MXL 9000 Exposed

Papa2face · Sep 15, 2005

I recently picked an MXL 9000 tube mic at Guitar Center. It is the same as the 76t. I reverse engineered the circuit and would like to share what I did. Here it is:

I haven't disected the power supply yet.
I assume some of the circuit here is to compensate for poor peformance from the capsule, tube, and transformer. If one were to upgrade the capsule, tube, and transformer, what parts of the circuit would need to be modified or removed?

Edited: Updated schematic.
Edited: Added voltage readings to schematic.

Kev · Sep 15, 2005

Welcome to Papa2face
good to see a little reverse engineering being done.

but I think I'll defer to one of the mic experts around here
to answer your question about component changes.

first it might be handy to know what you will use for an upgraded capsule, tube, and transformer

perhaps look to the Microphone META thread and perhaps a visit to Dave R's site.
I think he did an MXL mod.

Gus · Sep 15, 2005

interesting if your values are correct the 12at7 is running at a higher current than often used in tube microphones are both triodes in parallel?

This would make some sense of r8 and c6 values. The gain is the openloop gain of the tube in parallel with the capsule cap 55pf?/9pf. Maybe the output transformer is a low ratio like 2:1 or 4:1?

I need to look at the 12at7 load line.

What are C6,C5 and C3 brand and type of cap? That wuld be what I look at first. I have some other ideas

what is the B+ voltage?

Papa2face · Sep 16, 2005

Crap! All values except the 100m, 150m, and 1w resistors are off by a decimal place. These are 5 band resistors and I misread them. Disregard the schematic. I will update it asap.
Sorry.

The .1 and .01uF caps look like polypropelene. The polarized caps are electrolytic. 9pF looks like a some sort of foil.

Gus · Sep 16, 2005

install a NOS 12at7

try disconnecting c9 at the grid end Use a heatsink in case it is a polystryene. There are a few reasons

R7 looks to be on the high side I would expect 1.8K to 2.7K MXL might be shifting the tube away from the "normal" tube microphone bias point to change the harmonics, things change at loud levels at the ends

I need to think somemore about all the crap after the output of the transformer(r 20 and 21, C 20 and 21, L1 and 2)

If the transfomer is 6:1 or greater make c6 a 1uf polypro

Whats the B+?

Leave r5 for now it might work better with the transformer at 150meg. Increasing the grid R to 1gig is not always a good thing. Depends on grid leakage and low end stuff reponse with the transformer lams.

EDIT can we get a picture

Papa2face · Sep 19, 2005

I replaced the capsule with a similar one from Peluso. I swaped the tube with a NOS GE 6072. And I replaced the transformer with a Cinemag CM-2480 10:1. Very noticable improvment with the upgrades.
There was a drop in level with the new transformer so the old one was less than 10:1. I wonder what changes need to be made to the plate and cathode resistor values to bring the level back up.

I have added some voltage readings to the schematic.

Gus · Sep 19, 2005

Lift c3 the 9pf (I posted c9 meaning c3 before) at the grid

this will raise the level, you might even want to install the stock transformer again with this mod for a test. charge amp gain is about 55/9 is about 6. I would guess the stock transformer was about a 7:1.

other stuff to try

try a 2.2k to 2.7K for the cathode R (think higher B+, 1/2, of a AKG c24)

Then try the stock capsule again.

Gus · Sep 28, 2005

Papa2face

Any more information on your mods?

Papa2face · Sep 29, 2005

I haven't done anything else to it yet. I'm gonna try it for a couple weeks as it is currently. I might let some friends borrow it for thier projects. I'll try some of the changes you recommended. Would raising the value of plate resistor R8 give me an increase in gain?

The mic and the upgraded parts cost about $330. I'll have to compare it to mics of the same price range to see if it's worth the fuss to mod.

ohhey · Sep 29, 2005

[quote author="Papa2face"]I haven't done anything else to it yet. I'm gonna try it for a couple weeks as it is currently. I might let some friends borrow it for thier projects. I'll try some of the changes you recommended. Would raising the value of plate resistor R8 give me an increase in gain?

The mic and the upgraded parts cost about $330. I'll have to compare it to mics of the same price range to see if it's worth the fuss to mod.[/quote]

How much difference did just the tube make ? Can you describe what is better about the sound ?

Gus · Sep 29, 2005

Increasing R8 should not make any real kind of difference remember the load refected from the output transformer is part of the math.

The plate to grid cap is what is limiting most of your gain. If you want even more with that cap removed use a NOS 12at7 instead of a 6072a.

MXL 9000 Exposed

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