[ARC5] Solid State 6AL5

[Tom Lee]

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

Sent from my iThing, so please forgive the terseness and typos.

-- 
Prof. Thomas H. Lee
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On Feb 5, 2018, at 12:13 PM, Peter Gottlieb <kb2vtl at gmail.com> wrote:

> Well, I meant “acting like an ideal diode” near the origin. So my comparison question is: near zero volts, is a tube or a semiconductor diode “better” taking into account front to back ratio, small signal non-linearity distortion, capacitance, etc. 
> 
> Bottom line:  does replacing a 6AL5 with a 1N34A improve or degrade both low and higher level detection?
> 
> 
> Peter
> Kb2vtl
> 
>> On Feb 5, 2018, at 2:19 PM, Tom Lee <tomlee at ee.stanford.edu> wrote:
>> 
>> "Start acting like a diode" is not the way to look at it. Diodes always, by definition, look like diodes. :)
>> 
>> Thinking of diode detectors in terms of a threshold model ("turn-on voltage" of thus and so volts) is not useful (indeed, it's misleading) if we're talking about low-level detection (as in a crystal radio trying to pull in DX). Low-level detection occurs near zero voltage and zero current so what matters most in that instance is the shape of the curve near the origin. And that always looks quadratic (whence the term "square-law" detection), for any device, semiconductor or not.
>> 
>> It's only at large signal levels that the turn-on voltage concept makes sense. If that's the regime of interest, semiconductors always win. Just the contact potential between the metals used in a vacuum tube introduce a significant "turn-on" voltage minimum.
>> 
>> 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 11:08 AM, Peter Gottlieb wrote:
>>> What is the I-V curve like in a 6AL5?  Does it start acting like a diode at lower voltages than a Ge diode like a 1N34A?
>>> 
>>> 
>>> Peter
>>> 
>>>>> On Feb 5, 2018, at 1:47 PM, Kenneth G. Gordon <kgordon2006 at frontier.com> wrote:
>>>>> 
>>>>> On 5 Feb 2018 at 13:38, Bill Cromwell wrote:
>>>>> 
>>>>> Hi Ken,
>>>>> 
>>>>> Are the 1N34 germanium?
>>>> Yes.
>>>> 
>>>>> If so I wonder how newer silicon devices would
>>>>> work or even silicon power rectifiers.
>>>> I wonder too, but I did't try those at the time.
>>>> 
>>>>> I am getting more interested in
>>>>> this and I will try to compare the tube vs solid state agc circuits I
>>>>> have on hand here. We know that solid state devices can work in agc
>>>>> circuits because they do in existing radio gear.
>>>> Yes, but SS AGC circuits are, of course, designed differently from tube-types. I would think
>>>> that some sort of FET would work better than straight diodes, though.
>>>> 
>>>>> To the original question about just plugging SS diodes into those tube
>>>>> circuits - maybe not. It seems the whole idea is to NOT modify the radio
>>>>> and keep the option of plugging in the diode vacuum tubes. Mods may be
>>>>> required.
>>>> Yes.
>>>> 
>>>>> Related to this but with the shoe on the other foot I am interested in
>>>>> trying double balanced diode rings with 6AL5 and 6H6 tubes. Somebody is
>>>>> sure to tell me it can't be done. In that case I will find out why :)
>>>> Yes. That HAS been done. I have an article around here somewhere on doing just that. As I
>>>> remember it, there was some difficulty with balance issues.
>>>> 
>>>> Later,
>>>> 
>>>> Ken W7EKB
>>>> 
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