U47 Build using EF80

[gary o]

HI Guys,

Im obviously not getting something here as you seem to be saying to use the M49/M269 psu for the 6.3v correct?
I have tested the voltage on 3 of my m49/m269 psu's and the H+ is 24v-25v unloaded?
Im not getting how unloaded the U47 H+ supply is much less than this, if you could explain please.

regards

Spence.

I Spence Im sorry Im got so many mic circuits in my head I was forgetting yours is a U47 ish circuit that power supply wont work.....

Im gonna have a think tho ....

 

[gary o]

Hi Spence , Im confusing myself here ..... this is the circuit for your mic right
http://www.tab-funkenwerk.com/id68.html

So you do need a seperate supplt of 5.05V  (6.3) but underheated by AMI to mimic the V14 underheated

Also you have made this power supply for M49B & SM269 http://www.tab-funkenwerk.com/id68.html

So your M49B & SM269 uses a 5840 tube ? 6.3V but usually underheated 5.7V ish so you have a trimmer on the power supply I guess when loaded the 25V will drop down but to be safe I think you could put a dummy load across the 25V I guess you could jut check the heater voltges of the M49B & SM269 while working

Your M49B power supply has a choke built in ( you lucky bugger) I believe Dany found like I am finding & as Odbfs points out circuits like your EF80 U47 the M49B & Sm269 need cleaner power as dirty power like  my LM317 gets into the audio via cathode I believe..... still trying to understand all that thanks to odbfs... but I know from making six mic circuits last few days the three fixed biased like U47 are all noisy due to my LM317 type supply (I hope) hence my choke is on order

Sometimes if Im try to track noise  I use a battery to power the heater  I will then disconnect the heater transformer & so on to rule things out.

 

[0dbfs]

There needs to be a load on it. Are you using a 5840 on your m49? Compare
Data sheets for the two tubes and what the heater current is in both. That specifies what the load is and since it also specifies 6.3V you can use those tow values to figure what the resistance of the heater is at operating temp.
E=I*R
6.3V/I(in amps)=R

That will give you the resistor value in ohms to use for a "dummy-load" that you can use to pre-cal the psu to 6.3V. Just needs to be a high wattage resistor because it is a heater and will heat up.

When you measure "unloaded" it's not really unloaded. Your meter provides a path to ground through a high impedance. Probably 10M. That illustrates the fact that different loads will draw different current which provide different voltage drop across the psu resistors. The transformer is a 20V secondary I think.

Or just hook it up,measure, and tweak it to the specified 6.3, or 5.05V or thereabouts.

Cheers,
Jonathan

 

[Spencerleehorton]

Hi Gary,

yes the circuit im using is the alternate oliver circuit on the right, i will test the H+ with the M49 plugged in to see what it comes down to, i like your idea of the battery, good call.
Im also doing a U47 version using paper in oil caps to compare.

regards

Spence.

 

[Melodeath00]

FWIW, I have the PSU from Oliver himself, and the tube heater is at 5.67V measured at the EF800.

 

[0dbfs]

Data sheets say 5840 draws 160mA while ef800 draws about 275mA at 6.3V so if you load the heater supply of the PSU2649Passive with the ef800 it should be under what you measure with the 5840. Just re-tweak to desired voltage. Somewhere between Oliver's specified 5.05V and the tubes rated 6.3V. H+ supply in that one will be fine. Especially to test for noise.

Cheers,
Jonathan

 

[Spencerleehorton]

thanks 0dbfs, thats great, trying it out now.

regards

Spence.

 

[Spencerleehorton]

Hi Guys,

knocked this up to test very soon!!

regards

Spence.

 

[Spencerleehorton]

and this.

 

[Spencerleehorton]

Hi Guys,

ok just put the M49 psu H+ on the U47 and powered up and mic completely silent apart from hiss obviously when i turn it right up.
Sounds very nice now so this is defo the problem as hum is now gone.
Now how to change power supply?? do i just need a choke in there? if so where?
by the way the M49 psu didnt trim up past 3.2v?

regards

Spence.

 

[0dbfs]

Hey Spence, That's great news!

The VA rating of the 2649 PSU H+ trafo is 20V/6VA which I believe means the secondary will deliver 300mA @ 20VAC to the secondary.

If you measure V on each side of R2 (15R) in the 2649PSU we can calculate and confirm what the current draw is. The tube heater is rated 270mA which is pretty close to the 300mA rating of the transformers capability.

What PSU schematic have you been using for your U47?

-jb

 

[Spencerleehorton]

Hi,

I have used the Gyraf G7 psu schematic here:

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

I have used an LM350 in replacement of the LM317.

Is there a way to change the heater side of that schematic to remove the hum?
I assume its the choke which is helping and the higher caps to smooth everything.
I dont mind starting again with this psu as ive seen this one which looks like its the ticket version 4 is the one i think:

http://www.offsetguitars.com/forums/viewtopic.php?f=27&t=42991&start=15

let me know if you know how to fix what ive done or i need to start over.

regards

Spence.

 

[0dbfs]

Here is mention of using the mk7 PSU with an EF800:
http://groupdiy.com/index.php?topic=43683.0

The MK7 PSU does work with a PF86 which also has 270mA heaters:
http://groupdiy.com/index.php?topic=30124.0


Good info here too on another ef800 u47 style:
http://groupdiy.com/index.php?topic=42566.160

Cheers,
jb

 

[Spencerleehorton]

HI Guys,

came across this:

External Capacitors A 0.1 F disc or 1 F tantalum input bypass capacitor (Cin) is recommended to reduce the sensitivity to input line impedance. The adjustment terminal may be bypassed to ground to improve ripple rejection. This capacitor (CAdj) prevents ripple from being amplified as the output voltage is increased. A 10 F capacitor should improve ripple rejection about 15 dB at 120 Hz in a 10 V application. Although the LM350 is stable with no output capacitance, like any feedback circuit, certain values of external capacitance can cause excessive ringing. An output capacitance (CO) in the form of a 1 F tantalum or 25 F aluminum electrolytic capacitor on the output swamps this effect and insures stability. Protection Diodes When external capacitors are used with any IC regulator, it is sometimes necessary to add protection diodes to prevent the capacitors from discharging through low current points into the regulator. Figure 18 shows the LM350 with the recommended protection diodes for output voltages in excess of 25 V or high capacitance values (CO > 25 F, CAdj > 10 F). Diode D1 prevents CO from discharging thru the IC during an input short circuit. Diode D2 protects against capacitor CAdj discharging through the IC during an output short circuit. The combination of diodes D1 and D2 prevents CAdj from discharging through the IC during an input short circuit.

check page 7 on here

http://www.onsemi.com/pub_link/Collateral/LM350-D.PDF

So i will try a 10uf strapped across the trimmer i'm using for H+ to reduce the ripple for the LM350 im using.

regards

Spence.

 

[Spencerleehorton]

HI All,

Have put in the 10uf at the 470R/2k trimmer point to ground and no change in hum, will put in diodes and see if the hum goes.

regards

Spence.

 

[Spencerleehorton]

HI All,

no change with hum with this psu, have tried another adjustable psu and hum still there so it must be on the HT supply.
Next step?

regards

Spence.

 

[Spencerleehorton]

HI All,

now a very bizarre thing is happening, my mentor has let me borrow a 24v psu which is adjustable and reliable i have patched this into the H+, i am still using the Gyraf G7 psu layout for the B+ which is about 120v, when i turn them both on i get the hum, when i turn off the B+ the Hum is still there and the mic still works and passes audio?
When i keep the B+ on and turn off the H+ the hum fades away with the psu shutting down.
I dont understand why when just the H+ is on which is 6.3v that the mic still works? it seems there is still a voltage from the 6.3v which is getting to the capsule? any ideas anyone?

regards

Spence.

 

[0dbfs]

Hey spence, it takes a little while for the filter caps to discharge. The plate and capsule don't draw much current so they will continue to get current from the filter caps until they are empty.

Did you include the pattern switch - 100k/100k divider from the g7 PSU schematic you referenced? That also acts as a bleeder-resistor and will drain the filter caps after power is shut off. Be careful! Without a bleeder you can risk a bad shock once you start poking around in there.

Cheers,
Jonathan

 

[Spencerleehorton]

Hi 0dbfs,

Yes i did include the 2 x 100k divider although i only take the +V rail and -V to the mic.
Any ideas on how to proceed with getting rid of this hum? i have managed to get rid of it by using the H+ from the M49 supply but dont know what i should do with this gyraf g7 psu heater supply?

regards

Spence.

 

[gary o]

Im kinda wondering if the G7 supply for this kinda mic is a lost cause...... hope not but maybe its the regultor & as these mics are more sensitive its picking that up, just a thought did you use same wireing for your powers supply swap test gotta make sure you rule all other factors out ...... I would probably try this if was mine you could borrow the M49B PSUs choke & insert it into the  G7 supply be be careful dont wanna fry it .....

Or just keep the HT part of G7 PSU & build the passive heater supply of the M49B PSU with another choke, I have choke on order so I can build a heater supply to use on all my cathode biased mics like this.