[ARC5] Solid State 6AL5

[Tom Lee]

The forward characteristics are indeed quite different. A solid-state 
diode has a famous exponential dependency of current on voltage (ideally 
increasing a factor of 10 for every 60mV increment, although real diodes 
require close to double that voltage increment to produce a 10x current 
increase). A vacuum-tube diode has a much lazier dependency -- current 
is proportional to voltage raised to the 3/2 power. This difference 
needs to be accommodated in any proposed mod. In some circuits (such as 
power rectifiers), this difference matters little, but in others, it 
matters a great deal.

--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 11:01 AM, Bob kb8tq wrote:
> Hi
>
> The “forward voltage” characteristics of a vacuum tube diode are *very* different than the
> forward voltage curves of a solid state diode. That will make a big difference in something
> like an AGC circuit. It also makes a big difference in a power supply. There also are subtle
> differences between vacuum tube diodes and even things like a grid used as a diode.
>
> Back in the days when we all shopped at PolyPacks, solid state diodes were not quite what
> they are today. Leakage, capacitance, and reverse voltage numbers are all a bit different on
> modern stuff than they were “back then”. What you may have from that era - who knows ….
>
> The simple answer is: Yes, you can convert a tube radio to solid state. It is not anywhere near
> as easy as it looks at first. There are a lot of things to consider and work out. It’s easier to
> do at audio than at RF, but still not quite “just drop this part in there” sort of stuff.
>
> Bob
>
>> On Feb 5, 2018, at 12:17 PM, Bill Cromwell <wrcromwell at gmail.com> wrote:
>>
>> Hi Wayne,
>>
>> I have not tried that approach for signal diodes. I wonder if there are substantial differences between tube and solid state circuits for agc detectors, AM detectors, etc. If I remember, when I get time I will look. One obvious (to me) difference is the reverse voltage (avalanche) and reverse leakage in the solid state devices. That could make a big difference in the required circuits.
>>
>> I have mounted rectifier diodes on octal plugs and directly plugged them into sockets in place of 5U4*, 5Y3*, etc power rectifier tubes. Discounting the different forward voltage drop in the vacuum devices the solid state devices perform as expected. If we pay attention to that reverse leakage and breakdown factor it *should* work for signal diodes.
>>
>> I always look at the ratings of parts 'downstream' from the silicon replacement diodes to assure they can tolerate higher voltages. I recall a time or two replacing some filter caps that might have already been right on the ragged edge.
>>
>> 73,
>>
>> Bill  KU8H
>>
>> On 02/05/2018 11:20 AM, Robert  Eleazer wrote:
>>> Anyone ever try to use a solid state 6AL5, or to make one using solid
>>> state diodes?  I have couple of solid state ones that I think came from
>>> Poly Paks.  From what I recall they did not do well when I tried them in
>>> a circuit.  The same comment applies to 6H6; have you tried a solid
>>> state approach?
>>>
>>> Wayne
>>> WB5WSV
>>>
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>> -- 
>> bark less - wag more
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