[ARC5] Solid State 6AL5
Bill Cromwell
wrcromwell at gmail.com
Thu Feb 8 12:45:39 EST 2018
Okay...
I have censored my own replies to this escalating thread ;)
73,
Bill KU8H
On 02/08/2018 12:34 PM, Tom Lee wrote:
> 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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--
bark less - wag more
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