?

Here you go:
http://www.tubedepot.com/sk-sh9b.html

regularjohn said:

Here you go:
http://www.tubedepot.com/sk-sh9b.html

Click to expand...

oh john, you are the best!

xoxo
debby