MK7 - tube mic project

[ioaudio]

I just measured my two finished modded psu:s and im getting these voltages:

HT: I can trim between 5.9-8v aproximately, so that seems fine.

B+: 145v

Pattern: 112v, 55v, 0v measured directly on the switch were i mounted the voltage divider resistors.

Is that ok voltages?

I just hope that ill have better luck than IMO with my modded psu:s, since im building 4 mics.

/J

loaded?

 

[imo]

Thanks for those suggestions. I am quite happy with the mic, but in the spirit of gaining knowledge, trying to understand more about the various components involved. I am limited for the time being with this capsule (outside of the chinese capsule that came with the mic). I will definitely try the bigger resistor on the grid. How would this affect the low end of the mic? Is it a filter formed with the capacitance of the tube?
I am curious to know what it is with the VF14 that allows so much low end through. The 47's i have sang through have a ton of it, and not in a modern way. Is it the lower plate resistance going into the BV8? I guess i am wondering if using the EF86 with the BV8 tends to have a basic quality that is what it is and doing things like increasing the output cap (which in my case tends to lose the focus of the mids) is simply muddling up a design that is optimized as it is
Ian

 

[Jonkan]

All my voltages are unloaded.

I still havent got my capsule mounts, but the mike amplifier are all finished. -Sorry if this may be a stupid question, but can i measure the voltages without the capsule attached to get an idea of what the loaded values are?

/J

 

[ioaudio]

alternatively use a 150 volts / 0,0008 ampere = 187500 Ohm load resistor
no harm done when running the mic without the capsule (except hum like hell)

ian ,
just as a note, the bv7 used in the mk7 is not the same as a bv8. it´s primary inductance/impedance is higher to match the different tube used.
but there are many more tubes which play nicely with the transformer. i just want to encourage to try different things.

 

[ubxf]

Is E80F a good candidate?
thanks
francois

 

[Jonkan]

Today i finished two of my four mics (i finally got the capsule holders ive been waiting on), and also two psu units. Ive tried both mics with both psu units, and the mics pass sound.

One of them is very noisy, but I attribute that to the tube i have in it (i just threw in two leftover ef86 tubes that i had lying around at home).

Now heres the problem, both of them have a small hum to them when the psu is turned on. This happens with both of my psu:s, and both mics. The psu:s are right now being run without the chassis lid, but that shouldnt matter, right?

If i turn off the psu i can hear how the mic sounds for a couple of seconds without hum until the caps have discharged. Btw, they sound great these few seconds!!

Any ideas on how i could solve this? Preferably without having to rebuild the whole psu. Where do i start looking for errors?

Ioaudio, did you have to make any mods regarding to grounding schemes anywhere..Like in the cable, mic, psu etc?

The heater voltage coming out of the psu is a steady 7.6v under load as stated on your psu mod schematic
b+ =145v
pattern: 111.5v, 54v, 0v

Should i measure for ripple on the heater?

Any ideas on how to proceed is greatly appreciated!!
Thanks!
/Jonas

 

[Jonkan]

Update: I tried setting my multimeter to AC, and took measurement over the heater supply output during load.

The meter says i have 16v AC on my heater.

That cant be right, right?

I have a very cheap meter, so im not entirely sure that is working correctly...But atleast it could be an indication that something might be wrong here, or?

/J

 

[rodabod]

I wonder if it migth be worth AC-coupling your multimeter with a cap? CAn you test it with a DC source (eg. a battery or something?)

 

[Jonkan]

I just tried measuring ac from a known working "storebought" DC supply at 12v and my meter says 12.3v DC, 33v AC....so im thinking it might be broken.

The DC measurements seem to be working though.

/J

 

[rodabod]

Measure the B+ ripple, preferably with a 'scope.

 

[Jonkan]

I actually do have a scope, but I have never used it, so im not totally sure how to do it.

Last time i used an oscilloscope was about 10 years ago in school.

Is it allright to just connect 150v to it?

What should i look for on the screen?

Any tips are appreciated!
/J

 

[Jonkan]

Ok, so ive read up a little on oscilloscopes. Still a bit confused as i couldnt get my probe compensated properly...or more like the square wave output of the calibration signal had square edges however i set the probe.

Anyway.

I measured on two places now. First the B+ from the output of the psu pin 1, mic connected. Scope set to AC, 10mv/division with X10 MAG button activated (which if I understood correctly makes each division mean 1mv, right?)

Secondly the heater from the psu pin2, also with mic connected and the same settings on the scope.

Heres a pic of my results:

 

[rodabod]

Hi Jonkan,

Well, if as you see we are seeing effectively 1mV per division, then it doesn't seem that bad, particularly the heater.

It would be easier if you could get the trace a bit larger in maginitude; do you have any division smaller than 10mV? Also you didn't mention the "timebase". How many milliseconds per division are we at? I'd be trying something like 10mS/div or so. Whatever gives a good readable trace.

 

[Jonkan]

10mv is actually the smallest on my scope.

I think i had like 1ms/division on these pics. Will try another measurement when i get home with something bigger and post that. I think that when i used a higher value, the thing just looked like a straight but blurry line really.

Any ideas on where i should measure in the mic if it turns out the psu is actually working correctly?

Another question i was hoping you know the answer to: "On" my V per division knob theres also a "variable" adjust that has a "CAL" setting. Is it correct to keep it on the CAL setting for the measurement? What does this knob do?

Theres also the same type of knob for the timebase, that i used to make the lines stop moving horisontally on the screen.

Thanks for helping me!
/J

 

[MagnetoSound]

Theres also the same type of knob for the timebase, that i used to make the lines stop moving horisontally on the screen.

Thanks for helping me!
/J

Hi Jonkan,

You may be able to lock the trace to the input, rather than trying to adjust for zero movement all the time.

If your scope has a 'sync' or 'trigger' selector, try setting that to whichever X-input you are using and see whether that stops the scrolling. You might need to adjust the knob you were just talking about to find the 'locking' point.

Best regards,

Dan

 

[dkelley]

I bet it would sound even better if you remove the mesh and stuff like that!!!



figured I'd have fun with one of my first posts on this site (I'm a lurker normally)...

cheers all

Don

 

[Jonkan]

Did a complete measurement of my voltages inside mic and psu:

The values first stated in parantesis is the ideal values, then

after that are my actual voltages.

mic voltages:

(1.3v) pin 7+2+3+4= 1.27

(5.7v) between pin5 and pin 7+2+3+4= 5.67v

(6,95) heater after 2.2r= 6.95v

(7.35) heater before 2.2r=7.39v

B+ after 20k resistor= 132v

(60v) between 910k and 1m (capsule polarisation voltage)= 42v

Should i lower the 20k resistor where the B+ goes into the mic, or perhaps redo the divider network?

Im very bad at calculations, but i got around 800k for the first resistor in the network to get it to 60v instead of 42 v. which would make the other resistor 1m1. Is that correct?

How picky is it that the total resistance for the network is 1m+910k ohms? I cant really find 1m1 resistors, so can i just move the 1M resistor to the lower part of the network instead?


Pattern voltages before 33m resistor= 104.5v, 51,4v, 0v


Psu voltages:

(150v)= 149v
(7.6v)heater output= 7.6v
pattern (120v,60v,0v) measured on lorlin switch before 100k resistor= 114.4v ,56v


What do you think?
/Jonas

 

[Jonkan]

I have an idea whats causing the hum, but i cant test it until i get my pf86 tubes (using ef86 right now)

In my psu theres less than 3volts between output and input of the lm317t regulator. According to datasheets the regulator wants to see atleast 3v difference.

When using the pf86 tubes there will be a little more than 3v difference since the heater voltage is lower.

Maybe if that doesnt help i can use some low drop schottky diodes like Gus suggested. What current rating is recommended?

Im hoping this is it anyway, so that i can begin using these badboys! first impression is that they sound awsome except for the hum.

Just have to figure out how to make the tbone mics look a little prettier also.

/J

 

[ioaudio]

hi

>> In my psu theres less than 3volts between output and input of the lm317t regulator. According to datasheets the regulator wants to see at least 3v difference.

that would be good. there´s a wide veriety of power transformers out there, and you cannot predict voltages though they have the same specs except you buy a proper transformer. sometimes we are not lucky and get a TRAFO which spits out less than the average, and we have to use tricks like

>>  low drop schottky diodes like Gus suggested. What current rating is recommended?

what do you guess?

>> When using the pf86 tubes there will be a little more than 3v difference since the heater voltage is lower.

but the current is higher.

>> Just have to figure out how to make the tbone mics look a little prettier also.

me too, absolutely. cheap awful bastards.

 

[pH]

Fill in the logo with Bondo and spray the main body with Hammerite.