U47 Build using EF80

[Spencerleehorton]

Some pics of where im at with it.
to do list:
change transformers as 9v only 100ma so need at least 300ma+, 15v only 3va make at least 6va.
0.01uf 250v cap need to go on mic pcb
470uf/200v caps on psu pcb
connect and wire in EF80
Connect capsule
wire into mic body and connector
put psu into case and wire in IEC, on switch and pattern select

 

[Spencerleehorton]

.

 

[Spencerleehorton]

back of mic pcb

 

[gary o]

Proper DIY I like it  looks like a U87 circuit I did the other week that board is brilliant for DIY

 

[Spencerleehorton]

yeah its all pretty ugly but means to an end, once it all works i can then etch a pcb out and make it look nice, then i can start on the U47 body for it to go into!!
Just sourced some better transformers and a big metal box for the psu to go into!

regards

Spence.

 

[gary o]

I agree your build is a beauty queen compared to my recent U67 circuit cramed in a choc tin, I like to make em like this quickly to evaluate them in U67 case its too noisy might be my tubes so will re hash into a M269 to test

Your PSU will soon be ready then

 

[Spencerleehorton]

i certainly wouldn't call mine a beauty queen by any stretch!!!! but i'm definitely a lot neater with everything now then when i started doing all this, but gotta long way to go!! (i.e Chungers builds are always so elegant and beautiful.)
For the psu i can only find a 2 x 15v (7VA) toroidal and a 2 x 12v (4A) just using one winding from each should be ok right?
Just wondering what i need to change for using the 12v instead of a 9v? or would this be ok to use?

regards

Spence.

 

[gary o]

Which PSU are you building Spence is there a schematic...... Im no expert but would like to have a look.

 

[Spencerleehorton]

hi gary o,
im building the gyraf g7 psu:

http://gyraf.dk/gy_pd/g7/gic_s.gif

regards

Spence.

 

[gary o]

I usually build a little ajustable LM317 circuit for mics with seperate heater supply you can buy the ready made from fleabay

I made a 105V supply from maxes MK47 schematic (first post) but that can supply more milli amps than you would need for just B+ as was designed to supply filiments too, I used a larger than needed transformer that I had, I adjuted the resistors to arrive at 105V.

I also copied a AKG C28A power supply circuit quite standard and simple .... again adjusted resistor values & used a transformer I had, I like these designs because you can use bleeder resitors to supply different HT voltages for different mics

 

[gary o]

You should be able to get away with just one of those transformers but will need to do some ohms law to work out the voltages you need 105V not 160V so maybe the 2X12V would be better as you dont need so much voltage ......Im not great with ohms law maybe a clever chap cxan chime in & help us out I dont want to lead you in the wrong direction, If it was me I would make some thing with the 12V trans & experiment with resistors to get the voltages I want as I say Im not great with ohms law.

 

[Spencerleehorton]

HI Gary,

from what ive been led to believe the psu needs to be 160v unloaded (mic not plugged in) 105v mic plugged in.
I will start fiddling!!

regards

SPence.

 

[Spencerleehorton]

this is something to be aware of:

It is also worth pointing out that under-running a heater can poison the cathode and is therefore as damaging as overrunning.

food for thought.

regards

spence.

 

[Pip]

It can in some tubes. But as the VF14 in the original U47 is underheated so can others be, or not.

Looking good!

 

[Spencerleehorton]

this is the psu in early stages.

 

[gary o]

Hi Spence as I said Im in a little over my head with power supplies but yes I see what your sayingunder load it will drop but Im not sure how with this power supply it may not be same as say Maxes MK47 power supply which does work like that I think mine measures way above 160V but under load drops down to 108V if I remember correct 3V over & your mic will only need 105V for plate drawing less milli amps than a real U47 or MK47 which will be drawing current to heat the filiment as well as plate... to do the maths you will need to look at EF80 data to see how many milla amps the plate needs ...... what you could do is build power supply as is build the mic measure the resistance of the mic itself then get a resistor to act as dummy load connect across your power supply to make sure the load brings voltage down far enough.... from what Im told & experiments the tube are very forgiving... if voltage not right adjust the resistors still using dummy load before trying with the mic....

Take a look at MK47 power supply has built in dummy load resistor 2k3 5Watt to simulate the mic being connected, I guess 2K3 5watt resistor would be ball park for your mic

I think the heater supply will be fine will supply 6.3V or less, regarding underheating as Pip says there ton of info on it here & google a lot of classic mics under heated their tube, M49s AC701 between 3.8V 4.2V Maxes MK49 6028s are underheated to match a V14 ... 5840 mini tubes usually 5.7V instead od 6.3V

Found this in another forum ....The U47 underheated its VF14, and Oliver Archut recommends underheating an EF14 to 5.05V (should be 6.3). From what I've read, I believe underheating can change impedance characteristics, but I'm a little out of my depth there as far as understanding the finer nuances of tube behavior outside normal operating range.

 

[Spencerleehorton]

HI everyone,

have powered up the psu and getting 6.3v for the heater and 220v for B+ have to put a 20k trimmer on place of the 10k to trim the voltage down but the good news is no big bang or smoke!!!, also might higher the 2k trim pot to 5k to get voltage down on heater.
mic is all put together just gotta try and make something for the switch for omni/cardioid is there a simple way of making the original thing? i will investigate as im sure ive seen something that looked quite simple out there.

regards

Spence.

 

[gary o]

I converted my U67 circuit into SM269 cathode connected to heater supply similar to the AMI EF80 circuit, my usual LM317 heater supply circuit similar to Gyraf PSU is not quiet enough Im wondering if you may run into same problem Spence hope not.

I may end up making a passive supply with a choke for SM269 ...... the U67 circuit I tried bafore M269 was bit noisy as well with the LM317 heater supply ....

To learn & for fun Im going to change M269 to M269C version has cathode resistor & bypass cap Im guessing it will be quieter.... I can then compare U67 M269 M269C fro best sound/noise

 

[Spencerleehorton]

HI Gary,

sounds good you got a schematic you can show me?

regards

Spence.

 

[Spencerleehorton]

HI all,

for the B+ im gonna put a 1k 5w before the first 470uf/200v so to take the voltage below 200v then change the 10k for 20k trimmer and put on a 2.3k 1w resistor for load and adjust down to 105v
the heater will need a 5k trimmer and adjust this down to 5.05v
i'm also thinking about putting a variable resistor for the 29ohm and having a play with the cathode bias.
i will put up a schematic of what i've done once everything looks good.

for the switch i'm gonna make a flexible pin and a sliding mechanism so when it slides across it pushes the pin down to make contact with the other side of the capsule, then cut a hole in the body and line it all up.
hopefully this is something like the original?

regards

Spence.