Also tubes that start out noisy often will get quite again after 24h...... There is no rule of noise. Normally the tubes are burned in without listing first then they are tested for microphonics and noise... If tube stay noisy after 48 h then they can reconditioned 2kV via a 10M resistor on plate (2nd 3rd), grid as cathode no heater. After one week there is a 50% chance that the tube is quite again.
Interesting. I don't think I have anything that makes 2kV, but it's cool to learn about nonetheless.
2kV . . .
makes me shudder . . . .
Also tubes that start out noisy often will get quite again after 24h...... There is no rule of noise. Normally the tubes are burned in without listing first then they are tested for microphonics and noise... If tube stay noisy after 48 h then they can reconditioned 2kV via a 10M resistor on plate (2nd 3rd), grid as cathode no heater. After one week there is a 50% chance that the tube is quite again. Best regards,Oliver
What is the effect on the tube cathode/plate/grids during this procedure? Do I understand correctly that you are flowing current from plate through grid (grounded grid?) and not connecting the cathode or heater for a week of "Kur"?
It really is a jungle out there. One eBay seller advertises "microphone grade" EF800s, sells them for $30 a pop, with language in his listing that casts aspersions on competitors' tubes. When I politely asked him whether or not his tubes are burned-in for any length of time as part of the testing, instead of answering my question he asked ME a question--which microphone I wanted to use them in. When I explained my mic, he quite rudely told me he wouldn't sell me a tube because he didn't want his tube (and I quote) "to be used as a guinea pig for homemade mics" because he "doesn't know my qualifications as an electrical engineer (sic)." However, he told me "don't take it personally," so I guess that means it's not rude or insulting.
I never did receive an answer to whether or not his "microphone grade" tubes were ever burned in or tested in an actual microphone. And we all know what that means.
Meanwhile, the search for a tube continues...
how bizarre . . . I thought business was business!
If that's the same seller as i'm thinking of, I bought three EF800s from him, all of which were unusable in a Wunder CM7GT. (At the time they were only going for $10 each, so not a huge loss, but still frustrating.) I was told later by a trusted source that he engaged in some downright fraudulent practices under a different Ebay ID. I'm curious to know if it's the same guy...
It really is a jungle out there. One eBay seller advertises "microphone grade" EF800s...
Yes it is! It is unclear that ANY tube sales company is testing for microphone level quality at all right now. Having spoken with many tube suppliers here in the States, my understanding is the majority of them are taking a tube sticking it on a tube tester and as long as the numbers fall within spec, it's considered a tested tube. As pointed out on this thread, that may be sufficient for many hi-fi stereo or amplifier uses, it is not enough for determining without a doubt that a tube is suitable in a microphone circuit.
My guess is that despite the growing DIY microphone market and the increased offerings of new tube microphones that still won't be enough incentive to change the testing behavior of the various tube merchants. Between the time, the electricity, the investment to set up proper testing stations and the management of doubling your inventory SKU's (EF800 standard, EF800 microphone grade) it is likely that the vendors will continue to manage their stock in their current format.
If you are ordering tubes for microphones from Ebay, it's a good idea to have other uses at hand for all of the (many) not ready for prime time.
After rephrasing my question one more time, I got a final answer back from the guy. He believes if I'm having noise problems it's probably "dust on the capsule" because "microphones have to be made in clean-room environments." He did, however, tell me that he burns in the tubes for 12 hours, but contends that "the user has to burn-in the tube on their very mic, not some other mic. This carves a unique sonic signature... into the tube" like "breaking in a pair of shoes."
Which would be the first I've ever heard of that, but admittedly I don't know everything.
Although I never mentioned how I was heating my tube, he volunteered that less than 5.8v on the heater "destroys the tube." Which is contrary to what some venerable minds here and elsewhere have said.
He then offered to install and test a tube for me if I sent him my mic(?) which seems like a nice offer, but after his initial rude, cryptic, and condescending behavior, I'm not sending him anything. He has not established that kind of professional trust.
Will he take the tubes back if they turn out not to be microphone grade since they are advertised as such?
I don't know the specifics right this sec but I thought there were other options for the tube in that schematic. Maybe trying some others would also help?
I can say that I don't think the mic carves a sonic imprint into the tube but I do think that since mic's operate on the fringes of tube spec's that their specific sweet-spots are each inherently different and that they probably "need" to be tuned manually for optimal results. I could be wrong about that too because obviously there are MFG's that supply high quality mics without hand tuning the circuit of each one.
The guy sounds catechumenal to me.
That could mean they are very microphonic.
One eBay seller advertises "microphone grade" EF800s
The guy sounds catechumenal to me.Avoid.
Whatever that means, I bet it's bad!
Avoidance is already a foregone conclusion with this particular character.
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