Thanks for all the replies again, on and off the list.
Paul,
In the case you describe, if there was a flashover arc to the cathode
and the filament wasn't
at the same DC potential, I think a secondary arc from cathode to
filament would probably
happen anyway, given the low breakdown voltage between the two. That's
partly speculation on my
part, and partly based on experiments with my BVT.
What I find interesting (truly, not only being sarcastic) is that such a
basic issue wouldn't be covered
in the manufacturer's data. Perhaps that's because it is indeed so
basic? Don't know. After 40 years or
more of manufacturing 8877's, I'm just surprised it's even a question at
this point. :-)
My reason for the pondering was the potential Alpha 76A conversion
I've discussed here, as well as
a possible dual 8877 conversion of a Henry 3KA, which has a somewhat
outrageously overbuilt PS
considering it only runs a pair of 3-500Z's. (4KV @ roughly 1A! ) I got
to staring at schematics
again, which irritates my wife and doesn't always end up with desirable
results... ;-)
Thanks and 73,
Jim N7CXI
On 8/3/2012 6:02 PM, Paul Christensen wrote:
> Jim,
>
> I struggled with this issue when modifying my 77Dx. The current GG
> configuration is like the Henry 3KD with no connection between cathode and
> filament. This is unlike the AL-1500 where the two elements are tied
> together. Both models use a filament choke.
>
> I ended up using Siemens gas discharge tubes (GDT) between the two filament
> terminals, and again between each side of the filament and chassis. I
> believe the GDT value is ~180V but I would need to verify. Next time the
> amp is open for maintenance, I may tie the cathode and filament together.
> My understanding is that is Eimac's recommendation, but after a search, I've
> never seen it in their writings although I've seen it expressed by at least
> one amp "expert." The reasoning? Presumably there is little voltage
> tolerance that the 8877 can withstand between cathode and filament. How
> much? I don't know and it's not detailed in the Eimac datasheets.
>
> OTOH, if a HV arc occurs to the cathode and the filament is tied, do we want
> the filament "taken out" especially of the fil. transformer is a CT to
> chassis to complete a low-resistance DC pathway? Or, is there some
> semblance of safety in keeping the filament isolated from the cathode? I'm
> not sure there's a right answer and I'm skeptical of any "expert" who
> emphatically states their reasoning without solid data to back it up.
>
> Paul, W9AC
>
> ----- Original Message -----
> From: "Jim Barber" <audioguy@q.com>
> To: <Amps@contesting.com>
> Sent: Friday, August 03, 2012 8:15 PM
> Subject: [Amps] 8877 cathode-filament question
>
>
>> Looking at 8877 HF amp schematics published on the net I came across two
>> designs that
>> raised a question: The two I'm talking about are the Henry 3KD Premiere,
>> and a QST article also
>> for a single 8877. (don't know the date, sorry)
>>
>> (1) In the Henry schematic, the filament is powered through a RFC, but
>> the filament transformer has
>> no grounded center tap. There is also no apparent circuit between the
>> filament and cathode, as the filament
>> appears to float above the chassis.
>>
>> (2) In the QST schematic, the filament is powered directly from a
>> transformer,
>> (no RFC, just bypassed) but again with no grounded secondary center tap
>> or other
>> obvious circuit between the filament and cathode.
>>
>> My problem with these is simply that with the filament floating I can't
>> see what keeps the
>> filament-cathode potential from exceeding a safe value, whatever that
>> might be. I remember
>> an article somewhere stating it shouldn't exceed 150 volts, but I can't
>> find it and memory
>> is a tricky thing. The newer CPI, older Eimac and the Penta Labs data
>> sheets don't discuss
>> it that I've seen
>>
>> Comments?
>>
>> Thanks,
>> Jim N7CXI
>>
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