[ARC5] Solid State 6AL5

Tom Lee tomlee at ee.stanford.edu
Mon Feb 5 17:05:05 EST 2018 

Yes, for typical operating current densities the vacuum tube rectifier 
will have a much higher drop. As you say, replacing with a solid-state 
rectifier could result in enough of a B+ boost to pop filter caps in 
some cases. The moral of the story is that there are enough differences 
that one cannot casually replace one with the other.

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 1:43 PM, J Mcvey wrote:
> The tube diodes have a high forward drop vs the silicon which is only 
> 0.6V or so.
> This is something that has to be considered when replacing the HV 
> rectifiers with solid state. Usually a series resistor or zener  to 
> mimic the 5U4 forward drop characteristics  is used. otherwise the 
> voltage may exceed the filter capacitor ratings in some cases.
>
> The same may be true in some AGC circuits where the expected drop is 
> not present. I would get the 6aL5 drop and use a zener, in4148 combo.
> The zener, of coarse is REVERSE breakdown, so the cathodes 'point" in 
> different directions but the in4148 is in the same direction of the 
> original cathode.
>
>
> On Monday, February 5, 2018 3:35 PM, Tom Lee <tomlee at ee.stanford.edu> 
> 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
>
> Sent from my iThing, so please forgive the terseness and typos.
>
> -- 
> Prof. Thomas H. Lee
> Allen Bldg., CIS-205
> 420 Via Palou Mall
> Stanford University
> Stanford, CA 94305-4070
> http://www-smirc.stanford.edu <http://www-smirc.stanford.edu/>
> 650-725-3383 (public fax; no confidential information, please
>
> On Feb 5, 2018, at 12:13 PM, Peter Gottlieb <kb2vtl at gmail.com 
> <mailto: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 
> <mailto: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 <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 <mailto: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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> >>>> https://www.avast.com/antivirus
> >>>>
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