[Amps] 8877 cathode-filament question

[Paul Christensen]

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 at q.com>
To: <Amps at 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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