[ARC5] Solid State 6AL5

Wed Feb 7 20:17:16 EST 2018

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 (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
>>>
>>>
>>
>>
>
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