[ARC5] Solid State 6AL5

Wed Feb 7 20:34:23 EST 2018

Hi Mark,

You're absolutely right. I've never run any receiver long enough for 
cathode stripping to have been a lifetime limiter, so your data makes 
sense. The one non-PA case for which cathode stripping (or, more 
accurately, its cousin) has shown up is in dc-coupled circuits, such as 
old vacuum-tube op-amps (e.g., the K2-W). Getting low offset voltages is 
challenging enough under ordinary circumstances, but "cathode stripping" 
makes it worse. These parametric shifts (which are actually not due to 
stripping but to a drift in interface states at the cathode surface) 
would never be noticeable in ordinary receiver circuits, but they wreak 
havoc with low-level dc-coupled circuits. Aside from offset, they add 
peculiar artifacts to the step response.

I suspect the true reason for worries about B+ coming up too fast is 
actually the marginality in many designs, rather than stripping. If the 
rectifier comes up before the rest of the circuitry wakes up (and drops 
less voltage by virtue of its solid-stateness), the temporary lack of 
loading can cause the B+ to overshoot design values substantially and 
possibly pop the filter cap.

But cathode stripping just sounds so much more...technical. :)

--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 2/7/2018 5:17 PM, Mark K3MSB wrote:
>
> Hi Tom
>
> The issue of cathode stripping comes up every now and then on the AM 
> related groups as a lot of those guys like to play with the high power 
> tubes, and a lot have broadcast experience with such tubes.In every 
> case this issue came up the consensus was the same – unless you’re 
> dealing with high power tubes it’s not an issue.Typically tubes that 
> one needs to be concerned about will specify a time-delay between 
> application of heater voltage and high voltage.For the tubes that most 
> of us deal with, there’s been no demonstrable data to support that 
> cathode stripping is an issue.
>
> In the absolute, will using solid state rectifiers shorten tube 
> life?Probably.Is it worth worrying about?Probably not.I’d rather 
> shorten the life of some $2 tubes than have a transformer secondary 
> short to ground taking out a potentially irreplaceable part (or one 
> costly to rewind).Halllicrafter HT transmitter series transformers are 
> notorious for this failure mode.
>
> While one typically uses a dropping resistor when building a solid 
> state rectifier replacement they are not always necessary.I’m working 
> on a Johnson Valiant and the voltage drop on the HV 866 rectifiers was 
> so small it wasn’t worth it.For the Bias and LV circuits I certainly 
> used a dropping resistor.
>
> Concerning your comment of circuits designed with small margins, 
> you’ll run into this even if you don’t use solid state rectifiers as 
> line voltages are higher now than 50 years ago.
>
>
> 73 Mark K3MSB
>
>
>
> On Wed, Feb 7, 2018 at 5:10 PM, Tom Lee <tomlee at ee.stanford.edu 
> <mailto:tomlee at ee.stanford.edu>> wrote:
>
>     One should definitely take care when replacing vacuum tube
>     rectifiers with solid-state ones. The much higher efficiency of
>     the latter can lead to overvoltages, so if the B+ supply was
>     designed with small margins to begin with, the solid-state
>     rectifiers can produce a nice bang and let out all the magic smoke.
>
>     Someone earlier pointed out another important consideration: If
>     the B+ comes up well before the tubes warm up, that can cause
>     "cathode stripping" and accelerate the wearout of the tubes. So
>     even if you solve the overvoltage problem (e.g., by adding zeners
>     or resistors), there still remains the cathode stripping danger.
>
>     For AGC/AVC circuits, too, there can be many problems. I've
>     analyzed a number of AVC loops, and a fair fraction actually have
>     little right to work. To the extent that second-order effects seem
>     to keep them from going unstable, any changes from the original
>     design run the risk of making the loop fall off the cliff --
>     Murphy decrees that it can only go that way, despite the seeming
>     50/50 partitioning of outcomes.
>
>     Me, I like the warm glow of thermatrons, so I leave them in except
>     in a very few circumstances (ratio detectors often benefit nicely
>     from a switch to silicon, for example).
>
>     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 2/7/2018 12:50 PM, Richard Knoppow wrote:
>
>         FWIW, when I got one of my SP-600-JX's it had a solid state
>         rectifier in place of the 5R4. I removed it because I found
>         all the voltages were too high. This was not from high line
>         voltage, I checked that, it was the rectifier. When replaced
>         with a standard 5R4 all were OK. This is a molded case made of
>         resin of some sort on a standard tube base. I forgot about
>         this until this thread and have no idea where to look for it
>         but it had a label and was not home made.
>             As fare as using solid state diodes for AVC, I think
>         perhaps the difference in minimum voltage and effects of
>         contact potential might require a change in the bias for AVC
>         delay. I have not tested this. You may be aware that it has
>         been standard practice since about the mid-1940s to put a
>         dropping resistor in the filament of 6H6 and I think also 6AL5
>         tubes when used as noise limiters. This affects the contact
>         potential and reduces the effects of hum from heater to
>         cathode leakage. Obviously it would have no effect on solid
>         state diodes. BTW, I have never found any technical paper
>         about the filament resistors but all receiver manufacturers
>         began to add them about the early to mid-1940. You would think
>         the standard engineering texts would have something. Maybe I
>         missed it but if anyone knows please tell me.
>
>         On 2/7/2018 12:29 PM, J Mcvey via ARC5 wrote:
>
>             I'm curious...
>
>
>             On Wednesday, February 7, 2018 12:39 PM, John Watkins
>             <jpwatkins9 at yahoo.com <mailto:jpwatkins9 at yahoo.com>> wrote:
>
>
>             I have the metal case off of one of my mil 6AL5s, it is
>             encased in an amber colored epoxy.  I could remove enough
>             to see exactly what is in there and provide a few pictures
>             if it would be of interest.
>
>             John WD5ENU
>
>
>
>
>
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