Interesting bit of information

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Re: Interesting bit of information

Post by Glenndk on Sat Dec 30, 2017 4:27 pm

How about replacing the glass triodes with nuvistor triodes - even for condenser microphones? I have not checked transconductance nor any other charateristics:

http://en.wikipedia.org/wiki/Nuvistor
Quote: "...
Nuvistors are among the highest performing small signal receiving tubes.
...
It was also later found that, with minor circuit modification, the nuvistor made a sufficient replacement for the obsolete Telefunken VF14 tube, used in the famed Neumann U 47 studio microphone.[1]. Tektronix also used nuvistors in several of its high end oscilloscopes of the 1960s[2], before replacing them later with JFET transistors.
..."

https://en.wikipedia.org/wiki/Neumann_U47
Quote: "...
Custom tubes equivalent to the VF14 have been manufactured specifically for the U 47, by Telefunken[2][3] and others.[4] Beginning in 1962, Neumann offered a fully functional Nuvistor, small signal receiving tube replacement kit for the VF14 and required minor circuit modification.[3]
..."

6CW4 Nuvistor High-Mu Triode Tube:
http://datasheets.hypertriton.com/6CW4.pdf
http://electrontubestore.com/index.php?main_page=product_info&cPath=68_78&products_id=1044

2CW4 Nuvistor Triode Tube
http://electrontubestore.com/index.php?main_page=product_info&cPath=68_78&products_id=2156

8056 Medium-Mu Triode Nuvistor Tube:
max 50V 0,45Wa
http://datasheets.hypertriton.com/8056.pdf
http://electrontubestore.com/index.php?main_page=product_info&cPath=68_78&products_id=1049

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Re: Interesting bit of information

Post by Glenndk on Sat Dec 30, 2017 4:18 pm

Hi Ruud

Phædrus Audio AC701 is a real triode tube; it is not a solid-state (silicium) replacement:
http://www.phaedrus-audio.com/AC701%20Data%20sheet.pdf

Ruud wrote:
Nowadays there is even a company that produces a solid state replacement for the AC701 microphone tube: 
http://www.phaedrus-audio.com/AC701%20electronic%20tube.htm

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Re: Interesting bit of information

Post by Glenndk on Sat Dec 30, 2017 3:38 pm

I also wish you a happy new year.

-

In this document on page 28, it is mentioned that a low power fetron is made of cascode consisting of 2N4882 (equiv. 2N5543) and a selected 2N3823:
http://hrsasa.asn.au/docs/Fetron.pdf

I can not find 2N4882 nor 2N5543, but I found these possible upper cascode FETs:

DN2540N5-G 400V 150mA TO-220 15W fast (depletion but lower fet might only have 1Vds) Cgs=200pF:
http://ww1.microchip.com/downloads/en/DeviceDoc/DN2540%20B060313.pdf

450Vds 140mA 2W fast (enhancement - needs a Vgs voltage source) Cgs=70pF:
https://www.diodes.com/assets/Datasheets/ZVN0545G.pdf

LND150 30mA 500Vds but slow 1,3 uS fall time (depletion but lower fet might only have 1Vds) Cgs=7.5pF (best? => fastest cascode?):
TO-243AA (SOT-89) 1.6W:
TO-92 0.74W:
http://ww1.microchip.com/downloads/en/DeviceDoc/LND150%20C041114.pdf

Maybe the fet source should be driven by this low impedance high current output op-amp?: LMH6723 370MHz -3dB:
http://www.ti.com/lit/ds/symlink/lmh6723.pdf
Quote: "...Current Feedback Operational Amplifier..."


Last edited by Glenndk on Sat Dec 30, 2017 5:09 pm; edited 3 times in total

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Re: Interesting bit of information

Post by Admin on Sat Dec 30, 2017 9:26 am

Fetron, now I am learninf something more :-)
I read years ago that FETs could replace tubes, but I never realised there was a marketable product produced.

I will look into it. Thank you very much.

Happy New Year to you and all from Harry - SM0VPO

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Re: Interesting bit of information

Post by Glenndk on Sat Dec 30, 2017 7:45 am

Admin wrote:
...
Last night I realised that an ECC81 takes 300mA to heat.
Since I am only using an anode current of 0.5mA I thought about lowering the heater current a bit more. I tried both heaters in series (12v) and fed them from the 6v transformer. Current is now less than 100mA and they glow. The valve works very well with the lowered emission and I have about 8v Pk-Pk to drive the tube plates :-)

Just a bit of interesting data.

BR Harry - SM0VPO

How about using Fetrons instead of tubes. No heater current needed. The lifetime are supposed to be 100 years. No microphony. Extremely robust. Semiconductors might replace tubes in the future... No warm up time. Faster "warm-up time" than todays computers. Computers takes minutes to warm-up :-)

https://da.wikipedia.org/wiki/Fetron

https://en.wikipedia.org/wiki/Fetron

https://www.radiomuseum.org/forumdata/upload/Fetron_TS6AK5W.pdf
Quote: "...
TS 12 AT 7 ~ ECC 81 49.-- 41.-- 36.80
..."

http://hrsasa.asn.au/docs/Fetron.pdf

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Re: Interesting bit of information

Post by Ruud on Fri Dec 29, 2017 9:03 am

Yes, sometimes pentodes are used in a triode configuration in condenser microphones.
I have used the 5840 (pentode) tube with reasonable success in a microphone.
What is very important is the noise generated by the tube itself.
We are dealing here with very small signals (sometimes lower than 1 mV.) and extremely high impedances (1 G.ohm)
From all the tubes I have tested, the 6S6B-V was the best.
Nowadays there is even a company that produces a solid state replacement for the AC701 microphone tube: 
http://www.phaedrus-audio.com/AC701%20electronic%20tube.htm
I have thought about how they could have done this. I suppose it is a source follower combined with a high-voltage NPN transistor stage.
But of course the "phreakz" (they are the worst!) want a real AC701 tube!

(One of the things I do is the repair and restoration of vintage studio microphones. For some reason people think that microphones from 1950-1960 are better than the ones produced nowadays.)
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Re: Interesting bit of information

Post by Ivan on Fri Dec 29, 2017 7:33 am

Hi Ruud,
what about Russian pentodes like 1ZH29B? If you connect the 2nd grid with the anode, they should behave like triodes. The filament voltage is smaller and DC.

BR from Ivan

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Re: Interesting bit of information

Post by Ruud on Thu Dec 28, 2017 8:13 am

The reason for the high price is that Telefunken only produced a limited number of them.
The AC701 tubes for condenser microphones have the suffix 'K' (German: 'Klingarm') which stands for 'non microphonic'.
The tube itself is a simple single triode with a 4 V heater.
There are many of those tubes with wires through the glass instead of pins.
Recently I bought a batch of 25 pcs. NOS Russian 6S6B-V tubes.
I have tested them all, and they all can be used for condenser microphones.
The noise of those tubes is very low. (Important for a microphone!)
But the "audiofools" insist on having a real AC701 in their microphone, and apparantly are willing to pay for it!
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Re: Interesting bit of information

Post by Admin on Wed Dec 27, 2017 8:46 pm

Ruud wrote:By the way: did you look at the price of a replacement AC701 tube?
https://tubedepot.com/products/ac701
Shocked
Oops, no I missed that.
Good grief, are they expensive? or are they expensive!! I presume the price reflects the demand and availability of them. Are they no-longer made?

I wonder if that particular package is (or was) common? I am not certain but I think I have some similar, wire-ended tubes, of about the same size. Not sure if they are here or in Spain. I will have a look for them. If they are similar, then can you use them? No promisses.

BR Harry

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Re: Interesting bit of information

Post by Ruud on Tue Dec 26, 2017 9:13 pm

By the way: did you look at the price of a replacement AC701 tube?
https://tubedepot.com/products/ac701
Shocked
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Re: Interesting bit of information

Post by Admin on Tue Dec 26, 2017 8:34 pm

Hi Ruud,
Thank you very much. I really appreciate your comments.

My thought was that the tube would last longer, but that was based on supposition and not technical knowledge.

I have already observed that the noise is somewhat lower (or I have a particularly good tube), but now I have learned a lot more from you. I don't have any experience of runing valve cathodes cooler than about 1000°C.

Unfortunately I need an ECC81 for the timebase and another ECC81 for the x-axis amplifier. I only have a 10-Watt transformer so I have to run the anodes at 500µA and a total heater budget of 2-Watts. I already have 300µA bleeders for every 300V stage (0.5-Watts) as well as the current drawn by the CRT.

But I will have it working as soon as I we have finished renovating, decorating, Christmas, tidying, fixing, changing curtains, cleaning, washing, Easter, etc.. Maj-Lis goes back to Spain in 2 weeks so I can maybe steal a few hours when she is not here.

Best regards from Harry - SM0VPO

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Re: Interesting bit of information

Post by Ruud on Tue Dec 26, 2017 4:37 pm

Interesting.
So two tubes in series on 6v, this means 3v for each tube.
It works, I did this once with one of my tube microphones.
Underheating a tube also reduces the self noise of a tube microphone.
But I was warned to be careful with that!
I ran a 6.3V tube on 4V, no problems. 
Some people told me that underheating a tube could exhaust the cathode, in other words: it would reduce the lifetime of the tube.
In my case this would have been interesting, because the old tube microphones used an AC701 tube, with a 4 V filament. I found a good replacement, but with a 6.3V heater. Of course it would be great if you could only replace the tube and NOT change the power supply, but this was seriously discouraged.
I thought it was good to let you know, for what it is worth...

By the way: (link) AC701 ...  (This is exactly the reason for finding a good replacement!)
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Interesting bit of information

Post by Admin on Tue Dec 19, 2017 3:05 pm

I am presently striving to get some time to work on the oscilloscope. The problem is that Christmas is in the way.

Last night I realised that an ECC81 takes 300mA to heat.
Since I am only using an anode current of 0.5mA I thought about lowering the heater current a bit more. I tried both heaters in series (12v) and fed them from the 6v transformer. Current is now less than 100mA and they glow. The valve works very well with the lowered emission and I have about 8v Pk-Pk to drive the tube plates :-)

Just a bit of interesting data.

BR Harry - SM0VPO

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