U-47/ ef14 voltage question

[nielsk]

Working on a U-47 converted to EF-14, all other voltages look pretty close but to get the voltage at the node of G2, G3, A (1,4 &5) & the output transformer close to 34 volts, I have to change the R1 100k resistor to 250k (B+ is 105 VDC).
It is right at 1.1 VDC at pin K (7)
Is this a problem, or is getting the voltage correct the most important thing?

Thanks!

 

[david-p]

Looking at the schematic and its European-style writing in red, I assume that this is the recommendation of Neumann, who should know what they are doing.

A different approach is taken by Oliver Archut, who also feeds 5v to the heater.  He retains the 29 ohms to ground, and feeds this from the 5 volt heater supply through 100 ohms to establish a voltage of 1.1 at the cathode.  http://www.tab-funkenwerk.com/id68.html.  The 29 ohms does not affect the gain, and the capacitor across the 2k2 resistor in your circuit gives the same effect.

1.1 volts at the cathode should set the anode (and cathode) current to 0.5 mA, and this agrees with the 2k2 cathode resistance.  It should also set the anode voltage which is given on the diagram -- unless the gain (mA/V) of the valve is low.  Can you measure the tube parameters?

You should find that the resistor values given in red will work with the 105 volts HT.  (Sorry, this may not be of much comfort to you.)  If the HT is 105v, and Vanode is 34v, changing R1 to 250k implies that the anode/cathode current is only .25mA and thus the voltage drop across the 2k2 cathode resistor should be .55, which does not agree with your measurement.  Check that you have everything wired correctly, just in case current is going somewhere other than it should be.  It may also be that your meter is not measuring the anode voltage accurately if its self impedance is too low.  As an alternative, measure the current through R1 and calculate the voltages.

David

 

[Johan]

have you doublechecked that the supply voltage really is 105 volt still, now that the mic is moded?
now that you supply the heater in an other way than before, there is considerably less current pulling on the 105volt supply. perhaps it is too high now. that would mess with the voltages you expect
J

EDIT: rereading Davids reply, I see he allready said the same, with other words..

 

[nielsk]

Thanks for the replies,
I have been over the wiring & circuit many times, I am pretty sure there are no errors.
B+ is 105VDC, H+ is 5.5 VDC, measured at the mic.
I am using a B&K bench type DVOM, as far as I can tell it measures very accurately.
It seems like from your note I should try changing the tube (this tube is NOS, but I do have another to try).
I do not have a way to test this tube, other than in the mic.
I did try changing the cathode resistor to 29 ohms, and a 114 ohm to H+ (5.5 VDC) gave me the 1.1 VDC
As is it is working, but prone to distortion on moderate to high levels. With the 100k (instead of the 250k that gives the correct anode voltage) it is more prone to distortion...

 

[david-p]

Trying a different EF14 cannot do any harm.

What I am not clear on are the voltages Va and Vk when you have the circuit as modified in red.  What are they in these circumstances, and what do these voltages become when you replace R1 with 250k?

Regarding the possible change of the HT voltage, referred to above, have mods been done to the power supply?  I seem to recall that the regulation in the original power supply is done by means of series resistors (is there a neon tube also?) and these were calculated for the approximately 10x greater current draw of the VF14.  The lower current will mean lower voltage drops in the line if the resistors havent been changed.

David

 

[PRR]

> right at 1.1 VDC at pin K (7)

1.1V across 2K2 is 0.5mA.

With original plate load:

> B+ is 105 VDC

0.5mA in 30K+100K is 65V drop. 105-65= 40V.

If you are not getting 40V at anode, something isn't right.

40V is not a bad place to be. The "34V" is clearly for the original VF14, not this EF14 conversion.

I do think the plate current should be similar to the original to keep the same max output level; 250K gives significantly less current (nearer 0.25mA).

 

[nielsk]

I have tried different tubes with vastly different results. Al supposedly NOS, with stock value resistors one is 43VDC, one 21VDC, and one 28.5VDC.
They all sound fairly similar, the 43 volt is the brightest.
The power supplies I built, the heater has to be quite stiff and extremely quiet, I have it at 1.5 amp capable and the ripple down so low I can't measure it. 12 volt 2 amp transformer, full wave bridge, 33,000 uF into 5 ohm / 1,000 uF RCRC then a 7806, then 100 uF, 10 uF then 10 watt 1 ohm dropping resistor, more filtering caps...
The 105 supply is more straight-forward, a 120 volt transformer, full wave bridge, CRCRC, TIP transistor w/ 105 volt zener reference, and so on...

 

[david-p]

Firstly, from your description I would say that your power supply is not the culprit: you have more than enough filtering to remove hum and other means to establish the supply voltages.

The readings on the three different tubes would indicate that they are all have different values of µ.  I am surprised that this should be so if they are all up to spec.  If they are all genuinely NOS, i.e. 50-plus years old, there is a possible ageing process involved here, the most likely effect being loss of cathode efficiency.

But even so, I am puzzled by these vastly different readings.  You dont give Vk, which surely cannot be 1.1 volts for each tube.

Another method of biassing is to puit two 1N4148 diodes in series to ground from the cathode.  This holds Vk at 1.1 volts for varying values of IA and is described here: http://www.foxaudioresearch.ca/The47.htm .

It would be worth trying this to see which tube sounds the best.

David

 

[gyraf]

The different anode currents could very well be due to the fact that grid pull-down (at 60M Ohms) is much larger than what's aimed for in the EF14 - remember that it was not designed or spec'ed for microphones like it's cousin the VF14.

So what you see is probably grid current offsetting the whole system. Try shorting grid to ground via say a 1M resistor, and re-check.

Jakob E.

 

[nielsk]

Interesting...
The Vk stays remarkably consistent, varying only a few mV from tube to tube.
The diode biasing looks worthwhile trying, does anyone have experience of the effect of the different methods on the sonic qualities?
From what I have been able to discover, the VF 14 was designed for military field radios (Quote from Neumann:The VF14 was originally developed in the mid 1930s as a standard universal pentode for military field radios) hence the low production unit numbers, and the only difference (Quote: The VF14 is identicle to the EF14 and UF14 with a different filament current.) is the separate filament:

Heater voltage:
VF14 = 55V
UF14 = 25V
EF14 = 6.3V
Heater current
VF14 = 50mA
UF14 = 100mA
EF14 = 470mA
Typical operating conditions for VF14, UF14, EF14:
Anode voltage = 250V
Screen voltage = 200V
Control grid voltage = -5V
Anode current = 12mA
Screen current = 1.9mA
Mutual conductance = 7mA/V
Anode dissipation = 5W

Are you suggesting operating the mic with a 1M grid pull down, or this as a test, then returning it to 60M?
For the benefit of my (ongoing, thanks!) education, what are the effects of changing the grid pull down resistor?

 

[KrIVIUM2323]

Are you suggesting operating the mic with a 1M grid pull down, or this as a test, then returning it to 60M?

I think Jakob is telling you to test with 1m and if ok keep it that way, but i may be wrong.
Take a look at the schematic of O. Archut mod: 1G for ef14 and variants.

what are the effects of changing the grid pull down resistor?

First i would say it probably change the bass roll off of filter created by rgrid/ tube input capacitance.
Second it manage grid current aceptable for the tube.
But like you i'm learning, so take it with a grain of salt. 

I remmember reading about different materials used in the VF14 vs other types UF/EF/etc. I think this as to deal with the grid and his ability to withstand 'low rgrid' compared to EF/UF. But i'm not sure.

I may be awfully wrong.  :

 

[nielsk]

Do you have a link to that schematic?
I have 5 different EF14 (U-47) schematics (Archut included) and all show 60M grid pull down resistor

 

[KrIVIUM2323]

Hi Nielsk,
In the link David-p give on second post in this thread.
Take a look at the second one (right  corner) called 'alternate tube schematic'
R1:1G
With bom separated from schematic and R numbering different from 'original' Tfk/Neumann schematic it's easy to get mistracked.

 

[nielsk]

Aha!
Thanks for that... I will try it

 

[KrIVIUM2323]

R5 in O.Archut alternate schematic is 1G too.
In original it's 100m.
Maybe worth to change it too.
Is it an original U47 you'are working on or a 're-creation'?

 

[nielsk]

Luckily there are two 1g resistors in my G7 I can "borrow" to try this...
This is an original short body Neumann U-47 that was modified to use an AC701, now being reversed to a more original form
I notice on the linked schematic that the output cap was changed to 1uF from 0.5uF (as in all the other versions and the actual original versions)
What difference will this make?

 

[KrIVIUM2323]

Same as changing Rgrid. 
But it can be a problem:

First your mic will go lower in freq response and depending on use you'll probably need to filter all that 'rumble' noise afterward so... probably wise to stick with the original one.

The output of the circuit form some kind of L/R.C filter and depending on the kind of cap you'll use you can modify Q factor of this lrc. Can be a real problem 'theorically'. If you try to change it probably wise to use Film  cap (because of nearly no esr).

Take the second point with a grain of salt too.

Nielsk could you share some pictures with us? I could be really interested in some of the internal wiring if possible.

 

[nielsk]

Here you go...

 

[nielsk]

One more view

 

[KrIVIUM2323]

8) THANK YOU!
I like the presumably 'rubber' mount for the tube!
How did you build it?