U47 with EF14 tube Build

[Spencerleehorton]

Here is my nearly finished U47 with the EF14 in it with stock values.

I ordered body, valve, transformer, capsule and 7 pin connector from aliexpress and cost about £200 with components

 

[MagnetoSound]

Looks ok! Is there another capsule on the way?

 

[MaxDM]

My 2 cents on this argument.

I had built a U47 copy, using Wagner's kit years ago and was determined to use an EF14.

I bought a bunch of Telefunken sealed NOS EF14's as well as EF12's (that were used in the 'bottle' mics). This was 20 years ago, so...

I underheated the EF 14 to try and get it to behave like a VF14, but the end result is that after some time, the tube would become noisy.

I was told that the EF14 has the filament which is composed of many individual smaller filaments in parallel, so that when it heats up, it tends to heat up in the center first. The VF14 has the small filaments wired in series, and this seems to work better with underheating.

The VF14 was chosen because the U47 was designed for 105 VDC (roughly?) post-war wall sockets. Like other parts of the world, at the time, some places used DC for mains electricity in some neighborhoods, etc.

To get a tube to work at 105 VDC, they had to find a tube with a high voltage filament.. hence they chose the VF14's which had to be selected at the factory.


The EF14 sounds aggressive and a bit rough, maybe due to the fact that, it is a 300 mA tube, with higher current capability than an EF12, which is the more 'correct' tube.

At full heater voltage the EF14 works fine, in terms of not going noisy, but it heats up the mic quite a lot, and the sound is quite hard and aggressive.

After a couple of years of fighting with the EF14, I put an EF12 in there, and the mic sounded much more useable, IMO. Smoother top end, quiet, did not heat up too much, less distortion etc.

I much prefer the EF12 or preferably EF12K, which is the low (mechanical) noise version.

A VF14 does not sound like an underheated EF12, IMO.. I've never tried a brand-new VF14, so that may be part of it, but the EF12, IMO is even superior to the VF14's that I heard, which were old and probably worn out..

 

[MaxDM]

find quite contradictory information:

here says the EF12 is not suitable for a U47

http://www.moxtone.com/mU47_U47_part3.html
AEG use them and say that both EF12 and EF11 are perfect?

http://microphoneboutique.com/product/aeg-moscow-mkl-49/
View attachment 112873

found a diagram

IMO, underheating an EF12 in this application is not necessary, and can substantially lower the life of the tube.. It will not sound more like a VF14, although I imagine someone might like the sound.

 

[Spencerleehorton]

yes im waiting for another M7 capsule and will tweak the bias down as the waveform is a little off and i believe that to be the anode voltage, going to try 82k and then a 50k in replacement of the 100k and see what the waveform is like and measure the voltage on the anode.
On my PSU the 250R 2W pot is playing up as well, so im replacing that so i can dial in the heater voltage more accurately

 

[Cqwet Dbdfte]

Try using a constant current for the heater, an L317 can be used is series mode. That also helps with inrush current on a cold heater.
Trans-conductance of the tube is temp dependent, which may be a way of finding the right one. An FLIR type camera may work, have not tried.
Underheating the filament was a hack to reduce noise, if I got the story correct.
Tube has to have low microphonics, as only one "mike" is planned here. Keeping mechanical stability of all the parts inside the mike should be considered.
I'm not a fan of bird-nest circuits.
Gig-Ohm resistors should be cleaned with a good solvent, acetone works for most electronics, but some plastics will be damaged.
Leakage currents around sockets and circuits should be avoided.
Plate load resistors could be wire-wound, usually quite large for their values, but has better tempco and lower noise than metal film and other types.
Bulk Metal Foil resistors have the best specs, but expensive.

 

[Spencerleehorton]

I’ve now built 4 x u47s and only two Psu.
Both of the psu feature lots of filter caps and choke to get clean b+105v and H+ 5.05v.

I’m interested in building another two psu and wondered on other options which could work, anyone have any other ideas?

 

[rock soderstrom]

I’m interested in building another two psu and wondered on other options which could work, anyone have any other ideas?

You could try a TL783 power supply for B+ or one with a capacitance multiplier. A voltage multiplier with a lot of additional RC filtering would also be conceivable.

I would be interested to know how much difference that would make with the rather simple U47 circuit in terms of sound.

 

[rock soderstrom]

Hey, how about a tube-based PSU? Winter is coming, a tube rectifier and tube shunt regulator could provide extra warmth in the studio.

 

[Spencerleehorton]

Well I have some TL783 so if you could share or point me in the direction of some schematics?
Defo willing to give it a go, would be nice to have something a little simpler psu wise!!

 

[rock soderstrom]

Well I have some TL783 so if you could share or point me in the direction of some schematics?

Cheek the datasheet (and google), Jakob uses the TL783 in his G9

 

[Spencerleehorton]

Well I’m pretty sure I have a couple of 170vac and 6.3vac txf, also have a couple of 24vac toroidals.
Going to see what components I have as it would be nice to be able to build these psu out of what parts I have.



This could work

 

[rock soderstrom]

This could work

Sort of, you'll probably need a series resistor in front of the TL783 to get rid of some voltage. Additional RC filtering after the regulator would also be better, essentially like Jakob does in the G9.

Edit: what is the function of this 22k 5Watt resistor?
Edit2: Khron is right, it was early in the mornig...

 

[Khron]

Edit: what is the function of this 22k 5Watt resistor?

Discharging the capacitors between the bridge rectifier & regulator? And, via the antiparallel diode with the TL783, also the capacitance after the regulator?

 

[rock soderstrom]

Discharging the capacitors between the bridge rectifier & regulator? And, via the antiparallel diode with the TL783, also the capacitance after the regulator?

Makes sense, a bleeder resistor for the capacitance. Makes working on this circuit less exciting

 

[Spencerleehorton]

for the 170Vac B+ side would this be ok?

Just need to know if i'll be able to use the 6.3vac for the H+?

 

[rock soderstrom]

Just need to know if i'll be able to use the 6.3vac for the H+?

Some do it with shotkey diodes and low drop voltage regulators, I do it with a voltage doubler and additional filtering. Check this out, you might get some inspiration from it. (this method is more flexible IMHO)

https://groupdiy.com/threads/diy-pr...ps-like-preamps-buffers-or-microphones.84324/
If you underheat, it may work without a voltage doubler with an UF4007 bridge rectifier and an ordinary LM317.

 

[Spencerleehorton]

yeah i was thinking of a voltage doubler as i have 3.8amps to play with on the 6.3vac winding, thank you

 

[rock soderstrom]

for the 170Vac B+ side would this be ok?

I am not an expert for Tl783 circuits, I would add a little resistance in front of the 100uf cap on the output side of the regulator to get some extra filtering.

Perhaps a basic load should also be installed here, since the tube draws almost no current and the regulator, according to the data sheet, needs a certain current flow to regulate cleanly.

For the bleeder I normaly use resistors in the 100k to 200k range.

 

[Spencerleehorton]

the only thing i would ask is on the heater side, would a parallel pot work so i could adjust the volatge to 5.05v or there abouts?