I've got a BPM tube mic with what I assume is a 6DJ8 in it (it has a
shield that forms a sort of S between the two plates). I recently
replaced a broken IEC jack and power switch. Now it's working again but
seems to be noisier than it used to be (though I haven't used it in a
good while). It's low enough that I could easily track drums but vocals
would be a problem. I'm trying to figure out where I should look for
the noise. Here's what I know.

The noise is not quite hum and not quite white noise. It almost sounds
like a mic turned way up in a quiet room, picking up air sound and
motor hum from far away.
The noise is uneffected by the 10dB pad on the mic body(the noise level
stays the same).
The noise goes away if the mic is not connected to the PSU.
The noise goes away is the tube is removed from the mic.

I don't have another 6DJ8 around, or I would have swapped it out.
Does anyone have a list of other likely culprits?

Thanks, Andy

apa wrote:

The noise is not quite hum and not quite white noise. It almost sounds
like a mic turned way up in a quiet room, picking up air sound and
motor hum from far away.

It's mostly low frequency rumble?

The noise is uneffected by the 10dB pad on the mic body(the noise level
stays the same).

Is this a capacitive pad?

The noise goes away if the mic is not connected to the PSU.
The noise goes away is the tube is removed from the mic.

I don't have another 6DJ8 around, or I would have swapped it out.
Does anyone have a list of other likely culprits?

I'd swap the tube out before doing anything else, but another very common
culprit is contamination and dirt causing leakage paths. A fingerprint
on the board can be a big problem sometimes with high-Z circuits like
this. Flux remover spray can help clean gunk off, but make sure it's
not the tube first.

It could also be gunk on the capsule, but if the pad switch does nothing,
my assumption is that the capsule is clean and the pad is a capacitive
one that shunts across the capsule.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Great. I'll get a new tube first and go from there. Thanks Scott.

Bill Ruys wrote:
"apa" wrote in message
roups.com...
Great. I'll get a new tube first and go from there. Thanks Scott.

I had a noisy tube mic too. I tried a few different tubes, but settled on
this one:
http://www.thetubestore.com/janphilips6922.html

It's a 6DJ8/6922, same as yours. It turned out to be the absolute quietest
and sounds very nice too. These tubes are NOS American made (circa 1980's)
and I think they are much better than any of the contemporary Russian or
Chinese tubes that are around in abundance these days.

You should know that currently there are a large number of NOS JAN6922 tubes
on the market which are rejects from a high-end audio manufacturer. At least
one microphone manufacturer has bought a palletload of the rejects. So do
watch out for a while until they work their way out of the supply chain.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

"Scott Dorsey" wrote in message
...
Bill Ruys wrote:
"apa" wrote in message
groups.com...
Great. I'll get a new tube first and go from there. Thanks Scott.

I had a noisy tube mic too. I tried a few different tubes, but settled on
this one:
http://www.thetubestore.com/janphilips6922.html

It's a 6DJ8/6922, same as yours. It turned out to be the absolute quietest
and sounds very nice too. These tubes are NOS American made (circa
1980's)
and I think they are much better than any of the contemporary Russian or
Chinese tubes that are around in abundance these days.

You should know that currently there are a large number of NOS JAN6922
tubes
on the market which are rejects from a high-end audio manufacturer. At
least
one microphone manufacturer has bought a palletload of the rejects. So do
watch out for a while until they work their way out of the supply chain.
--scott
--
That's the good thing about buying from a reputable dealer. The Tube Store
has a fantastic reputation with customers all over the world. They test
every tube for noise specs. I bought two of the JAN Philips tubes and they
are both *very* quiet.
http://tinylink.com/?E3DSB8vhzK

Bill.