[ARC5] Solid State 6AL5

Thu Feb 8 12:21:59 EST 2018

In light of current events I suggest we stop using the term cathode
stripping and replace it with cathode undressing.

Dennis AE6C

On Wed, Feb 7, 2018 at 5:34 PM, Tom Lee <tomlee at ee.stanford.edu> wrote:

> 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 94305-4070http://www-smirc.stanford.edu650-725-3383 <(650)%20725-3383> (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> 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 <(650)%20725-3383> (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> 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
>>>>
>>>>
>>>
>>>
>>
>> ______________________________________________________________
>> ARC5 mailing list
>> Home: http://mailman.qth.net/mailman/listinfo/arc5
>> Help: http://mailman.qth.net/mmfaq.htm
>> Post: mailto:ARC5 at mailman.qth.net
>>
>> This list hosted by: http://www.qsl.net
>> Please help support this email list: http://www.qsl.net/donate.html
>>
>
>
>
> ______________________________________________________________
> ARC5 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/arc5
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:ARC5 at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.qth.net/pipermail/arc5/attachments/20180208/610d5449/attachment.html>