[ARC5] Solid State 6AL5
Tom Lee
tomlee at ee.stanford.edu
Thu Feb 8 12:34:18 EST 2018
That's hilarious!
We've witnessed the birth of a new genre. Soon, we'll be checking out
Netflix for "Electrodes Gone Wild" and "Anodes Meet Cathodes".
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 2/8/2018 9:29 AM, Peter Gottlieb wrote:
> Or “cathodes going natural”
>
>
> Peter
>
> On Feb 8, 2018, at 12:21 PM, Dennis Monticelli
> <dennis.monticelli at gmail.com <mailto:dennis.monticelli at gmail.com>> wrote:
>
>> 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
>> <mailto: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 94305-4070
>> http://www-smirc.stanford.edu
>> 650-725-3383 <tel:%28650%29%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
>>> <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 <tel:%28650%29%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
>>> <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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>>
>>
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