ELA M251 Inspired Microphone - Build Thread

[mhelin]

The picture is not too sharp around that zero ohms resistor but at least it seems to be in correct place. It though looks like there might be quite a blob of solder on the right pad of the resistor, might be worth cleaning. How is the output capacitor btw?

 

[johnsonic]

It though looks like there might be quite a blob of solder on the right pad of the resistor, might be worth cleaning. How is the output capacitor btw?

Output capacitor measures fine. Will clean blob for safety! Thanks!

 

[murrayatuptown]

One sometimes needs, or prefers, to make in-circuit capacitance measurements over removing a component (for multiple reasons).

Two precautions: protect the measuring instrument by confirming capacitors have no stored charge (measure DC voltage across them) and don't jump to conclusions if the in-circuit capacitance measurements are not as you expect. In-circuit measurements are affected by the rest of the circuit (applies to R, L & C).

The impedance of the overall circuit may skew the displayed value and may cause an instrument to change its test frequency (for instruments that make that decision for you).

If possible, lifting (unsoldering) one lead of a 2-lead capacitor will reduce or eliminate most in-circuit measurement inaccuracies.

If the in-circuit measurement is fine, remember you made an in-circuit measurement in case something is still suspicious later. Occasionally you may need to verify the measurement on the component alone.

If you were to compare in-circuit capacitance measurements with very different types of instruments, you may often find significantly different measurements. A 'better' lab instrument doesn't guarantee more accurate in-circuit measurements. I saw an engineer in a factory once try three different capacitance meters to verify a bank of in-circuit (capacitors because some were found to be very far out of tolerance (high) at incoming inspection. He chose the instrument with the 'best' (for his comfort level) reading and set up a screening procedure for factory workers to check circuit boards for what seemed like a 'correct' circuit measurement.

He had asked my advice about why he got three very different measurements with three instruments. I told him he might well get another with a 4th type, and the polarity of the test leads in-circuit might cause a different effect. I understood the desire to not remove multiple components for measurements, but pointed out he was trying to make a correct decision that was obviously not meaningful.

He went forward and had factory workers document measurements for each serialized circuit board.

Luckily, the measurements were rejected later because the selected instrument had no current calibration record. Multiple people stubbornly did not learn anything from this effort.

 

[MicMaven]

So Matador is right, . . .
______________________________________________

Thanks for the uptake - My bad. I forgot about pilot tones and sub-carriers etc.
Thanks Alex. / James

 

[johnsonic]

So, I'm back and I think it's the tube socket. Am measuring right off the plate, and started with 6V of output with 500mV to the grid. After wiggling the tube it went up to 11.7V.

Repeating the test through the xlr output, I'm able to get it to over a volt, which starts to be in line with the C12.

Thanks for helping me troubleshoot this and learn something along the way!

 

[k brown]

Rustoleum Gloss Ivory is a nice '251-like' color for the body.

Have to leave it untouched for many days, though, before it's hardened enough to handle safely. Pay no attention to the words "fast drying" on the can - yes it 'dries' fast, but remains very soft and tacky for several days (unless I had a defective batch).

 

[Matador]

So, I'm back and I think it's the tube socket.

Wow. That would have been the absolute last thing to occur to me....glad you sorted it out!

 

[MicMaven]

yes it 'dries' fast, but remains very soft and tacky for several days (unless I had a defective batch).

I concur with Mr. Brown -- 1) It is a nice color, and 2) it does take longer than normal for that particular type of spray paint to dry because it goes on so thick and deep. One coat is supposed to be like doing two or more coats all at once.

Gratuitous Sidebar: I often use flat black or semi-gloss outdoor grill paint because it dries really fast - in less than 15 minutes - and I hasten the process and harden the result with with a heat gun. Of course, you gotta like flat black or whatever other color they paint outdoor grills, so perhaps you can file this one away for another project another day. Just mentioning a paint that really DOES dry fast.

Bonus Serving Suggestion: I know guys who restore old radios with metal cases and they sometimes bake spray painted metal cases it in the oven for something like 20 minutes before dropping it into a bucket of cold water to harden the finish. (Disclaimer and Caveat: Research this to be sure it works with your particular paint before you try it, but I have seem some really good results with the process.)

Good luck. It is a lovely color. James / K8JHR

 

[k brown]

I did wonder about 'baking' it.

 

[Matador]

A lot of local places will powder coat for reasonable rates, especially if they can combine with someone else's project, provided you can live what whatever color they selected.

I had two done in a dark grey for $20 at a local metalworking place several years ago, but I needed to make sure the surface was rough sanded and clean so there wasn't any prep work other than to wipe degreaser, coat, and bake.

 

[MicMaven]

A lot of local places will powder coat for reasonable rates,

YES. The guys who restore old radios sometimes do that and they have had excellent results. As you say, you may have to take black or grey or wait for the color you want, but it works as you describe. I get so dag blamed impatient to finish a project, I look for ways to hasten the result to just ... um ... done yesterday! James - K8JHR

 

[johnsonic]

Hi all - just wanted to check back in here.

I was able to temporarily get the output off the tube to be higher by wiggling it. Would measure higher for a moment and then fall back. Went back through, reflowed all the solder joints, scuffed up the contacts in the tube socket female parts, and can't get this mic to deliver a decent output. Through testing have verified it's not the tranfo, not the capsule, not the tube, not a bunk xlr connect, not preamp or input lane, not voltages at any of the test points in the schematic.

I have not replaced the tube socket, but did test resistance from outside points on the pcb all the way to the tube leg (had it slightly pulled out), all came back as an continuous circuit. With the tubes (had a test tube and a primo tube) in that same exact position still saw low output.

There are also a few other comments about low output in this thread - could it possibly be something else?

 

[Delta Sigma]

Hi all - just wanted to check back in here.

I was able to temporarily get the output off the tube to be higher by wiggling it. Would measure higher for a moment and then fall back. Went back through, reflowed all the solder joints, scuffed up the contacts in the tube socket female parts, and can't get this mic to deliver a decent output. Through testing have verified it's not the tranfo, not the capsule, not the tube, not a bunk xlr connect, not preamp or input lane, not voltages at any of the test points in the schematic.

I have not replaced the tube socket, but did test resistance from outside points on the pcb all the way to the tube leg (had it slightly pulled out), all came back as an continuous circuit. With the tubes (had a test tube and a primo tube) in that same exact position still saw low output.

There are also a few other comments about low output in this thread - could it possibly be something else?

Did you inspect your tube socket pins? Sometimes a bent tube pin won't line up properly and will bend the tube socket pins when you insert the tube.

 

[Trickyricky]

Hi all - just wanted to check back in here.

I was able to temporarily get the output off the tube to be higher by wiggling it. Would measure higher for a moment and then fall back. Went back through, reflowed all the solder joints, scuffed up the contacts in the tube socket female parts, and can't get this mic to deliver a decent output. Through testing have verified it's not the tranfo, not the capsule, not the tube, not a bunk xlr connect, not preamp or input lane, not voltages at any of the test points in the schematic.

I have not replaced the tube socket, but did test resistance from outside points on the pcb all the way to the tube leg (had it slightly pulled out), all came back as an continuous circuit. With the tubes (had a test tube and a primo tube) in that same exact position still saw low output.

There are also a few other comments about low output in this thread - could it possibly be something else?

Just completed my build last week and I’m also having lower than expected output. The mic sounds great, but It requires double the gain of my u67 and other tube condensers. I need to go through everything when I get a chance.

 

[Delta Sigma]

Just completed my build last week and I’m also having lower than expected output. The mic sounds great, but It requires double the gain of my u67 and other tube condensers. I need to go through everything when I get a chance.

Double the gain, as in 3dB? Or you crank the gain pot quite a ways around; like 10dB?

Some mics' output level will be lower. Depends if you mean a few dB or a lot.

 

[Trickyricky]

Double the gain, as in 3dB? Or you crank the gain pot quite a ways around; like 10dB?

Some mics' output level will be lower. Depends if you mean a few dB or a lot.

I had to crank the gain an additional 20db

 

[JMPGuitars]

A lot of local places will powder coat for reasonable rates, especially if they can combine with someone else's project, provided you can live what whatever color they selected.

I had two done in a dark grey for $20 at a local metalworking place several years ago, but I needed to make sure the surface was rough sanded and clean so there wasn't any prep work other than to wipe degreaser, coat, and bake.

The local guys I use sandblast before coating, no prep work needed. The only thing is you gotta tell them what to mask so you don't lose shielding connections on the shell.

 

[JMPGuitars]

My buddy wanted the ELA M 251, so I converted my C12. I couldn't find any good info, so I designed my own layout:


I placed a couple pieces slightly different than the drawing, but it's electronically the same:


For anybody that wants to do this, I removed 2 traces from the C12 PCB. I cut the ends of the trace, then I lift one end up, and the trace will peel right off. Not too messy at all.

In the diagram, the blue lines are new connection wires added to complete the circuit.
The thin tracer lines are just to show the PCB's existing traces so you can follow the circuit.

There's a couple spots I cleaned up after taking those photos, from the older joints. It sounds good, and it's super quiet. Check it out:


Thanks,
Josh

 

[oongbac]

I successfully changed my C12 to 251!
Much appreciate @JMPGuitars!

 

[freshtapescent]

Flip the transformer around, it’s probably backwards.