U47 Build using EF80

[Melodeath00]

I am quoting Oliver here. It should be relevant since U47 with EF80 will be very similar. "min. of 16V and everything above 220uF will work." Positive lead to cathode, negative lead to ground. I am not sure if this is the same as what Jonathan has said. I don't know if it changes the mic to self bias or not. Maybe that's not what Jonathan is saying.

I am just a noob, but from what I understand, any stray AC from the cable or PSU would be shunted to ground through this cap and not modulate your audio in the tube.

 

[Spencerleehorton]

Hi all,

I have abandoned the fixed bias approach as this only really related to the VF14 valve, now going with self bias and works like a charm.
No hum, 2k2 with 220uf on cathode, mic sounds amazeballs!!!
have built both the passive and rectified psu so will compare to see if any difference.
Will start putting together another body for the PIO caps version and then compare both mics.
thanks again to everyone for their help.

regards

Spence.

 

[Melodeath00]

Is the 2k2  a resistor from cathode to ground, but bypassed by the 220uF?

 

[Spencerleehorton]

HI Melodeath00,

the 2k2 resistor is put in parallel with the 220uf to ground.
simples!!

regards

spence.

 

[0dbfs]

Cool. The 2k2 provides 1.1V?

The higher the value the cap the lower the hpf -3db corner freq.

I guess balls=good.

Cheers,
jb

 

[Spencerleehorton]

Hi All,

yes 2k2 provides 1.1V and i give the H+ 5.05v and its all good, running a higher B+ to get more sensitivity.

regards

Spence.

 

[OPR]

I've been working on a U47 build with ef80 tube Thiersch blue line capsule. What I did for a fixed bias was to use two 1N4148 diodes in series to get the 1.1v into the cathode. Sounding amazing! BTW!

It's such a nostalgic feeling listening to the mic as It gives you that sound you've been hearing and trying to get for your whole life! It's like ahhh no wonder I couldn't get  there 

For those who want to know how it's done, you'll need to add two 1N4148 diodes and connect them in series with both of the  the cathode(stripped sides) connected to ground and the anode connected to the tubes cathode.


I'll probably experiment with the PSU some more to supply the voltage to see how it sounds but the diodes are quick and easy!

 

[firehazard]

Hi,

this is a really great build thread to read. Thanks for sharing!

So much useful information for anyone who is new to the idea of scratch building a mic.

Like me

Just to let you know Spencer that there are people wishing you luck and hoping you won't need it.

Take care now...

 

[Spencerleehorton]

HI Firehazard,

thanks for your words, I have completed this build with several self etched pcbs, psu and mic and mic and psu works great (started with vero board).
I am in the process of having some pcbs made for me as a friend wanted a couple of these mics as they sounded so good, even with a basic capsule and transformer (which will be upgraded later).
I am in the process of building U47 style bodies and perfecting nickel plating (finishing off my M49 bodies as well)
will post pictures as soon as i'm done as i have documented this build from the start.

regards

Spence.

 

[Spencerleehorton]

Hi All,

few new pics

bottom caps

 

[Spencerleehorton]

all the bits.

 

[Spencerleehorton]

another one.

 

[Spencerleehorton]

HI GUys,

little update, just finished the body for this to go in, nickel plating is working again after quite a few problems.
Now to finish the M49 bodies!!

regards

Spence.

 

[Spencerleehorton]

.

 

[Spencerleehorton]

.

 

[Patrick from Davis]

In case people wanted to know more about setting the self bias points (cathode biasing), I will point you to a DIY guitar amp page that has some tube circuit theory.  Most, if not all, of the preamp tubes in guitar amps are biased in this manner.  Page 5 has the circuit that is referred to, and the second paragraph of page 6 is where the nuts and bolts of the self bias resistor is explained.  Basically knowing your voltages at a few points can let you set/use the plate load resistor (for current) and the self bias/cathode bias resistor (for voltage) and be done with it.

http://www.ax84.com/static/p1x/p1-ex-theory.pdf

Patrick

 

[RuudNL]

Interesting article! Especially for those who grew up in the transistor era. (And for those who forgot some things about tubes...)

 

[micaddict]

Tubes are the future. 

 

[OPR]

Have people found the Microphonics to be an issue with their ef80's? I'm using Nos Mullard tube which sound pretty good but it's a bit microphonic for my liking! so I switched for nos EF800 which are dead quiet but seem to sound totally different, a bit dark flat and lacking HF detail but maybe they will improve after burn in...

 

[Melodeath00]

Do you have any clips of the microphonic EF80? I'm still yet to hear a microphonic tube microphone.