MXL 9000 hum & mod

[Matt Nolan]

The distortion would be really high especially on hot sources.

Maybe not. That 9pF feedback cap is going to bring the gain down a fair bit.

 

[0dbfs]

9pf also reduces output impedance so would be interested to understand transformer windings.
Looks like the capsule backplate see's about 65V.
Any info on what R2/C2/R6 setup does?

Thx!
-jb

 

[Matador]

Maybe not. That 9pF feedback cap is going to bring the gain down a fair bit.

I don't think feedback can correct the gain curves down at those low currents (down at the bottom of the plate curves, where the transconductance changes rapidly at each operating point.  At some point, you have to assume linearity of the gain element inside the feedback loop.

 

[ForthMonkey]

Let's select tube! My friend sent me three tubes.

Siemens ecc81
RCA NOS 12ax7
Philps Miiwatt Nos ECC82

Which one is good for mic? Any idea?

 

[Matt Nolan]

The first one. The 12ax7 isn't ideal for mics and is perhaps too high gain. The ECC82 is too low gain. But from a selection of only 1 each, none of them may be any good.

Let's select tube! My friend sent me three tubes.

Siemens ecc81
RCA NOS 12ax7
Philps Miiwatt Nos ECC82

Which one is good for mic? Any idea?

 

[ForthMonkey]

The first one. The 12ax7 isn't ideal for mics and is perhaps too high gain. The ECC82 is too low gain. But from a selection of only 1 each, none of them may be any good.

Thanks.I was thinking like you.

This mic has 12AT7(ECC81).I'm thinking to use itI guess Siemens 12AT7 is better than stock MXL tube.Right?

 

[ForthMonkey]

Painted body.Photo added to first post.

Now waiting new tube and PSU caps.

 

[ForthMonkey]

Changed PSU caps and hum's gone!