MK7 - tube mic project

[Bri]

Has anyone else had problems with the sound of the MK7?
I have two MK7 builds...different capsules, different tube manufacturers, different bodies.
When singing through them, they both have similar brittle, metallic, sizzling anomalies and sound...depending on volume, frequency, etc.

After troubleshooting a little, I concluded that it's EF86 tube microphonics.
The reason I concluded it was the tube, is that out of all the tubes I have(two Valvo's, two Mullards, one Bugle Boy, and one GE), the GE has a bit less of the problem.

Are EF86's really this problematic?
Is there anything else in the mic, like the coupling cap for example, that would cause such a thin, grating/tizzy character?
Just keep buying and trying different tubes?

I appreciate your thoughts.
Thanks,
Brian

 

[Rob Flinn]

Brian not sure if this helps with your problem, but it could be something to do with the capsule mount if it's overly toppy or lacking bass. 


Take a look here http://prorecordingworkshop.lefora.com/2012/05/15/problems-with-upgrades-of-conderser-capsules/

 

[Bri]

Thanks Rob.
I think I'm okay regarding leak current...well I hope I am anyway.
My capsule mount sits atop a quarter inch thick disc of Teflon.
The Teflon disc is connected to the metal plate using nylon screws.
Although, a noticeable increase in rubber smell does emanate from the capsule area when the mic is on.
Current leaking into the rubber post from the backplate?
The nature of capsule mounts?

--Brian

 

[Bri]

Two of my voltages are not ideal, relative to Max's schematic.
My heater voltage is 5.2v and my tube bias voltage is 1.8v.
Can this effect the sound of the mic...in a negative way?


Thanks,
Brian

 

[MagnetoSound]

Hi Brian,

I think your tube voltages are probably fine, but a slightly lower value cathode resistor should bring them to spec quite easily. What is the heater supply voltage at the connector, and after the 2R2?

After troubleshooting a little, I concluded that it's EF86 tube microphonics.
The reason I concluded it was the tube, is that out of all the tubes I have(two Valvo's, two Mullards, one Bugle Boy, and one GE), the GE has a bit less of the problem.

Are EF86's really this problematic?

The problem is, there are very few good ones left available that will be usable in such a demanding application - even for guitar amp use a lot of what is left now are rejects due to microphony.

Time was when PF86 were much cheaper and there were plenty to choose from, but the sellers have cottoned on now and prices are a bit steep IMO for what is, essentially, a TV tube.

The brands you have mentioned are general purpose types, not designed or guaranteed to be suitable for mic use. Look for low-noise variants - EF806S by Tesla can be very good, or the same tube by Telefunken if you can afford it.

If you just want to cut to the chase, there are companies offering graded tubes with money back guarantee for high end mics, but you will pay through the nose for the service - maybe worth it if it saves you a lot of time and hassle, though.

 

[Bri]

Thanks Dan, very helpful.

Yeah, I realized recently that all the used tubes I find being sold, were likely merely what others didn't want.
I will try and find a couple NOS Tesla's then.

Question.
To what extent is the effect of the particular tube being used?
I mean, I put in a Valvo last night, and my voltages changed relative the the GE I had in there.
Heater is now 5.7v(measured between pins 4 and 5), and the tube bias is 1.23v.
Quite different from the voltages when the GE tube was in.

Also, with this Valvo tube, the extreme rush of noise when changing polar patterns is back again.
So I guess that was tube related after all....seems to occur only on the nosier tubes though.

Is the particular tube really responsible for all of the voltage changes and switching polar pattern noise?

--Brian

 

[MagnetoSound]

Heater is now 5.7v(measured after the 2R2, and also between pins 4 and 5), and the tube bias is 1.23v.
Quite different from the voltages when the GE tube was in.

Heater resistance can vary somewhat from one tube to the next, and over time. Not enough to be an an issue in many circuits, but here - since the heater current is way dominant over the cathode current - the bias is forced up or down depending on the heater current.

Also, with this Valvo tube, the extreme rush of noise when changing polar patterns is back again.
So I guess that was tube related after all....seems to occur only on the nosier tubes though.

Is the particular tube really responsible for all of the voltage changes and switching polar pattern noise?

When changing polar patterns, there is a wide variation in voltage across the 1nF coupling cap. This cap has to either develop or release a charge across the grid leak resistor.

I am not certain as to why one tube may be more susceptible than another to this current transfer, but my guess is that the particular inter-electrode capacitance of the tube comes into play.

 

[Bri]

Thank you for the very illuminating and helpful information, Dan.
I have a couple Tesla EF806's on the way....
I'll report back then.

Is it normal for the capsule(front) to be significantly louder and more bassy when in figure-8 pattern as opposed to when in the cardioid pattern?
Is this merely the variation in capsule manufacturing?

My K7 exhibits this effect, yet my Dale M7 capsule(front side) sounds virtually identical in both figure-8 and cardioid.
It's unfortunate because my K7 capsule(the front) is at its best when in figure-8 pattern.
I'd like to have that same full sound when in cardioid pattern.

Also, regarding the K7, when in figure8, the rear of the capsule is mostly mids, "overly toppy", not too usable.
Yet the front and rear of my Dale M7 capsule sound very similar in figure8 pattern.
Same ideal voltages for each mic.

Just wondered if this is the nature of the specific capsule, or perhaps something else.

Thanks,
Brian

 

[0dbfs]

Front and back should be very similar in my opinion. Fig 8 does have more proximity effect though. I made two of these and used pf86's. Purchased five and the first two had no issues. I was expecting more rejects.

If the front/back of the K7 are that different Ben might exchange or reskin. Did that one come from the GB? I would get in touch to query anyway. He would know.

Cheers,
jb

 

[Bri]

Congrats on your builds JB.

If the front/back of the K7 are that different Ben might exchange or reskin. Did that one come from the GB? I would get in touch to query anyway. He would know.

This is what is confusing to me in regards to what would cause a capsule to behave this way.
I'm hesitant to bother Ben about it and then ultimately find out that it has something to do with my build.
Yes, group buy.

What makes me hesitant is that when I switch the leads, whichever side is the front at that time is full and loud, and whichever side happens to be the rear of the capsule is attenuated and mostly upper mids.
So, the anomaly doesn't follow one specific side of the capsule, physically.
It's whatever side happens to be "front" and whatever side happens to be "rear."
Front always good, rear always bad.
I'm primarily talking about fig8, but it is similar in omni.

My backplate voltage is 61v.
The pattern voltage in omni is 119v.
Caps and resistors test correct, etc.

Does this still sound like a capsule function anomaly?

Thanks,
Brian

 

[MagnetoSound]

I am suspicious that your 1nF might be leaky. A few meg resistance across this cap could cause the effect you describe. Did you change it?

 

[Bri]

Actually no, I didn't change it.
I was just about to, had my spares out...
I removed it from circuit and tested it.
Since it read 1nf exactly, I assumed it was still good.

So you're saying even though a cap reads the correct farads on a meter, there can still be a problem with it?
I was suspicious about such things, but since I don't know all the inner workings of a capacitor, I figured I was likely creating fictitious, paranoid possibilities.

I assume you're saying the cap can very well act much different in circuit than when being tested by a meter?

If so, I will be changing that cap today.

--Brian

 

[MagnetoSound]

Yes, capacitance and resistance are two different things. Unfortunately, a resistance leak may not show up in a capacitance test.

Not saying this cap is definitely the problem, but you have to eliminate the possibility, as the symptoms seem to fit.

 

[0dbfs]

Would be worth measuring resistance on the 1nF and replacing/re-testing. I agree though that if the issue does not follow the physical side of the capsule and rather follows the circuit front/rear then it's probably not the capsule. Interesting though that you find the issue significantly less pronounced with the m7 over the k7.

May also want to check the rear capsule polarization portion of the circuit if swapping the 1nf does not localize the fault.

Just curious:
If the 1nf is "leaky" would it potentially pass DC to the grid ckt that would also affect bias point depending on currently-selected rear polarization value? ie; observed discharge noises when switching patterns.

Cheers,
jb

 

[pasarski]

Is it normal for the capsule(front) to be significantly louder and more bassy when in figure-8 pattern as opposed to when in the cardioid pattern?
Is this merely the variation in capsule manufacturing?

My K7 exhibits this effect, yet my Dale M7 capsule(front side) sounds virtually identical in both figure-8 and cardioid.
It's unfortunate because my K7 capsule(the front) is at its best when in figure-8 pattern.
I'd like to have that same full sound when in cardioid pattern.

Also, regarding the K7, when in figure8, the rear of the capsule is mostly mids, "overly toppy", not too usable.
Yet the front and rear of my Dale M7 capsule sound very similar in figure8 pattern.

Have to ask the obvious, how did you test? Same procedure with both capsules? Just want to point out that if you're speaking to the mic with cans on rear will sound different, but you probably knew that already.

 

[Bri]

I just went down to my studio to troubleshoot.
I was going to test a few things relative to your helpful comments, before I replaced the 1nf.
I've had the mic on for about four days now.

Well, I put my headphones on and noticed the overall noise level was curiously lower than yesterday.
I start changing polar patterns, and was startled...there is suddenly no rushing or crackling noise.
I tested the capsule rear attenuation problem in Fig8 mode, and that problem is gone as well.

BTW, pasarski, yes I was using headphones and singing into the mic.
And no, the phase issue would never have occured to me, so thank you for bringing that to my attention.
I did experiment with the phase in and out, just now, but the differences were subtle(less bass and fullness) compared to the initial, significant attenuation problems I've been troubleshooting.

So I seem to have an intermittent problem then.

Is the 1nf capacitor supposed to be microphonic?
I was tapping on it, hoping to get it back to being problematic, and was surprised how incredibly microphonic it is.
I thought maybe it was just vibration transmitting up to the capsule thru the connected capsule wires.
However tapping the 1nf very lightly is much more microphonic than tapping the individual capsule wires moderately.
Is this normal?

I'm going to go change it now anyway.

Thanks again for all your helpful information and advice.

--Brian

 

[0dbfs]

Is the 1nf capacitor supposed to be microphonic?
I was tapping on it, hoping to get it back to being problematic, and was surprised how incredibly microphonic it is.

I don't have a specific answer for you without testing the theory on a couple mic's but the capsule itself is a capacitor connected directly to the grid too and it's probably considered to be slightly microphonic.

Cheers,
jb

 

[MagnetoSound]

Some caps are more microphonic than others. (Terrible envy problems. ;D )

A polystyrene cap (hopefully what you have here) will make some noise when you flick it, ceramics much more so. As jb says, this part of the circuit is prone by nature.

 

[gevermil]

anyone have some extra 1nf and 10nF 160V styroflex caps ?

or , know where I can find em ?

thanks
gart

 

[loopermc5]

Hello all so my heater voltage is fine with no tube as soon as I plug in the tube poof it's gone I have tested the tubes they are fine checked and rechecked the psu and mic pcb what could be the causes for this? it's all so simple and yet i can figure it out any hints would be great as my head hurts at this point.

thanks?