D-EF47 Tribute To Oliver Archut U47 Build Thread.

[JMPGuitars]

I've decided to build one of these, and I just finished up the power supply. Voltages are good with dummy loads.

Question: what's the purpose of running the heaters at 5.05V? Typically within 10% is the safest, and for an EF800 that's 5.7V minimum.

 

[Delta Sigma]

5.05 came from Oliver I believe. Underheating in general is very common in mics. Google underheating. Or maybe start here: https://repforums.prosoundweb.com/index.php?topic=36769.0

 

[JMPGuitars]

5.05 came from Oliver I believe. Underheating in general is very common in mics. Google underheating. Or maybe start here: https://repforums.prosoundweb.com/index.php?topic=36769.0

That thread was entertaining, but not very useful. They posed the question and went off topic for 3 pages.

After further googling, it seems people think that this improves the s/n ratio & linearity in a mic. But this is still dangerous for most tube types, including the EF800. Going beyond 10% variance is usually bad, and the EF800 spec says to be within 5%. The 5.05V is just above 20% underheating.

This page points to more speculation about running 20% below for better linearity: https://vacuumtubes.net/How_Vacuum_Tubes_Work.htm

But I have yet to see any objective scientific / verifiable proof of anything beyond risking the lifespan of the tubes.

All that being said, I'll try it out anyway because I ordered 10 tubes used on ebay for pretty cheap. I'll test them on my tube tracer and put the 2nd best one in the mic.

 

[Purplenoise]

That thread was entertaining, but not very useful. They posed the question and went off topic for 3 pages.

After further googling, it seems people think that this improves the s/n ratio & linearity in a mic. But this is still dangerous for most tube types, including the EF800. Going beyond 10% variance is usually bad, and the EF800 spec says to be within 5%. The 5.05V is just above 20% underheating.

This page points to more speculation about running 20% below for better linearity: https://vacuumtubes.net/How_Vacuum_Tubes_Work.htm

But I have yet to see any objective scientific / verifiable proof of anything beyond risking the lifespan of the tubes.

All that being said, I'll try it out anyway because I ordered 10 tubes used on ebay for pretty cheap. I'll test them on my tube tracer and put the 2nd best one in the mic.

Maybe somebody more knowledgeable could chime in but I think it also has to do with impedance the transformer sees...at 5.05 I think the impedance is closer to what the bv08 wants to see. I could be totally wrong though.

 

[rmburrow]

Maybe somebody more knowledgeable could chime in but I think it also has to do with impedance the transformer sees...at 5.05 I think the impedance is closer to what the bv08 wants to see. I could be totally wrong though.

Follow what the late Oliver Archut says.  I had a long phone conversation with him years ago.  You may recall the VF14 tube in the U47 mic has a 55 volt heater but it is operated around 36 volts.  There is negligible audio power involved in these mic circuits.  Essentially, these "anode followers" provide impedance transformation from the capsule (at 100 megohms or higher) through a suitable transformer to connect to external equipment.  Tube noise is probably lower also (not as many electrons emitted per unit time from the cathode when the heater voltage is lowered).

Operate a EL34 or some other amplifier tube that handles power at significantly low heater voltage and cathode bombardment may eventually wreck the tube.  Same with rectifier tubes (especially mercury-vapor types used in transmitting equipment).  Common rectifiers like the 6X4, 5AR4, 5U4, 5V4, etc. should be operated at their rated filament/heater voltages.

 

[Purplenoise]

Follow what the late Oliver Archut says.  I had a long phone conversation with him years ago.  You may recall the VF14 tube in the U47 mic has a 55 volt heater but it is operated around 36 volts.  There is negligible audio power involved in these mic circuits.  Essentially, these "anode followers" provide impedance transformation from the capsule (at 100 megohms or higher) through a suitable transformer to connect to external equipment.  Tube noise is probably lower also (not as many electrons emitted per unit time from the cathode when the heater voltage is lowered).

Operate a EL34 or some other amplifier tube that handles power at significantly low heater voltage and cathode bombardment may eventually wreck the tube.  Same with rectifier tubes (especially mercury-vapor types used in transmitting equipment).  Common rectifiers like the 6X4, 5AR4, 5U4, 5V4, etc. should be operated at their rated filament/heater voltages.

Thanks for chiming in. I think though that some tubes handle underheating better than others...obviously the vf14 has been proving its “underheating” superpower everyday for 50 years haha

 

[JMPGuitars]

Follow what the late Oliver Archut says.  I had a long phone conversation with him years ago.  You may recall the VF14 tube in the U47 mic has a 55 volt heater but it is operated around 36 volts.  There is negligible audio power involved in these mic circuits.  Essentially, these "anode followers" provide impedance transformation from the capsule (at 100 megohms or higher) through a suitable transformer to connect to external equipment.  Tube noise is probably lower also (not as many electrons emitted per unit time from the cathode when the heater voltage is lowered).

Operate a EL34 or some other amplifier tube that handles power at significantly low heater voltage and cathode bombardment may eventually wreck the tube.  Same with rectifier tubes (especially mercury-vapor types used in transmitting equipment).  Common rectifiers like the 6X4, 5AR4, 5U4, 5V4, etc. should be operated at their rated filament/heater voltages.

The VF14 is a very different tube than an EF800. I  think I'll record with the heater at 5.05V, 5.7V, and 6.0V and see if there's a real perceivable difference.

 

[Delta Sigma]

That thread was entertaining, but not very useful. They posed the question and went off topic for 3 pages

Ha ha, I didn't even read it. There's usually some good info from that crew (and almost always some bickering).

 

[JMPGuitars]

I finished the EF47 build. Now I wait for the tubes to arrive from Germany. In case anybody is curious/wants a quieter mic, I marked the caps for the outer foil direction in the circuit.

 

[Ricardus]

I finished the EF47 build. Now I wait for the tubes to arrive from Germany. In case anybody is curious/wants a quieter mic, I marked the caps for the outer foil direction in the circuit.

How are you determining that? Did you build Mr. Carlson's box?

Another guy I follow on youtube (XRayTonyB) does a lot of vintage stereo restorations, and he recapped a device PURPOSELY putting outside foil the wrong way. The the unit's noise floor was actually quieter than when he re-installed them correctly and measured again.

 

[JMPGuitars]

How are you determining that? Did you build Mr. Carlson's box?

Another guy I follow on youtube (XRayTonyB) does a lot of vintage stereo restorations, and he recapped a device PURPOSELY putting outside foil the wrong way. The the unit's noise floor was actually quieter than when he re-installed them correctly and measured again.

No, I just hold the capacitors tight in my fingers. The difference is generally obvious, and when it isn't I have measurements on the scope screen.

I've found it does help. But lead dress and solder connection quality is more important. I'm guessing his experiment wasn't fantastically scientific.

 

[Ricardus]

No, I just hold the capacitors tight in my fingers. The difference is generally obvious, and when it isn't I have measurements on the scope screen.

I've found it does help. But lead dress and solder connection quality is more important. I'm guessing his experiment wasn't fantastically scientific.

Well, how would we design a proper experiment to measure this... I'd be curious to know if there are truly any measurable differences.

 

[JMPGuitars]

Well, how would we design a proper experiment to measure this... I'd be curious to know if there are truly any measurable differences.

You would need to use the same DUT (device under test), and you would need to use the same capacitor(s) in the device.

The problematic factors I can think of quickly would be:
risk of overheating the capacitor;
inconsistent solder joint quality;
DUT would need 3rd party inspection for consistency;
accuracy of foil testing;
correct orientation in circuit / consistency of same;
must be tested in an anechoic space;
may need external source to introduce unwanted signal and test DUT's resilience (oscillations, etc...);

For a scientific(ish) difference, you can do bode plots and see if there are differences there, as well as SNR and other tests. To be really scientifically accurate, it would need to be on a larger scale with multiple devices.

Then to test the subjectivity of the sound, you would need a double-blind test to see if there is a perceivable difference with multiple participants.

Another thought is that if the shielding is doing it's job well, it may not directly impact SNR (and certainly not necessarily consistently). If the effect of the shielding operating correctly is to make it less susceptible to oscillations and outside interference, that's very specific noise, and not necessarily basic noise you would hear under normal circumstances.

My first thought when you said that about his noise floor being lower with the caps allegedly being backwards is that 1. maybe his orientation in the circuit was wrong; or 2. maybe he did a better job soldering them in the wrong way. There's too many factors to have such a simple contrary opinion.

Thanks,
Josh

 

[shabtek]

Is it shielding or capacitive gradient/coupling?

 

[JMPGuitars]

Is it shielding or capacitive gradient/coupling?

If the outer foil is connected correctly, it acts as shielding. There's no capacitance issue on non-polarized capacitors. Not all foil capacitors are designed this way, but most of the ones we would use in mics (or guitars and amps) are.

The attached image is combined screenshots I took testing a .68µF film cap. Obviously one orientation shields better than the other. The only difference is which end of the cap has the probe tip vs. ground clip.

 

[Delta Sigma]

If the outer foil is connected correctly, it acts as shielding. There's no capacitance issue on non-polarized capacitors. Not all foil capacitors are designed this way, but most of the ones we would use in mics (or guitars and amps) are.

The attached image is combined screenshots I took testing a .68µF film cap. Obviously one orientation shields better than the other. The only difference is which end of the cap has the probe tip vs. ground clip.

I haven't considered this before. I'm curious if it makes a difference in an assembled mic. A guitar amp often has poor shielding (open chassis in a wood box). A mic should have a shielded body and cable bonded to ground.

 

[Ricardus]

My first thought when you said that about his noise floor being lower with the caps allegedly being backwards is that 1. maybe his orientation in the circuit was wrong; or 2. maybe he did a better job soldering them in the wrong way. There's too many factors to have such a simple contrary opinion.

Thanks,
Josh

I have confidence he had them correctly oriented when they were supposed to be incorrect, and correct. He's a solid engineer.

 

[Gus]

That thread was entertaining, but not very useful. They posed the question and went off topic for 3 pages.

After further googling, it seems people think that this improves the s/n ratio & linearity in a mic. But this is still dangerous for most tube types, including the EF800. Going beyond 10% variance is usually bad, and the EF800 spec says to be within 5%. The 5.05V is just above 20% underheating.

This page points to more speculation about running 20% below for better linearity: https://vacuumtubes.net/How_Vacuum_Tubes_Work.htm

But I have yet to see any objective scientific / verifiable proof of anything beyond risking the lifespan of the tubes.

All that being said, I'll try it out anyway because I ordered 10 tubes used on ebay for pretty cheap. I'll test them on my tube tracer and put the 2nd best one in the mic.

Lowering the heater voltages should lower the transconductance and cause other changes

IIRC There was a good Mullard article I can not find again about cathode alloy and oxides(N type semiconductor). I am guessing the alloy and oxides used are a big part of under heating tube plate out to transformer microphones

A good link
https://www.john-a-harper.com/tubes201/

 

[JMPGuitars]

I haven't considered this before. I'm curious if it makes a difference in an assembled mic. A guitar amp often has poor shielding (open chassis in a wood box). A mic should have a shielded body and cable bonded to ground.

Mics can always use better shielding, and should avoid ground loops.

The amps I build, I shield the chassis. Sometimes with a plate, sometimes with copper tape. The cap orientation still helps.

I also use filtered inlets now, which I included in my EF47 PSU.

 

[JMPGuitars]

Lowering the heater voltages should lower the transconductance and cause other changes

This is true. Some tube datasheets include information for what drop in transconductance can be expected. For example, the 5840 datasheet says with 5.7V heaters, you can expect a 10% drop in transconductance.

The interesting thing is that low transconductance can also be a sign of a dead tube. For example, EF800 datasheet says if initial transconductance value drops to 5.4mA/V, that's a dead tube (EoL).

I wonder if with lower heater voltages / operating proportionally at a lower transconductance 1. really increases tube life as some believe; and 2. if a tube that was considered EoL on standard voltages would be usable at lower heater voltage.

Assuming an EF800 and the drop being proportional (and it isn't always), then a 20% drop would be 6mA/V, which is still within a good range. If one of the tubes I receive isn't great, I'll try and measure the transconductance at lower heater voltage and see what comes up.