Official C12 Clone - Build and Support Thread

[59flame]

Got it working sounds great and quiet too! thanks sax monster i appreciate the info.

 

[Matador]

I tried playing with the headroom of the amp circuit by grounding the grid and watching the output on my scope... funny thing is that the bias is having no influence on the chopped wave form (positive half, do to +/- flip flops throughout?)

The output wave looks nice at real world levels.  The top half of the wave is chopped when I increase the signal source.  I remember another member had the same results. 

Have not been able to get a nice symmetrical clipping.

Now I'm only doing this to fine tune.. the amp circuit is "working fine" (or is it?)...  can anyone verify, with pictures, a symmetrical clipped waveform on the output side of the big cap and at what input voltage etc... thanks guys!

You should be able to pump over 1V peak into the tube and get a clean sine wave:  if you can, then all is well.

How large of an input signal can you inject?  You may need in the 3-4V PTP range before you can start seeing these effects.

 

[MicDaddy]

See photos.. sorry I should have adjusted for better resolution on the first.

Note the clipping happens on the positive half of the waveform

edit:  was the "clipping often happens negative half of waveform" meaning the input signal negative half of cycle?  So on the output we see the top half clip?

 

[Matador]

A (few) questions:

1) What is the cathode DC voltage after biasing?
2) What is the DC plate voltage after biasing?
3) Which tube?
4) What is the bias voltage set to?
5) Where are those scope shots measured at?  Are they right at the plate with your scope set to AC coupling?

 

[Mr.GrinchSD]

Ok so lets see if someone can help me I am using the cinemag transformer I have triple checked all connections everything seems good but I get quite a loud hiss in the mic and when changing polarity it will make a bunch of loud noise before finally settling on the correct pattern and if figure 8 mode the back capsule is defiantly darker, been trouble shooting for about 2 day's now AHHHHHH using a very nice capsule and also electro harmonix tube???

Any ideas

Hard ground the grid with a small temporary jumper wire to make sure you are debugging the right thing.  With the grid hard grounded, only tube self noise should be coming out.  If it's still hissing, then you need to look elsewhere.

Ok I hard grounded the grind wire and now it is quite as a mouse, meaning I hear no noise at all not even signed coming through, so what next?

 

[MicDaddy]

1) cathode is 1.1V wrt P4/bias
2) 120V
3) 6072A GE
4) -1.1V
5) Ch1 right at signal injection grid side of c13, Ch2 right after c12.  (All testing conditions with microphone completely assembled; capsule, transformer, less the body tube)
6)After further poking I did notice that my heater was waaaay low of 6.3, since I've bumped the heater up and dialed in B+, bias, now we are clipping on the negative half of the waveform after c12 at ~60+Vpp which is almost 3Vpp on the input.  I can hear the distortion at these insane levels but haven't looked through an FFT.

I won't try to reproduce the problem, especially on this tube.  Maybe tomorrow throw an EH in there and under-heat and see if I can reproduce.

 

[andyfromdenver]

Just found a super cheap donor, giving this project the big green light
Just wanted to say thank you thank you thank you to Matador, Chunger, AKG, and many others for making this possible
About to head to Chunger's shop.
First order of business me thinks (see pic)
8)  aww yeah

 

[Matador]

6)After further poking I did notice that my heater was waaaay low of 6.3, since I've bumped the heater up and dialed in B+, bias, now we are clipping on the negative half of the waveform after c12 at ~60+Vpp which is almost 3Vpp on the input.  I can hear the distortion at these insane levels but haven't looked through an FFT.

Interesting!  It's hard to get symmetrical clipping on a tube, since it will happily run with the grid a bit more positive than the cathode.  So the positive signal swing on the output will always hit the rail before the input signal swing runs out of headroom.

However the scope picture you posted looks about right:  you will have rounding of both the upper and lower halves of the output signal when biased properly.  A 3V peak input level is quite hot in any case.  8)

The load lines at http://www.trioda.com/tools/triode.html suggest that running at a slightly more negative bias might help improve distortion (perhaps -1.5V?), but the closest tube they have is the ECC82 in their charts.

 

[saxmonster]

Just found a super cheap donor, giving this project the big green light
Just wanted to say thank you thank you thank you to Matador, Chunger, AKG, and many others for making this possible
About to head to Chunger's shop.
First order of business me thinks (see pic)
8)  aww yeah

Where did you find it at?  Whats badass, hahahah

 

[Mr.GrinchSD]

I hard grounded my mic and its quiet as a  mouse, no noise comes through at all, it's sounding good but when I have it powered upland I change the polarities it makes a bunch of noises a lot of air, sometimes even when in Cardioid it will make a few noises AHHHH also when in figure 8 both sides sound different BLEH I fell like I'm almost there but not quite I really want to get this mic working perfectly and start some recording ALEADY haha but I know these thing;s take patience. SOOO any ides I have checked all wiring 3 times and also cleaned all the flux off very well, the one thing is when installing the left 5000pf on the back I slightly burned the side with my solder iron (DUMMY) I know those are sensitive but just barely it still reads 5000 and I don't think that would be causing the problems I'm having .

 

[Matador]

I hard grounded my mic and its quiet as a  mouse, no noise comes through at all, it's sounding good but when I have it powered upland I change the polarities it makes a bunch of noises a lot of air, sometimes even when in Cardioid it will make a few noises AHHHH also when in figure 8 both sides sound different BLEH I fell like I'm almost there but not quite I really want to get this mic working perfectly and start some recording ALEADY haha but I know these thing;s take patience. SOOO any ides I have checked all wiring 3 times and also cleaned all the flux off very well, the one thing is when installing the left 5000pf on the back I slightly burned the side with my solder iron (DUMMY) I know those are sensitive but just barely it still reads 5000 and I don't think that would be causing the problems I'm having .

Ok, if it's quiet with the grid grounded (and you can hear a ping when you rap on the side of the tube with a pencil erasure), that tells you that the self noise of the tube (and everything downstream) is ok.

I would start by forcing the mike to cardioid only and remove the polarization of the rear diaphragm  from consideration.  Leave FC connected to ground, make sure FB and RB are tied together and are going to the backplate connection on the capsule.  Completely remove the connection to the RC from the PCB or the capsule.  This will leave the mike in cardioid only mode.

If that sounds ok then we can go from there.

 

[Tim Campbell]

Grinch check your PM,s.

 

[Tim Campbell]

Noise when switching patterns can mean contamination  in or on the capsule. Differences in tone between halves could be phase cancellation due to monitoring with headphones, a poor connection or badly matched capsule halves.

 

[Mr.GrinchSD]

I hard grounded my mic and its quiet as a  mouse, no noise comes through at all, it's sounding good but when I have it powered upland I change the polarities it makes a bunch of noises a lot of air, sometimes even when in Cardioid it will make a few noises AHHHH also when in figure 8 both sides sound different BLEH I fell like I'm almost there but not quite I really want to get this mic working perfectly and start some recording ALEADY haha but I know these thing;s take patience. SOOO any ides I have checked all wiring 3 times and also cleaned all the flux off very well, the one thing is when installing the left 5000pf on the back I slightly burned the side with my solder iron (DUMMY) I know those are sensitive but just barely it still reads 5000 and I don't think that would be causing the problems I'm having .

Ok, if it's quiet with the grid grounded (and you can hear a ping when you rap on the side of the tube with a pencil erasure), that tells you that the self noise of the tube (and everything downstream) is ok.

I would start by forcing the mike to cardioid only and remove the polarization of the rear diaphragm  from consideration.  Leave FC connected to ground, make sure FB and RB are tied together and are going to the backplate connection on the capsule.  Completely remove the connection to the RC from the PCB or the capsule.  This will leave the mike in cardioid only mode.

If that sounds ok then we can go from there.

Ok i was wrong with grid grounded if i turn it up high enough to hear the ping in my head phones with the tap of a pencil there is still bit of white noise coming through so is the normal?

 

[Mr.GrinchSD]

I did what you said remover the RC an mic sound good it did have a few popping noises like a db increase once or twice when i fired it up then it seemed to stop but still has white noise?????

 

[Mr.GrinchSD]

Ya it will make a noise as if air were hitting the capsule, but only i the beginning it seems then the mic sounds really good besides the (white noise)

 

[Matador]

Ya it will make a noise as if air were hitting the capsule, but only i the beginning it seems then the mic sounds really good besides the (white noise)

Hard to say if normal:  tube mikes aren't always the quietest.

Can you post a short clip, along with how much gain is applied during the recording?  Some tube blow noise is fairly normal.

 

[ciscan.81]

when I have it powered upland I change the polarities it makes a bunch of noises a lot of air

Hi Mr.GrinschSD

My recently completed mic makes similar noises when changing patterns (a kind of soft ticking/popping whooshy type sound lasting around 5-10 seconds - different to tube noise).

After burning in the 6072 for a couple of days the mic is otherwise very quiet and sounds great.

I assumed the pattern-changing noise was a cap or something charging to the new voltage...it only lasts a short time and doesn't really bother me.

Should it? Is it normal? Anyone else getting it?

Waiting a few seconds between pattern changes for the mic to settle I can live with provided it isn't an indicator of some other more sinister problem lying in wait!

Thoughts?

 

[Matador]

Are you saying that the sounds/noise only happen during the transition between patterns, and that the mike sounds normal after it settles?

The RC time constant on the pattern supply is quite long:  20-30 seconds if memory serves.

 

[Mr.GrinchSD]

No after it settles in figure 8 both side sound very different