DiY AMI U47

[chaserose1]

Chase,

Did you clean everything possible with 90% isopropyl alcohol? PCB, resistors, acrylic decks, etc? Any bit of dirt or skin oil will manifest as noise.

And congrats on a successful build!

Robert in Austin

Thanks Robert,

I cleaned again. I was very diligent with this second build attempt to watch for flux contamination, the additional cleaning did not seem to help (but I'm sure it didn't hurt)-- so I changed the tube and now after running it for 8 hours or so I am reporting that the mic is much, much quieter.  I do have a question to those that have an operational ef800 AMI.  Have there been any problems with tube life- (going noisy after running the tube with the lowered heater for a time, I know some have questioned this with an ef800).  I would like to know what I might expect and what others have experienced as far as performance longevity.

Thanks

Chase   

 

[AusTex64]

I'm using EF802 and so far, so good. I understand this to be a very robustly made tube, but it's still a tube. Seeing as the rated life is something like 10,000 hours, it might take a while beofre problems manifest. Or they might manifest quickly, it's the luck of the darw. But I think you increase your odds dramatically by using Telefunken tubes, they are the best of the best IMHO. Oliver Archut told me "If you look over GE and RCA tube development data, they were very interested in how to make tubes cheaper. Telefunken was concerned with how to make tubes better".

 

[BoomRoom]

AusTex64 and Melodeath00 - I am trying to assemble a freshly received AMI U47 kit (at a quite terrible time to do so without you know who around).  Can you give me a BOM list of what you ended up using for the R and C values.  The parts I have seem to mostly conform to the CS-4 schematic, but I have a few parts that are not correct and a few missing.  I am set up with ef800 and will move to ef802 when the tubes show up.  I would just like to get a list off of a working build that you have done.  Thanks.

-Jeff

 

[AusTex64]

Hi Jeff,

I used what's on the schematic posted on the AMI website. What parts appear to be missing?

Did you see my layout drawing posted in this thread somewhere? That might be helpful.

Get er done!

Robert Mokry
Austin, TX

 

[duantro]

Hi, I have built this AMI alt tube/ Lucas CS-4 version mic that the late great Oliver Archut shared and is being discussed here. I am trying it with an EF 14 tube and with a slightly modified (trimmers added) MK7 psu. Could someone look at Robert's drawing that I have appended, and see if there is anything missing?  For some reason I am having the heater voltage dropping down to 3.5v and can't seem to figure it out.
Cheers!
Edit: sorry the file was huge! Maybe this is a little better. Also fixed rc connection to the relay wiring error in v2.

 

[duantro]

The only ef14 pinout that I could find is the one I found from radio museum and a German pdf. Can anyone confirm this pinout is correct?  I'm thinking that my tube wiring is incorrect or the power supply isn't supplying enough.
Edit: by disconnecting the 5 v relay I still get close to the same response so don't think this is an issue. I'm using the Edcor power transformer  recommended for the MK7 psu. It is rated at 500mA Is this too close to the 470mA draw?

 

[duantro]

I have found that I had the output transformer wired up wrong.  Fixed that and  can confirm ef14 pinout, as I'm getting sound finally! I am using an Antek power transformer with 2A heater windings.  I can get the heaters up to 4.65v now.  I've paralleled 2.2r with the last two 4.7r on the heater side of the psu. I'm now getting 4.92v. Is this close enough? I'm going to take it to the studio and give it a listen.

 

[duantro]

I'm getting some ocean waves and slight static. I'll let the tube burn in more and hope it goes away.  I have an rft ef14 I'll try if the telefunken keeps up with  the racket!

 

[AusTex64]

Let your tube run continuously for a week then listen. It can take a while cor tubes to settle down and burn in.

 

[duantro]

After a 3 day burn in period, the tube  still sounded like a conch shell full of the ocean. I swapped out the telefunken with an rft ef 14, and it sounds amazing and quiet.  I noticed the center plastic pin looks somewhat destroyed on the telefunken. Not sure if it contributes to the defectiveness. I got this from a fellow gdiy member.

 

[Moby]

After a 3 day burn in period, the tube  still sounded like a conch shell full of the ocean. I swapped out the telefunken with an rft ef 14, and it sounds amazing and quiet.  I noticed the center plastic pin looks somewhat destroyed on the telefunken. Not sure if it contributes to the defectiveness. I got this from a fellow gdiy member.

Hey  Duantro. Center pin is not important,  but before you trash your tube try to clean and degrease the pins. They look pretty dirty from the pic. Cheers

 

[duantro]

Can anyone suggest method to minimize the pop when switching out the back capsule with the relay? As of now I have a 2 pole 6 position Lorlin switch setup as 3 positions. Position 1- back capsule 0v/ (pole 2) relay has back capsule disconnected, Position 2-b.c. 60v, b.c. connected via relay,  Position 3- b.c. 120v. Would it be smarter to make the switch a 4 position, with the relay disconnecting on the 4 th position? This could maybe allow the voltage to dissipate some before the relay takes the capsule out of the circuit?
Edit: I found the discussion earlier in the thread. I forgot the 10k resistor to ground!

 

[Melodeath00]

duantro, any more info on your switch and relay? That's a mod I still haven't incorporated yet.

Last night I tried underheating the EF802 from 5.68V (the default AMI PSU setting) down to about 4.9V. I did a spoken word comparison with my C12 both before and after the additional underheating, and took recordings of some pink noise coming through my monitors. The C12 did not change, so it was my "control."

I have to say I heard no obvious difference after the underheating, so I'm not sure where I will leave it.

 

[duantro]

duantro, any more info on your switch and relay? That's a mod I still haven't incorporated yet.

Last night I tried underheating the EF802 from 5.68V (the default AMI PSU setting) down to about 4.9V. I did a spoken word comparison with my C12 both before and after the additional underheating, and took recordings of some pink noise coming through my monitors. The C12 did not change, so it was my "control."

I have to say I heard no obvious difference after the underheating, so I'm not sure where I will leave it.

Hey Melodeath00,
Yeah the relay works great. I get a small click when the relay turns the back plate out of the circuit, nothing too bad though. I mounted the relay on the high impedance plate, underneath the top plate in the mic.

This is the 5v relay I used. http://www.mouser.com/ProductDetail/Panasonic-Industrial-Devices/DS2E-S-DC5V/?qs=sGAEpiMZZMs3UE%252bXNiFaVCLW1ZA8WQBLmDvvwbgQLyY%3d

It switches from a 2 pole Lorlin switch at the psu- Position 1 = 0v on pattern with pole 1/ the relay grounded via pin 5 of cable, on pole 2, Position 2 = 60v on pole 1, Position 3 = 120v.

 

[dmp]

I'm finishing up a mic build right now with an EF800 and the heater voltage is too low (~1v).
I am using one of Dany's choke psu, which should be able to power this without a problem.
I have two telefunken EF800s and they both measure only ~5 ohms between pins 4 &5. It should be more based on the spec (6.3v, 275 mA = ~20 ohms)
Any ideas?

 

[pH]

Hey dmp, did you ever figure out this problem with low filament voltage?

 

[Phrazemaster]

Hello,

I am happy to report that I now have a AMI U47 that is working well after such a bad outcome with the first attempt, it did require a complete do over/rebuild.  Voltages are correct using the alternative circuit component selection.  It is a little noisy however so I will attempt using different tubes to see if that will improve.  It does have a nice low end which is what I was hoping to achieve.  Thanks for all the info on this forum subject, I doubt if I would have ever completed the build successfully without information shared here. Thanks guys/gals.

Sorry for the topic change- just wanted to report and say thanks

Chase 

I know this is old but I just wanted to say, to anyone if it's helpful, sometimes it is best just to start over. I built my M49 PSU and mic very carefully, and I only had what I can describe as gremlins in it (even PocTop said so). Sometimes it sounded great but other times there was terrible noise, despite extreme cleaning/care. So I rebuilt the PSU completely, reorienting components etc, and the noise went away. Still had issues with the mic itself, and ended up getting a new PCB and rebuilding the whole mic.

Result? Flawless sound. Very clear, low noise floor. Fabulous mic.

But it took me 6 months of wrangling!

Mike

 

[4earstudios]

Hey Folks, i recently built the ami u47 with thiersch capsule and ef800, and the ami built PSU with switchable characteristics, after having troubles which derived from not using teflon cables ( it was very microfonic, and handling noise was awful, also noise when using upside down) i am only havin one issue : a 100hz hum very quiet but noticable - its around the level of the tube noise - i wanted to ask you if you also have this hum ,  or what you did about it..thx

 

[dmp]

It's leaking from the power supply somehow - most common in this mic is the heater circuit since the mic bias uses the heater rail. Could also be a poor ground layout.

 

[dmp]

Hey dmp, did you ever figure out this problem with low filament voltage?

Totally missed this - sorry -  I did figure it out. I think I swapped the transformer for the heater circuit.
It needed more VA to not sag down