U47 Build using EF80

[Spencerleehorton]

HI Gary,

Yes i think having a choke is indeed the way forward, i just havent established whether the hum is on the H+ (most likely) or on the B+, i do have a couple of these coming which are nice and cheap and i have one of these in one of the M49 psu and is nice and silent. it has 2 x 100mH coils so wired in series gives about 300mH as it more than doubles.

http://www.ebay.co.uk/itm/390565303198?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

regards

Spence.

 

[Spencerleehorton]

Hi Guys,

Ive started to learn eagle and wanted everyones thoughts on this, would like to know what is the least i would be able to get away with on the B+ side as regards the inductors, do i need to put two chokes in there or will one be ok? and would 550mH be ok on each?
I will probably not use the trimmer on the H+ as long as i get the correct values in there.
any other values i need to change?

regards

Spence.

 

[0dbfs]

Third quoted link here has an EF800/alternate U47 build which uses a modified mk7 PSU. It works.

I would read through those and study up on why/how it works.

Especially if you intend to complete a number of these different builds since they will likely need maintenance and things throughout their lives.

The 200k should bleed the filter caps but the mic still makes audio during that period because it is still receiving current from the caps. How long does it take to die out after turning b+ off?

Best Regards,
Jonathan

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

 

[gary o]

Hi Spence what I meant in my earlier posts is to power your new U47 mic with M49B supply on filimet & G7 suppy for B+ at same time ...... If hum goes away then it was caused by your G7 6.3V you called also do the opposite but would have to tweak B+ down from 120V only gotta waste 15V could use couple resistors ohms law.... this way you can find out what your gonna need to build, if it turns out to be the filiment supply you could try adding a choke BUT I would guess that the reg is making the noise so a passive including choke would be the ticket

Im not really qualified to know about the chokes but would gues they would work as long as the ampage is enough I see maxex & Danys chokes are 0.3 H 500ma, max 10H 50ma B+ choke....

I think the choke you have in M49B PSU 0.3H is about £8 from mouser but the poxy postage is expensive.

 

[Spencerleehorton]

HI Guys,

the choke i have in the m49 psu is only £5 from ebay!!!, i will try and make a H+ with choke and compare.
I will also try the other thing you suggest about using the B+ form the M49 and the H+ from the Gyraf psu if no hum then i suspect the hum is from the B+ on the gyraf.

regards

Spence.

 

[Spencerleehorton]

HI Guys,

slight change of plan with this, gonna keep the gyraf B+ supply as is, will run H+ supply from 12V battery if no hum will introduce some changes to take ripple out as its probably this LM350 which is the source of the hum.
Will also build the other supply to compare as i think these are both good lessons to have both passive and regulated and understand these circuits.
this is a good page to review:

http://groupdiy.com/index.php?topic=30124.0

regards

Spence.

 

[ricardo]

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 ......

There's nothing wrong with the G7 supply.  The regulated heater supply should be quieter if done properly.

Your problem sounds like an earthing problem at the PSU end.  You are somehow sending hum & charging current down the heater lines.

Can you post a sound bite of the hum?

 

[Spencerleehorton]

Hi Richardo,

Yes i do have a audio clip which i can put on here later, its around 120hz, i ran the H+ supply from a 12v battery and no hum so this eliminated transformer coil induced hum as these were still connected in case.
This also ment that the hum was coming from the H+ circuit as per the Gyraf G7 H+ schematic.
I am going to change the H+ to include the following to see if it improves.
take out the 2 x 1000uf/35v and put in 4700uf/16v either side of the LM350
already have a 22uf/16v on the adjust of the LM350
put in the protection diode from output to input on LM350
put in 2 x 2.2R 1W resistors on the output along with 2 x 10000uf/35v

hopefully this will eliminate any hum on the H+ supply.

regards

Spence.

 

[ricardo]

I am going to change the H+ to include the following to see if it improves.
take out the 2 x 1000uf/35v and put in 4700uf/16v either side of the LM350
already have a 22uf/16v on the adjust of the LM350
put in the protection diode from output to input on LM350
put in 2 x 2.2R 1W resistors on the output along with 2 x 10000uf/35v

If 120Hz, its even more likely an earthing/grounding problem.  Your 'improvements' to the Heater supply regulator will do squat to help.

Have a look at http://www.valvewizard.co.uk/Grounding.html.  Read it carefully and look at how you've wired the whole heater supply.  Look at where the charging currents for your supply caps go.

Think what happens if ALL your 'ground' lines have significant resistance .. cos they DO.

Then read the article again and draw up your wiring diagram as he does.  ie showing EXACT connections to each supply capacitor and the high current paths.

Try and find an EXACT layout & wiring that Gyraf recommends for his PSU .. to compare with yours.

 

[Spencerleehorton]

HI Ricardo,

as i said above we have done a comprehensive test with a 12v battery to test the circuit and it is the LM350 which hums with the 9V transformer, so more caps have been put in place as per above, i have tried to test but seem to have short somewhere, so i need to go over this tomorrow, hopefully get it working and will be hum free.
Thanks for that article on grounding thats a great read, i'm satisfied that the grounding scheme i have is good.
Will update once hum is gone.

regards

Spence.

 

[Spencerleehorton]

HI ALL,

well it seems the slighest bit of hum from a LM317/LM350 gets in the cathode and cant be removed!!!
I have moved over the passive/Choked psu and am now enjoying hum free operation, have done a PIO caps version as well.
Will make some U47 bodies up now and plonk them together properly now all the test have been done.
at least the Gyraf G7 psu will be of some use when i put together that mic as well!!

regards

Spence.

 

[0dbfs]

That's really good news spence. Which heater supply and component values did you end up using? What voltage do you end up with between the heater pin and gnd?

Cheers,
Jonathan

 

[Spencerleehorton]

HI 0dbfs,

so far in hum free operation i have used a 9v transformer but i only can get tops 5v, which drops a little after a few minutes.
using 4700uf/35v then 2R2 5W resistor, inductor (Epcos 100mH x 2 = 300mH), 4700uf/35v, 2R2 5W, 10000uf/35v, 2R2 5W,10000uf/35v, 20ohm wirewound pot, 330R.
When i turn off the power the mic sounds amazing when powering down, clean as a whistle so i've still got a way to go to get it really clean, might do self bias and try other transformers and put in another choke to see if it can clean it up a little more.

defo getting there and on the right track now.

regards

Spence.

 

[0dbfs]

Yeah cool. 9V is probably too low then. I modeled the PSU2649 using Duncan psu designer and it showed about 3V and change with the same load. That's a 20V trafo but only 6VA so probably dropping voltage at its current limit driving the 275mA ef800 heater.

The g7 heater supply would be fine with self bias since the heater ckt doesn't connect to the cathode in that case. But then it's getting further from a u47 than the ef800 fixed bias.

You could probably use a 5k pot and 47uf bypass on the cathode to tweak for 1V1 on the cathode to test a self/auto-bias setup with the g7heater supply. Then measure the pot and swap in a more precision resistor. If your already in bench mode

Cheers,
jb

 

[Spencerleehorton]

HI jb,

i dont fully understand about self bias so if your able to be a little more specific from the G7 supply i could change this on one of the mics. pictures are good as well if you have them?
The passive supply still has a little hum ive noticed when turning mic off (caps still loaded for a few secs) the mic then sounds amazing, would love to get it sounding like it sounds when the supply is just turned off, as this is completely hum free.
This is a steep learn curve but im getting there!!!

regards

Spence.

 

[Spencerleehorton]

HI All,

Little update, EF80 is microphonic so need to change that, have changed connector and all handling noise is and hum is gone!!
Will test PIO caps version.

regards

Spence.

 

[0dbfs]

Hey Spence,

For some reason I thought you were using an EF800 @ 275mA but I see you mentioned an EF80 @ 300mA.... Just a little more current but that also better explains the drop with the PSU2649 since it's rated at 300mA.

Anyway, cool that changing the connector sorted out some hum. That was probably a grounding/shield issue I imagine.

Self Bias:
- Connect H+ to the heater pin and the other heater pin to GND. Tweak your H+ supply to desired voltage. Oliver recommends 5.05 on his schematic and the tube is rated for 6.3 so in that range to taste.
- Resistor between cathode and GND with a value such that cathode-gnd voltage is 1.1V or in that area to taste... This makes your grid negative with respect to the cathode.
- Bypass cathode resistor with a cap to gnd. 25-47uF starting point. The value of that cap & resistor plus tube operating points will define the LF -3dB point. RDH4 manual has some equations on calculating that as well as around the web.

Fixed Bias:
- In this setup there are some voltage dividers in the heater circuit which provide a positive voltage reference to the cathode. Same thing though, grid is then negative relative to cathode. Positive voltage to cathode in that case is directly from the heater so any ripple remaining in H+ is then fed to the cathode and therefore modulated as part of the grid-to-cathode voltage and audible.
- Heater voltage value is more critical because the fixed-cathode bias is derived directly from the heater. ie; difference between 6.3 and 5.05 may change the negative grid value which I believe is spec'd @ -1.1V (or cathode @ +1.1V)....

Hope that helps!
Cheers,
jonathan

 

[Spencerleehorton]

HI Guys,

back to square one now, i have built the passive B+ and got hum, so went back and used the Gyraf B+ and still get hum!!!
something is seriously not right, my brain is scrabbled now so gonna let the dust settle and look over it all again.
Need to check all my grounding i think, most of my problems have been to do with this poxy connectors and this cable.
gonna make up the lead again and make sure everything is secure.

0dbfs it does say in the title U47 build using EF80!!!!!
thanks for explaining the self bias think ive nearly got it now.

regards

Spence.

 

[Melodeath00]

I haven't read the whole thread, but have you tried a bypass cap at the cathode? If that clears up the noise, it would tell you there is hum on the heater line.

 

[Spencerleehorton]

Melodeath,

what value for the bypass cap? same as what jonathon has said? also what type and what voltage?

regards

Spence.