[help] Trouble with Aurycle A5500MP Build. Low heater voltage?

[constellates]

This is my first time building a microphone so I went with a budget kit from Aurycle. Here's a link to the specific kit where you can see a layout and schematic at the bottom. http://www.aurycle.com/aurycle-a5500MP-large-diaphragm-tube-condenser-diy-microphone.html

I believe I have everything connected correctly, but when I plug it in I'm not getting any real sound. If I crank my speaker/headphones I can hear it picking up some radio waves but there's no actual sound coming from the capsule as far as I can tell.

It also appear that the tube (12ax7) is never fully heating. There's a faint glow, but it doesn't get nearly as warm to the touch or visibly glowing as it does when I put it in my guitar amp.  If I measure the voltage (with all parts/tube connected) between the heater supply wire and the ground I get around 4.4v. I believe 6.3v +-10% is standard for this tube. The heater supply wire emerges out of the bell surrounding the transformer so I can't tell if this wire is coming from the transformer or directly from the 7pin socket. If I measure what I believe is the heater supply pin directly at the power supply I get about 12v.

I found a video where someone mentions the schematic shows pin 5 of the tube connecting to ground instead of pin 4 which causes only half the tube to heat. I've tried connecting this both ways and it does change whether a single or both sides of the tube heat but doesn't fix the voltage issue.

Right now I'm feeling pretty stumped as to what to try next. Any suggestions or additional info I could provide? The heater supply part of the circuit seems so simple. Is it even possible I have a wrong component or something causing the voltage issue?

 

[dchang0]

In a typical 12AX7 setup, pins 4 and 5 are soldered together, then to one side of an AC filament supply at 6.3V. Pin 9 gets the other side of the AC filament supply. According to the GE datasheet, current draw is 0.3A with the two triodes' heaters in parallel.

In guitar amps, I don't recall ever seeing just pin 4 or 5 tied to ground. In most cases, people use an AC filament supply with a true center tap or a virtual center tap--the center tap is what's tied to ground. I would expect mics to treat 12AX7s the same as a guitar amp would.

Looking at the schematic for the Aurycle, it seems that they are using only one of the two triodes in the 12AX7. Note that pins 6, 7, 8, and 5 are all tied together and to ground. That looks to me like they are disabling that half.

That means they are really only using the one filament from pin 4 to pin 9 and that is by design. I figure the problem you are facing then is not the wiring but the low voltage: 4.4V is definitely not enough to heat the tube properly. Not obvious from the schematic is whether the filament supply is AC or DC--if I had to guess, it is AC. If they had a clean 6.3VDC, they could feed the filament directly without C11 in parallel.

 

[Gus]

I looked at the schematic only 1/2 of the tube is used.  If pins 1, 2,3 are used pins 4 and 9 are for the heater and 5 can be grounded
China supplies I have looked inside often have a 7806 with a Si diode in the ground leg for about 6.6VDC regulated inside the supply, you will have less voltage in the microphone due to cable IR drops.

Is the transformer wired backwards?

Have you checked the cable for miswiring?

Did you use nitrile gloves when handing the capsule, high value resistors and caps by the capsule?
Do you have a good temp controlled iron so you don't overheat the components when building the microphone?

Have you searched this forum and the web to learn how to test the power supply? There have been threads with this information. Ohms law and knowing how to calculate DC power is what you need. This will help test the heater supply.

Do  you know how to work safely on circuits with tube voltages?
What type of meter do you have? cat rating?, does it have a transistor checker etc.? transistor checkers are not safe on meters IMO

Power down and disconnect the microphone from the power supply make sure the caps are discharged.
Check for connections and shorts by using a copy of the schematic and marking what you find on the schematic
This tube microphone is a simple circuit not many connections so checking the wiring might be the fastest way troubleshoot the build
Next I would check if the components are damaged.

After you do the above post what you find then the next tests needed will be more clear.

 

[constellates]

Looking at the schematic for the Aurycle, it seems that they are using only one of the two triodes in the 12AX7. Note that pins 6, 7, 8, and 5 are all tied together and to ground. That looks to me like they are disabling that half.

I looked at the schematic only 1/2 of the tube is used.  If pins 1, 2,3 are used pins 4 and 9 are for the heater and 5 can be grounded
China supplies I have looked inside often have a 7806 with a Si diode in the ground leg for about 6.6VDC regulated inside the supply, you will have less voltage in the microphone due to cable IR drops.

That makes sense now that only half of the tube is being use.


Gus, thank you for the thorough break down. I'll go through your list and try to post back soon. Here's what I know off the top of my head:

The capsule shipped mounted in the body with the headbasket on. So I've never touched it, but I didn't use nitrile gloves while handling the high value resistors and caps. Should I now do something to clean those components.

I am using a temp controlled iron (Hakko fx-888d).

I've tested the power supply based on limited instructions on Aurycle's site, but I'll look for more info here.

I know the basics of working with high voltage electronics. I'm clearly not an expert but have built two tube guitar amps previously.

I have this meter https://www.amazon.com/INNOVA-3320-Auto-Ranging-Digital-Multimeter/dp/B000EVYGZA/ref=lp_15707471_1_4?s=automotive&ie=UTF8&qid=1482511076&sr=1-4
It's cat II rated. I don't think it has a transistor checker. It definitely doesn't have a spot with sockets to insert transistor pins.

 

[dchang0]

Is capacitor C11 wired in the correct orientation for polarity?

The part you actually received might not care about polarity, but the schematic uses a polarized symbol for a capacitor in that spot (versus say C5, which uses the non-polarized symbol for a capacitor).

 

[constellates]

Thanks for all the help everyone. Tonight I finally had time to track down the issue. There was a problem with C11. I had upgraded that cap to a better quality version and the leads were slightly too big for the pre-drilled holes in the PCB and I think I damaged the connections getting it to fit. I swapped it for the stock C11 component and the heaters are now running at a totally acceptable 6.55v DC.

 

[dchang0]

Nice work!