EF800 tubes going noisy
Just curious about peoples' experience with the EF800 as a microphone tube. I made a microphone with one and it sounds great. However, I have a hard time getting one that stays quiet over a long period of time. I have about a 50% initial success rate, but after about 50 or 100 hours of use, I've had 2 different ones go noisy... slight periodic 'popcorn' like discharges.
Is this just a fact of life with this particular tube, or am I doing something wrong?
I'm giving the tube +105 on the plate and underheating it slightly to about 5.2v filament.
Also, is there a known good supplier of tested, guaranteed-good examples of this tube? I'd be willing to pay a slight premium for one that would be reliable.
Thanks!
I'm not sure I have a specific answer for you but most everything out there suggests that microphone grade tubes are fairly hard to come by.
That being said, I have at least eight tube mics that use EH6072's which have been in service, powered on, and in daily-use for about the past four years 24/7 with only one or two developing "tube-issues" recently.
Best,
jonathan
I'm not sure I have a specific answer for you but most everything out there suggests that microphone grade tubes are fairly hard to come by.
-jburtner
I kind of suspected as much.
Try not underheating them. Most tubes degrade fairly quickly when underheated.
I might try that. Thanks.
Try putting some real GE 6072's in there and report back.I have at least eight tube mics that use EH6072's which have been in service, powered on, and in daily-use for about the past four years 24/7 with only one or two developing "tube-issues" recently.
-jburtner
I would search for information about cathode alloys and oxide coatings and what happens with heating voltage.
People have posted about heating voltages at different forums over the years and things to think about with tube microphone circuits.
Gustav
Hello
Brad,
Do you have a source we can reference to? I ask because I have found contradictory liturature on the subject.Try not underheating them. Most tubes degrade fairly quickly when underheated.
-tim
Yes we have a good rejection ratio on that tube as well BUT we don't resell the bad ones.
Hello Brad, they have about a 2 out of 10 failure rate. One of the biggest problem that some mic makers sell their rejects on e-bay, so finding good stock is the hardest part today. Best regards,Oliver
-oliver_archut
Hello David,
seems like that I still have to work on my English skills, those two sentence are not linked. I did not try to imply that you or any of the manufactures I work with do that. There are quite a few mic makers that use the EF800 and I do know that some of them get the rejects back into the NOS loop, same for most of the mic usable tube.
For myself the EF800 mic rejects that are not microphonic can be used with no problem in mic pres, etc.
Sorry for the confusion.
Best regards,
Oliver
Oliver and David,
Do either of you (or anyone else) know of a source for GOOD microphone-grade EF800s (not rejects)?
I'd be interested in a piece or two, if so.
kindest regards,
--brad
Two
of my most trusted tube dealers closed due to old age last year. I was buying
tubes from them since the mid 80s, their sold their stock to other tube dealers,
unfortunately people that I would not buy from, but they offered the highest
amount. Needless to say last year Joe and I bought a bunch of tubes before they
closed, but for the one offs it is getting harder and harder to find good
sources for a decent price. Sorry to say I can not recommend anyone right
now.
Confusion may be my fault. I wasn't implying anything towards Oliver.
I should have been more clear to Tim, and ask, "do you have any literary citation to support your statement", as I have found such to support both sides of the nominal and underheating argument.
Two of my most trusted tube dealers closed due to old age last year. I was buying tubes from them since the mid 80s, their sold their stock to other tube dealers, unfortunately people that I would not buy from, but they offered the highest amount. Needless to say last year Joe and I bought a bunch of tubes before they closed, but for the one offs it is getting harder and harder to find good sources for a decent price. Sorry to say I can not recommend anyone right now.-oliver_archut
Hmm, that's unfortunate news. Too bad I basically have to take my mic apart to swap tubes! Since it's a U47-type body (built from one of your kits!) the noval glass tube barely fits and doesn't really allow for easy swapping. Otherwise I'd be less bummed about just sorting through a stock myself.
you could build a jig into a bud box and use your mic's psu and a dummy polystyrene cap in place of the capsule.......
Hmm, that's unfortunate news. Too bad I basically have to take my mic apart to swap tubes! Since it's a U47-type body (built from one of your kits!) the noval glass tube barely fits and doesn't really allow for easy swapping. Otherwise I'd be less bummed about just sorting through a stock myself.
-soapfoot
Got one in today that's working great... for now. I plan on leaving it on for about 100h to see if it starts acting up. So far, though, so good.
Well, no dice on this one.
Is it semi-normal to have an untested tube start out performing well but then start acting up after a period of hours? I'm just starting to wonder if I should suspect something else.
Well, no dice on this one.
Is it semi-normal to have an untested tube start out performing well but then start acting up after a period of hours? I'm just starting to wonder if I should suspect something else.
-soapfoot
Semi-normal. We have not done any statistical record keeeping and analysis so this is not scientific, but our general rule of thumb is that 85-90% of tube problems will show up after the first 24 hours of burn in and about 95% will show up after the first 48 hours of burn in. Even then, there is no guarantee that your <insert well made NOS tube here> will last the lifetime that was the published spec of the day.
It's one of the reasons that we shy away from selling tubes outright to folks.
