MXL 9000 hum & mod

[ForthMonkey]

Yes,i checked and it's humming around 100 Hz.So this hum coming from PSU?

By the way it was humming before mod,too.

 

[ForthMonkey]

I opened PSU and found this cap!

10000uf 16V

There are two 10000uf 16V caps.I guess they are filtering but one of them is dead.

 

[Khron]

Well, i was close - it WAS, after all,  the heater supply that was the issue

 

[ForthMonkey]

Well, i was close - it WAS, after all,  the heater supply that was the issue

I will order new 10000uf 16V cap and change.

Now waiting for Siemens 12AT7.My friend will send me.It's way better than stock tube,right?Will effect sound?

 

[Khron]

If possible, try to make sure the new cap is rated for 105C. In fact, just to be on the safe side, i'd replace both of them.

 

[ForthMonkey]

If possible, try to make sure the new cap is rated for 105C. In fact, just to be on the safe side, i'd replace both of them.

What happens if i change it with smaller value caps?I have no 10000uf right now.I have to order and wait around a week.What if i use 470uf?There is big difference but i just want to know...

 

[Khron]

470uF is about 20 (that's TWENTY) times LESS than 10000uF, isn't it? Do you really think that wouldn't make a difference?

If you had said 4700uF, that would've been a bit more... realistic

http://www.changpuak.ch/electronics/power_supply_design.php

Trafo sec = 8v / 0.5A
Regulation = 10%
Load = 30 ohm (for the worst-case 300mA draw if both 12A_7 heaters are powered)
Storage cap = 470uF / 10000uF / 20000uF (this last one because there are two in your power supply, most likely in parallel)

See if and what differences show up, particularly on the red trace

 

[ForthMonkey]

470uF is about 20 (that's TWENTY) times LESS than 10000uF, isn't it? Do you really think that wouldn't make a difference?

If you had said 4700uF, that would've been a bit more... realistic

http://www.changpuak.ch/electronics/power_supply_design.php

Trafo sec = 8v / 0.5A
Regulation = 10%
Load = 30 ohm (for the worst-case 300mA draw if both 12A_7 heaters are powered)
Storage cap = 470uF / 10000uF / 20000uF (this last one because there are two in your power supply, most likely in parallel)

See if and what differences show up, particularly on the red trace

Got it,man.  ;D Just wanted to know...

I will go with 10000uf.And will change both of them.

Thanks!

 

[Khron]

That's kinda why i'm not a big fan of "spoon-feeding" ideas and advice for mods and stuff - it's often important to know the "why"-s behind doing certain things, not just the "because"-s

Or rather, it's kinda like that saying, "Give a man a fish and you will feed him for a day; teach a man to fish, and you've fed him for the rest of his life", or something like that

That, and i'm the skeptical / cynical type ;D But when it comes to cold hard technical facts... No, blind following random internet ideas is not the way, imho

 

[ForthMonkey]

That's kinda why i'm not a big fan of "spoon-feeding" ideas and advice for mods and stuff - it's often important to know the "why"-s behind doing certain things, not just the "because"-s

Or rather, it's kinda like that saying, "Give a man a fish and you will feed him for a day; teach a man to fish, and you've fed him for the rest of his life", or something like that

That, and i'm the skeptical / cynical type ;D But when it comes to cold hard technical facts... No, blind following random internet ideas is not the way, imho

You are absolutely right! But as i said i'm not tube and mic guy.Even i'm afraid of high voltage but i get that mic for cheap and then searched about mod.Tried some mods.For now i'm happy.Waiting for caps and new tube.Then i will see what's happening after new caps and new tube.

 

[0dbfs]

Good find on the swollen cap and seems like a likely culprit! Gotta wonder why it blew. Can you measure the voltage at and around that node with mic connected just to confirm that it's well rated  @ 16V?

Maybe it was previously set to a 120V line and somehow ended up on a 220V line? If so you may want to also check and/or pro-actively swap the B+ filter caps too - since you want to get it working and keep it that way If you're ordering parts that is...

Cheers!
-jb

 

[ForthMonkey]

Good find on the swollen cap and seems like a likely culprit! Gotta wonder why it blew. Can you measure the voltage at and around that node with mic connected just to confirm that it's well rated  @ 16V?

Maybe it was previously set to a 120V line and somehow ended up on a 220V line? If so you may want to also check and/or pro-actively swap the B+ filter caps too - since you want to get it working and keep it that way If you're ordering parts that is...

Cheers!
-jb

Should i check caps when PSU is on?  : I don't want to work with on when it's on...Just changing components when it's off.  

I ordered 10000uf 16V caps.After replacement i will see result.

 

[Khron]

What brand and/or series are/were those caps? Photos of the markings would help.

 

[0dbfs]

Changed R7 to 2k2 and increased the level!

I'll comment on this -

The note on the schematic at the cathode node shows 3.38V. What is it now with the 2k2 value R7?
You should also measure the voltage on both sides of R8 (150k Rp) so you can figure the voltage drop before and after your 2k2 mod...

The cathode resistor sets the tube operating points current and as a result also the voltage drop across Rp and therefore the plate voltage....

That 3.38V (pre-mod) on the cathode also means that the grid (input) is sitting at -3.38VDC with respect to the cathode... If you add your AC (audio) from the capsule you can get an idea of headroom / etc through the tube..

I would tend to want to add some tech notes and measurements to your empirical observations about "how good it sounds"... Send a signal into the circuit at various levels and graph the output as you implement your mods...

And spend a lot of time listening - if it's "better" that's all that really matters but that's also subjective so measuring gives us some basis in what we might think we're hearing...

Cheers and good luck!
-jonathan

 

[ForthMonkey]

What brand and/or series are/were those caps? Photos of the markings would help.

Xiahui brand and CD110 series i guess.

 

[0dbfs]

Should i check caps when PSU is on?  : I don't want to work with on when it's on...Just changing components when it's off.  

I ordered 10000uf 16V caps.After replacement i will see result.

Please be careful and follow regular procedures for measuring voltages in gear powered on. ie; one hand behind your back, probe in your right hand (not left). Use clips to connect GND.

If you;re not comfortable with it then don't do it. Read up on it a little because it is helpful and safe if you follow some standard testing procedures..

Edit - Tube mic's do have high voltages but very low current. Still need to always be safe but guitar amps have much higher voltages and currents so they are very dangerous.

 

[ForthMonkey]

I removed 10000uf 16V caps.

I checked bad cap and good one with multi-meter.Touched with multi-meter pins to positive and negative legs and i charged it and then reversed pin and multi-meter beeped with both of them.Looks like both of them are ok...But i removed them to install new caps.Waiting for them.

 

[Khron]

Good find on the swollen cap and seems like a likely culprit! Gotta wonder why it blew. Can you measure the voltage at and around that node with mic connected just to confirm that it's well rated  @ 16V?

Xiahui brand and CD110 series i guess.

I rest my case

 

[Matador]

I'm not sure I understand why MXL designed around a high transconductance tube, then run it starved at 0.5mA.  Especially when you can easily get several milliamps from a passive plate supply.  The distortion would be really high especially on hot sources.

 

[Matt Nolan]

I'm not sure I understand why MXL designed around a high transconductance tube, then run it starved at 0.5mA.  Especially when you can easily get several milliamps from a passive plate supply.  The distortion would be really high especially on hot sources.

Tube mics are typically run around 0.6 to 1mA - mostly for noise optimisation, I think. 0.5mA isn't far from there, especially with a slightly under-heated tube.