[ARC5] Solid State 6AL5

[John Watkins]

I have a few of these pulled from some military radios.  Haven’t used any in years.  They seem to work fine in place of the regular 6AL5 tubes, just no soft warm glow.  Can’t remember which mil radios I pulled them from or why.  I think it was some Collins aircraft stuff.

John WD5ENU 

Sent from my iPad

> On Feb 6, 2018, at 15:28, Tom Lee <tomlee at ee.stanford.edu> wrote:
> 
> Hi Richard,
> 
> In the context of detection, nonlinearity is the desired attribute, hence the somewhat tortured grammar. But there was a method to my madness.
> 
> As to semiconductors, it isn't accurate to say that the forward voltage is independent of current. It most certainly depends on current, just as a vacuum tube's forward drop does. It's simply that an exponential is so much more dramatic that we are fooled into thinking that solid-state devices have a fixed forward voltage. In fact, it will increase about 100mV for every factor of 10 in current (for ordinary diodes; ideally, it should be 60mV per decade); that's a 16% increase in forward voltage, and that's without including the additional parasitic resistance that all real devices possesses. Compared with a vacuum tube, it may seem "constant" but one must be careful about quantitative aspects in certain contexts.
> 
> --Cheers,
> Tom
> 
> -- 
> Prof. Thomas H. Lee
> Allen Bldg., CIS-205
> 420 Via Palou Mall
> Stanford University
> Stanford, CA 94305-4070
> http://www-smirc.stanford.edu
> 650-725-3383 (public fax; no confidential information, please)
> 
>> On 2/5/2018 1:35 PM, Richard Knoppow wrote:
>>    I am a little confused, here you say that semiconductors are more linear than vacuum tube diodes due to the 3/2 law leading to poor _nonlinearity_ which to me means better linearity. Is this sentence supposed to read _poor linearity_ if so it makes more sense.
>>     Also note that semiconductors have a constant voltage drop, something like mercury vapor rectifiers. Ge is about 3/4 volt, Si about 1 volt. It is independent of current. Vacuum tube rectifiers have a voltage drop which is dependent on the current but is not like a linear resistor since it does not vary linearly with the current drawn, perhaps this is where the 3 halves power law comes in. For this reason the output voltage of a semiconductor rectifier is usually greater than a vacuum tube diode and can be corrected for only one value of current.
>>    BTW, some early detector circuits using solid state semiconductors had a source of bias voltage, usually a battery, to improve sensitivity. When I was making crystal sets in the dim, distant, past I didn't know about this and never tried it but it shows up in early books on "wireless".
>>> On 2/5/2018 12:33 PM, Tom Lee wrote:
>>> Hi Peter
>>> 
>>> Assuming that impedances are matched, a vacuum tube diode will always be less sensitive than a semiconductor diode as a detector -- the 3/2-power law leads to poor nonlinearity. That more-linear characteristic is one reason there are some audiophiles who insist that tubes sound better.
>>> 
>>> The 1N34 is a good detector partly because matching impedances to it is straightforward. Even though a silicon device has a better slope near the origin, the extremely high impedance there can't be matched in practice, so that potential lies unrealized. Adding a tiny bias current helps, but purists dislike the extra bits.
>>> 
>>> Cheers
>>> Tom
>>> 
>> 
> 
> 
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