U-47/ ef14 voltage question

[david-p]

Nielsk,

The original U47 does have quite a low value (60M) for the grid resistor.  With a capsule capacitance of 80pF the bass will start to roll off (-3dB) at 33 Hz in cardioid.  When both capsules are paralleled in omni mode, the bass will roll off an octave lower.

As an aside, I am at present not certain why 1G should be the usual value for Rg these days: I have always used it without questioning it; but I will think about this...

IF there is grid current, and I think this is unlikely to be the case with all three of them, your tubes are out of spec.  In this case, changing Rg to 1MB will change Vg, which, in the absence of grid current, should be 0v -- and also Vk and Va.  It is hard to measure these changes directly: one needs a very hi-Z dc meter.

I am actually building a U47 clone with polar diagrams switched in the power supply, using the Skylar EQU47 body and an EF14 tube (I also have three of these).  My power supply up to now has been a lash up, but I am building a proper one this weekend.  In testing my bread-boarded power supply I had consistent results with two of my tubes.

I am basically using the right hand circuit here: http://www.tab-funkenwerk.com/id85.html; but in order to obtain the fig-8 polar diagram I have raised the basic HT at the power supply from 105v to 120v, changed the value of the dropper resistor R8 and made R7 2Mohm, to compensate.  Va is still 34v and the capsule polarizing voltage still 60v.  I have not tried the diode biassing method, and am not sure what the advantanges would be.

When I have a finished power supply I will report on my findings in more detail.

David

 

[Johan]

..could the problems that started this thread have something to do with the fact that the original VF14 ran with underpowered heater?
perhaps playing with that part of the equation on your powersupply?.. with the voltage drop from 105volt through that 1780 ohm resistor, it only got ~35 volt for the 55volt heater. roughly 65%...I know that the swedish made SELA mic's (neumann knock offs made for tv studios, using EF95 tubes) ran their 6.3volt heaters at ~4.5volt, or ~70%...perhaps, there is something to consider?
J

 

[nielsk]

As I understand it, the diode biasing is supposed to deliver 1.1 volts no matter what other conditions exist...and that the 5 volt heater scheme is supposed to emulate the "under-powering" of the VF-14 heater in the original and is the key to the EF-14 sounding the same in the circuit.
As far as the mount I fabricated, that was rather involved... I found an aluminum ring that snugly fit the socket, and put some tiny sheet metal screws through it pointing out. Then I peen'd the ring around the socket Then I took a piece of foam and wrapped it around the ring, with the screws poking into it. I then took a piece of large shrink wrap & shrunk it around all of this. The "doughnut" fits snugly inside the mic frame, and has 3 more tiny sheet metal screws through the frame into the shrink / foam assembly. There are also machine screws that prevent the "doughnut" from moving up or down, but they are actually not necessary.

 

[david-p]

..could the problems that started this thread have something to do with the fact that the original VF14 ran with underpowered heater?
perhaps playing with that part of the equation on your powersupply?.. with the voltage drop from 105volt through that 1780 ohm resistor, it only got ~35 volt for the 55volt heater. roughly 65%...I know that the swedish made SELA mic's (neumann knock offs made for tv studios, using EF95 tubes) ran their 6.3volt heaters at ~4.5volt, or ~70%...perhaps, there is something to consider?
J

According to several who have trodden this path, the EF14 heater is underpowered by the same amount as the VF14 (i.e. takes the same power) when Vh is 5 volts, which is what Nielsk's circuit uses.

HOWEVER, I have read that there is a difference between the VF14 and EF14 in terms of the compostiiton of the cathode material.  This may (or may not) affect the sound.

David

 

[nielsk]

I just tried the two 1n4148 in series, I am getting 1.16 volts. Maybe a different diode? I have some 1N4448, I think some 1N917 here...

 

[KrIVIUM2323]

As far as the mount .... but they are actually not necessary

Nice process! I like the smartness of it.

takes the same power

What do you mean about that sentence Dave-p?
The heater draw (sink?) same amount of current? It don't seems logical to me. 

As i understood this underheating lead to increased Ri (internal resistance of plate) to match VF14 circuits conditions approx  8500r.

1.16 volts

right ballpark no?

 

[nielsk]

1.16 VDC is in the ballpark, but close enough to 1.1 to be a home run?

 

[KrIVIUM2323]

Don't know, it's tube after all... 5/10% tolerance... Honestly i don't know. Close enough for trying i would say.

As an aside, I am at present not certain why 1G should be the usual value for Rg these days: I have always used it without questioning it; but I will think about this...

Good question. Never been able to find a clear answer about that too.

HOWEVER, I have read that there is a difference between the VF14 and EF14 in terms of the compostiiton of the cathode material.  This may (or may not) affect the sound.

I think you're right. Cathode material is different. I am lucky enough to had Flea47 in previous job and all i can say is that the tube went noisy after some times (ef14) with some plop appearing from times to times. I've seen other models which didn't have this  problem. So definitely the tube as suspect (rather than the mic as a whole, which was quite good by the way).
The original VF14 was selected by Neumann (vf14-m if memory serve) so maybe the cathode material- his quietness -was the parameter for selection.
Anyway i had access to original longbody and never heard a plop with them.

 

[david-p]

1.16 VDC is in the ballpark, but close enough to 1.1 to be a home run?

Surely!

D

 

[david-p]

What do you mean about that sentence Dave-p?
The heater draw (sink?) same amount of current? It don't seems logical to me. 

As i understood this underheating lead to increased Ri (internal resistance of plate) to match VF14 circuits conditions approx  8500r.

What I mean is that the product of heater current and voltage is the same for the EF14 if you run it at 5 volts as it is for the VF14 when in the original U47 circuit.  It then follows, everything else being equal, that the heater is at the same temperature in each tube and that the emission of electrons from the cathode is the same, thus hopefully giving the same sound from the two valves.  I agree that there are a lot of conditions and assumptions, but several people seem to have been convinced by the sound of an EF14 when used as a replacement in an original U47.

David

 

[KrIVIUM2323]

Ok David, I understand what you mean explained that way.
Thanks for your answer.

 

[nielsk]

So now it has the 2 diodes biasing scheme & the 1G grid resistors, I am getting 32 volts at the output cap with a tube that is operating very low noise. I tried a Wima 1uF for output, changed right back to the .5uF Auricap. I'm not sure why, but it sounded plastic in the mids & just not exciting w/ the 1uF. Maybe there is something to the Auricaps...that makes them worth 10 times as much $
It is more to spec voltage wise, but I am going to try going back to the 100M & 60M grid resistors, I think it sounded better with them.

Later.. To me it sounds better with the 60M & 100M, a bit more hiss type noise but more presence and bigger, possibly a bit louder too. I'm sticking with it.

For those who are curious, here is page 2 of the doc on the 1st post..

 

[david-p]

Nielsk,

I have been looking again at both the original Neumann U47 and the circuit with red ink that you posted.

The original Neumann circuit ensures that the cathode is at 1.1 volts, by deriving it from the heater current, which is much more predictable than Ia.  The second configuration cannot hold Vk steady in the same way, and makes it depend upon the valve's d.c. parameters.  The diode method obviously works, but I personally would prefer to see a linear resistor instead of nonlinear diodes in the signal path.

For this reason, I think that Archut's circuit is to be preferred.  He also derives the 1.1 volts from the heater current, and the much higher value of If in the EF14 makes the 1.1 v of Vk even more stable than in the original Neumann circuit.  This, in my view, is a good thing.

As to the 60 and 100 Meg resistors, I think you may well be right to prefer them as opposed to 1G.  It seems that, given the propensity of 1G resistors to attract dirt and moisture and therefore become a source of noise, the use of them should only be contemplated in extreme need.  I will try the same values with my mike.

The other thing that I would say about the circuit is that the 1000 µF on the 5v line is probably not doing anything to earn its keep.  At 120Hz its impedance is .75 ohm.  The reduction in hum due to this is negligible, unless the cable has a high resistance.  For instance, the reduction in hum  would be 3dB if the cable has a resistance of 0.75 ohms Surely, your cable is less than .75 ohms.  In any case, with the power supply design that you described, the hum is probably inaudible without any filtering in the mike.  I suggest you see what effect removing this C has.  If it makes no difference, I would omit it.

David

 

[nielsk]

I was thinking that going the other way, as in a 10uf or even 1uF electrolytic, would be better on the H+ inside the mic
Thoughts?

 

[KrIVIUM2323]

I'm thinking as David-p for psu decoupling. Less capacitor is always good sonically speaking (specialy with electrolytics).

If you follow O.Archut path, (+5v heater and direct bias) you already have a decoupling cap in mic circuit ( 1uf right after 30k r). It serve at least 2 purpose and 1 of them is decoupling. As you supply heater with dedicated  psu, your b+ is really overrated and stiffier so this 1 Uf should be sufficient.

 

[nielsk]

I mean on the H+, not the B+... The heater P/S is the potential hum source...

 

[MagnetoSound]

For this reason, I think that Archut's circuit is to be preferred.  He also derives the 1.1 volts from the heater current, and the much higher value of If in the EF14 makes the 1.1 v of Vk even more stable than in the original Neumann circuit.  This, in my view, is a good thing.

Actually, no. In Oliver's circuit, the heater is not in series with the cathode resistor. The cathode bias is taken via R2 from the supply side of the heater and since the cathode resistor has the same value the bias current is the same 38~39mA.

 

[nielsk]

So is there any advantage to Oliver's approach vs. the diodes? It seems with the diodes there is less chance of any noise getting into the audio.
Is having 1.1 VDC at the cathode all that matters?

 

[david-p]

I was thinking that going the other way, as in a 10uf or even 1uF electrolytic, would be better on the H+ inside the mic
Thoughts?

I dont actually see any need for a C inside the mic on the H+ supply if the resistance of the cable to the power supply is less than 1 ohm, which it should easily be.  You have plenty of decoupling/smoothing in the power supply.  It is easy to disconnect the 1000µF to see whether it makes any difference.  In my opinion, if there IS hum on the H+ supply entering the mic, 1000µF would be too small to deal with it.

[quote author=Nielsk]
So is there any advantage to Oliver's approach vs. the diodes? It seems with the diodes there is less chance of any noise getting into the audio.
[/quote]

Yes, with the diodes there is no connection to the LT supply, and therefore less likelihood of hum.  I just dont like the idea of putting a non-linear diode in the signal path.  But perhaps this is an unnecessary fear of mine.

[quote author=Nielsk]
Is having 1.1 VDC at the cathode all that matters?
[/quote]

Yes, because this sets the grid bias, which in turn sets the anode (and cathode) current.  With the EF14 (or the VF14) this should be about 500µA, and this in turn sets the voltage at the anode.

[quote author=MagnetoSound]
Actually, no. In Oliver's circuit, the heater is not in series with the cathode resistor. The cathode bias is taken via R2 from the supply side of the heater and since the cathode resistor has the same value the bias current is the same 38~39mA.
[/quote]

Sorry, I was wrong: please ignore my remark!  What is significant, however, is that any hum or noise on the heater supply will arrive at the cathode (13 dB lower).  If this is a problem, a big C across the 29 ohms would cure it, but I would rather deal with the problem in the power supply itself.

I hope I have clarified the issues.

David

 

[telefunk]

I am basically using the right hand circuit here: http://www.tab-funkenwerk.com/id85.html; but in order to obtain the fig-8 polar diagram I have raised the basic HT at the power supply from 105v to 120v, changed the value of the dropper resistor R8 and made R7 2Mohm, to compensate.  Va is still 34v and the capsule polarizing voltage still 60v.  I have not tried the diode biassing method, and am not sure what the advantanges would be.

When I have a finished power supply I will report on my findings in more detail.

David

David,

Did you finish this PSU? I'm interested how you dealt with the polarity switch issue...

What values did you use for R8?

I'm REALLY interested because i'm in the middle of building the same schematic (Altenative with Switch in the PSU) and havent still managed to figure this out...