Official C12 Clone - Build and Support Thread

[DDay]

Thanks for the advice! I can't seem to find the 500n output cap on the BOM though.. :-[
Or the polarity switch. Apologies in advance for being such a novice.

 

[ln76d]

http://www.uf-nf-pf.com/

Use converter and ook do you still don't see it.
Since i wrote it's at the output maybe you could look at the output?
Man! Use your brain - it doesn't matter are you novice or not

 

[DDay]

Yes I saw it on the PCB, it's just strange C12 isn't in the BOM (http://www.musicalsparks.com/images/c12/c12_bom.pdf) but I guess neither is the output transformer or capsule. Thanks for the heads up on using an MP.

 

[Matador]

Yes I saw it on the PCB, it's just strange C12 isn't in the BOM (http://www.musicalsparks.com/images/c12/c12_bom.pdf) but I guess neither is the output transformer or capsule. Thanks for the heads up on using an MP.

Wow, I can't believe I left that one off.    I have a new version of the BOM, but have been too busy (aka. too lazy) to upload it.

Also, to answer the "don't use polyprop for C12...": it's like saying don't put salt on your food.   Many have built with the Erse polyprop in C12 and have been very happy.  I suggest you try out a few and pick the one that sounds best to you, and they aren't too expensive so you can try several.  Also try several values:  a few have stuffed up to 2uF for C12 with good results.

 

[ln76d]

Yes I saw it on the PCB, it's just strange C12 isn't in the BOM (http://www.musicalsparks.com/images/c12/c12_bom.pdf) but I guess neither is the output transformer or capsule. Thanks for the heads up on using an MP.

Wow, I can't believe I left that one off.    I have a new version of the BOM, but have been too busy (aka. too lazy) to upload it.

Also, to answer the "don't use polyprop for C12...": it's like saying don't put salt on your food.   Many have built with the Erse polyprop in C12 and have been very happy.  I suggest you try out a few and pick the one that sounds best to you, and they aren't too expensive so you can try several.  Also try several values:  a few have stuffed up to 2uF for C12 with good results.

It happens. I made once album graphics for CD release where two songs titles were in different order and it was released that way Shit happens

About the polypropylene - of course it's an obvious oversimplification
From the other hand most people will find "use polypropylene only" type information.
Someone which want to build microphones not only because it's cheaper and he "have to", to get tool only - will probably use his brain and look for his own way
Many people are here to build few things and bye then - so they wouldn't look for anything more.
My - Don't use MKP/polypropylene at the output! - is for those lazy guys which will probably choose between use or not use

Testing parts is one of the most important things - so of course i  subscribe at that with both hands!
My beloved method is rotary switch with few capacitors and making A/B comparison by listening and measurement (if possible).
From my experience, especially with small sized transformers, oversized capacitors and polypropylene (tested many from chinese to audiophile market parts) overall sound worse - it's not even related with low end response and its corner but full usable spectrum. Best option which i found (whatever type of microphone it's) are MP caps - and this is not related to any historical things. Of course you can find some MP which will degrade the sound due to out of specs - which is related with the age of these parts.

 

[DDay]

Cool, thanks Matador. Any chance you could put the lorlin rotary switch in there as well, not sure the correct one to get.

 

[Matador]

Cool, thanks Matador. Any chance you could put the lorlin rotary switch in there as well, not sure the correct one to get.

The Mouser part # is 10WA364 (Lorlin 1 pole, 12 position).  The Lorlin part # is CK1454.

 

[patterson]

I would be interested in seeing if there is a difference with a MP cap.  Is there anyone who has a source for these? the C-12 cap in the kit is over 600v. rated.  What would be the minimum reasonable voltage rating.

Thanks

 

[Matador]

I would be interested in seeing if there is a difference with a MP cap.  Is there anyone who has a source for these? the C-12 cap in the kit is over 600v. rated.  What would be the minimum reasonable voltage rating.

Thanks

If you wanted to be pedantic, with no tube installed it would sit at B+ forever, so 160V rating and up would work.  Under normal operation, it sits at (about) B+/2.

 

[ln76d]

Is there anyone who has a source for these?

ebay

 

[DDay]

The Russian k42 looks to be slightly too big.. Has anyone had any joy with them?

 

[ln76d]

Russian k42 - sounds like machine gun
Am using MBM model and it sounds good - also size is pretty ok for many applications.
I would look for german made - siemens, bosch, hydra etc.
These are best which i had.

 

[DDay]

I can't seem to get the B+ voltage below 135V (with a 180k resistor between pins 1 & 7), the trimmer has reached it's limit.

There's a post on here saying to never exceed 120V when using Tim's capsule.

I'm using a 12AT7 so substituted 33K resistors for R1 & R2.. could this be skewing B+?

 

[Matador]

For an 12AT7, you need a different test resistor.  The 180K is for 6072A tubes.

Here are the two common scenarios:

C12 (6072A) - 120V, 0.7mA = 180K @ 0.25W
C12 (12AT7) - 120V, 2.55mA = 47K @ 0.5W

If you don't have the correct resistor or wattage, in a pinch, two 100K 1/4W resistors in parallel can be used and will get you close.

 

[DDay]

Thanks,

Realised I missed R17 being changed to 47K so i'll buy a couple.

Just want to double check is there anything apart from the extra (plate) turret missing from the BOM for mic pcb version 1.4a?

Also is it ok to use a wire jumper instead of the SMT zero resistor?

 

[ln76d]

Also is it ok to use a wire jumper instead of the SMT zero resistor?

Yes

 

[digitall2000]

Albeit it is not my place to correct anyone on this site but i am trying to learn "tube stuff"
and any discrepancies make my little brain ache
it just so happens that i had printed out reply #875 the other day, just not all of it, missed the summary
I'm trying to fiure out what is need to calculate the actual tube circuit load so as to figure a load resistor for testing the PSU
on the 12ay7 you used the idle current of the tube which is where the load line crosses the bias voltage line, correct?
on the 12at7 you used a 2.5ma load line to achive a certain gain (60 v swing with 2 v input) and state the idle current is 1.1 ma
then here you are using the 2.5 ma to calculate for the test load (brain starts achin')

Is it where the load line crosses the bias line the you get the idle current and the plate voltage swing?
Which is the proper test load for the 12at7?
Is there a concise document that would explain this "tube curve stuff"?
By the way i have built two of these kits with 12at7 and they sound really nice !!!
So i also want to thaank you guys for all your hard work, patients and help
best reguards
lance

Bah Chunger posted before I could get to the PSU part.  :-[

12AT7 Conversion - The PSU

So the two big changes from above (from the PSU perspective) are:  a) the idle bias current draw increases from 0.7mA to 1.1mA, and b) the "cathode" resistance decreases from 1.4K down to 900ohms.  "B" is not an issue because we have R3 which can easily accommodate this change.  With the default 91K values of R1 and R2, the additional 0.4mA of current will drop cause an additional 72V drop in B+, which means there would be no way to get 120V out of the PSU.  So some re-calculations are in order.

Let's assume that R4 is set for half, which is 50K.  We have about 270V coming out of the rectifier diodes and we want 120V out of the PSU, which is a 150V drop across R1+R2+R4.  Total current should be 1.1mA + 0.25mA (through R6) which is 1.35mA.  150V drop at 1.35mA implies a total resistance of 112K.  Since R4 provides 50K, then R1+R2 needs to equal about 62K.  R1 = R2 = 33K sounds pretty good to me.  This gives an adjustment range of about 160V down to about 50V.

12AT7 Conversion - Summary

Microphone:  Change R17 from 100K down to 47K
PSU:  Drop R1 and R2 down to 33K.  Adjust R3 down to about 900ohms, then tweak with the actual tube.
Test load: decreases from 180K down to (120V/1.35mA) = 88K.  91K would work as a substitute for a real tube for PSU testing.

 

[Phrazemaster]

Yes I saw it on the PCB, it's just strange C12 isn't in the BOM (http://www.musicalsparks.com/images/c12/c12_bom.pdf) but I guess neither is the output transformer or capsule. Thanks for the heads up on using an MP.

Wow, I can't believe I left that one off.    I have a new version of the BOM, but have been too busy (aka. too lazy) to upload it.

Also, to answer the "don't use polyprop for C12...": it's like saying don't put salt on your food.   Many have built with the Erse polyprop in C12 and have been very happy.  I suggest you try out a few and pick the one that sounds best to you, and they aren't too expensive so you can try several.  Also try several values:  a few have stuffed up to 2uF for C12 with good results.

It happens. I made once album graphics for CD release where two songs titles were in different order and it was released that way sh*t happens

About the polypropylene - of course it's an obvious oversimplification
From the other hand most people will find "use polypropylene only" type information.
Someone which want to build microphones not only because it's cheaper and he "have to", to get tool only - will probably use his brain and look for his own way
Many people are here to build few things and bye then - so they wouldn't look for anything more.
My - Don't use MKP/polypropylene at the output! - is for those lazy guys which will probably choose between use or not use

Testing parts is one of the most important things - so of course i  subscribe at that with both hands!
My beloved method is rotary switch with few capacitors and making A/B comparison by listening and measurement (if possible).
From my experience, especially with small sized transformers, oversized capacitors and polypropylene (tested many from chinese to audiophile market parts) overall sound worse - it's not even related with low end response and its corner but full usable spectrum. Best option which i found (whatever type of microphone it's) are MP caps - and this is not related to any historical things. Of course you can find some MP which will degrade the sound due to out of specs - which is related with the age of these parts.

Just wanted to thank-you and kudos to you LN76 because you always provide such interesting and well-thought out ideas! Thanks for being such a great asset to this community.

Mike

 

[micaddict]

Well said, Mike.

 

[Matador]

I'm trying to fiure out what is need to calculate the actual tube circuit load so as to figure a load resistor for testing the PSU

The test load is not meant to (and cannot in reality) replace a real tube fore testing.  It's just meant to give a test load so you can dial in the voltages to be at a good starting point.  In reality, individual tubes never idle at the point specified in the datasheet.  The build thread is careful to denote that tubes should be trimmed before the capsule is connected, as some capsules say to never exceed 130V or somewhere similar.

The load line is not a current, it is actually a conductance (remember that the tube curve's you mention from my other post have voltage on the X axis, and current on the Y axis, which means the slope of the load line is actually proportional to I/V, which is conductance).  The idling point is the point along the  line where the tube sits at a given grid bias wrt. the cathode (aka the 'bias voltage').  So in reality, the actual idling point of the tube at DC depends not only on the B+, but also where the cathode voltage is trimmed to at that point.