EMRFD Message Archive 14054

Message Date From Subject
14054 2017-06-23 12:24:55 wb9kzy noise: 6AL5 versus semiconductor diodes
I was looking at the circuit diagram of my Heathkit IM-18 VTVM - noticed the 6AL5 dual diode tube.

The I remembered reading this issue of GE Ham News (July/August 1951) with the Signal Slicer phasing receive adapter circuit for SSB:

<  http://n4trb.com/AmateurRadio/GE_HamNews/issues/GE%20Ham%20News%20Vol%2006%20No%204.pdf  >

On page 10, 2nd column, the author mentions that he considered using germanium diodes instead of the 6AL5 but used the tube because the effective noise resistance of Germanium diodes is high compared to a tube diode.

This would prompt some questions: is this also true of Silicon diodes ?  What data sheet parameter might serve as figure of merit for effective noise resistance ?  Would the possible lower noise of a vacuum tube diode be worth pursuing for 21st century mixer circuits ?

I know, measurements are in order - that brings me back to fixing that Heathkit :)


Chuck, WB9KZY
14055 2017-06-23 23:49:53 Hubert Miller Re: noise: 6AL5 versus semiconductor diodes
A possibly interesting factoid. The U.S.N. 1948-1970s HF aircraft receiver AN/ARR-15 used Ge diodes
in the detector and noise limiter. The production revision AN/ARR-15A went to using tube diodes. The manual
has no explanation why.
14056 2017-06-23 23:53:33 Hubert Miller Re: noise: 6AL5 versus semiconductor diodes
Coincidently, i recently posted a note to another list asking about the IM-18. I have one with the Heath solidstate
plugins for the 2 tubes. I wondered if anyone knew actually, not in "general terms", what's inside these. I wonder also
if the innards are potted. If not, i would not mind sawing them open and doing reverse engineering on them. I notice
the manual also does not give all the tube operating voltages, so i'm not sure, without actually measuring, whether 
i can make the IM-18 totally battery powered. I have other nice VTVMs and i'd like to transistorize them also. 'Ham
Radio' magazine had an article in 1974 on doing this, and i do have the DVD set, but i have too much already to do to
take on this right now.
14057 2017-06-24 13:33:40 wb9kzy Re: noise: 6AL5 versus semiconductor diodes
Hue -

Thanks for the pointer to the Ham Radio article, looks like those ICs are still available on ebay.
14062 2017-06-25 08:14:38 Jim Strohm Re: noise: 6AL5 versus semiconductor diodes

Keep in mind that the semiconductors of 50 years ago were relatively new in their technology -- at the time, vacuum tubes had a good half-century of commercial development behind them. 

50 years later, those semiconductor processes look downright crude, and vacuum tube technical development has almost completely ended, except for some occasional, esoteric, and extremely interesting forays done -- in the semiconductor labs.

Right now you'll probably see that a 50-year-old 6AL5 has much less noise than a 50-year-old germanium diode.  Germanium is noisier than silicon, which is one reason the industry shifted to silicon (besides cost -- germanium is about 3 orders of magnitude more expensive for raw materials than silicon).  The cost trade-off was the difference in band-gap voltages between silicon and germanium. However, modern technologies have mostly overcome that, and the new stuff is vastly quieter than either 50-year-old technology.

But you might have to make a surface-mount board to use the new stuff, and impedance-match the rest of the circuit for the new detector.  That's not hard, though it requires nimble fingers and a fairly sophisticated magnifying system for us old guys.

What's next?  My company is playing with rotary and linear actuators for semiconductor foundry applications.  I'm going to bring a few of the lessons learned home, and see whether I can homebrew micro-scale actuators for doing surface-mount work ... under the watchful eye of my USB microscope and 24" monitor, of course.

Ain't technology wonderful? 


14063 2017-06-25 19:16:46 wb9kzy Re: noise: 6AL5 versus semiconductor diodes
Jim -

Thanks for the info.


14064 2017-06-25 20:14:17 K5ESS Re: noise: 6AL5 versus semiconductor diodes

This is quite a bit divergent from the original topic but does anyone know if there is modern solid state device that could replace the vacuum tube diode in the AC probe of the old (ancient) HP410B VTVM?




14065 2017-06-25 21:41:53 Jim Scott Re: noise: 6AL5 versus semiconductor diodes
This bring back a question that has knawed at me for years.

I have looked in my old vacuum tube manuals, and although they show a nice knee, my question is if the forward turn on point in a vacuum tube diode start as the diode becomes forward biased?  Consequently making it more sensitive than even a germanium diode?  (Forgetting the noise issue).  I'm just wanting to know if this is true, and if so, using a high impedance load, would it not be feasible that the vacuum tube diode might be a good performer?  At least through HF?

Certainly, there are ways to offset the EV of the detector diodes, and I have used them, but I've never made use of vacuum tube diodes so was wondering.


Jim Scott
Life Member ARRL
FISTS 15327
Joplin, MO EM27RC

The whole universe is waiting...

-----Original Message-----
14070 2017-06-26 20:51:14 biastee Re: noise: 6AL5 versus semiconductor diodes
If the vacumn tube diode functions as an RF detector, you may want to consider replacing it with a medium barrier Schottky diode, such as HSMS-282x.

A low barrier Schottky diode like HSMS-285x can detect lower RF level but is more susceptible to damage.

73, Chin-leong Lim, 9W2LC