[Amps] 8877 Filament choke

[Saandy Eban]

...for the most part, these amplifiers operate the 8877 in grounded grid
configuration. As such, the input circuit is supposed to be above ground
RFwise.
Now, the capacitance between the cathode and the filament is quite high in
this tube. This reactance shunts a sizeable portion of the RF drive to
ground and this is what that choke is intended to prevent.
If you have drive to spare you can leave it out of the circuit, but if
you're marginal on drive by all means use it.
There is a more pernicious problem which I will not address here, just
mention it: high capacitive reactance from the grid to the cathode can
create propitious conditions for oscillations! The mechanism is complicated
but it does exist. If the grid is not perfectly grounded, you can be in
trouble.
Notwithstanding regular operation' always try to reduce parasitic reactances
to the minimum! 
73 to all:
Alex	4Z5KS

-----Original Message-----
From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
Behalf Of Roger
Sent: Friday, November 16, 2007 5:34 AM
To: Bob Maser; Rick Stealey; amps at contesting.com
Subject: Re: [Amps] 8877 Filament choke




> Beats me.  I've never been on 6, not since 1961 anyway.  A lot of builders

> install a filament choke on a 8877 amp, example my Henry 4K Ultra, which 
> runs a single 8877, had a filament choke.  I'm not sure about an Alpha 77.

> I have always been told that in the HF region running a 8877, you don't 
> need one in the filament circuit but a 2.5uH choke is required in the 
> cathode to bias line.  Bill Orr shows this in his 21st edition (1978)Radio

> Handbook on

Thanks,

I just might have that edition down in the basement...some where.

Roger (K8RI)

> page 22.41.  If you don't already have some of Bill's old handbooks start 
> looking for one if you're serious about amp building.
>
> Bob W6TR
> ----- Original Message ----- 
> From: "Roger" <sub1 at rogerhalstead.com>
> To: "Bob Maser" <bmaser at tampabay.rr.com>; "Rick Stealey" 
> <rstealey at hotmail.com>; <amps at contesting.com>
> Sent: Thursday, November 15, 2007 5:41 PM
> Subject: Re: [Amps] 8877 Filament choke
>
>
>>
>>
>>
>>> With an indirectly heated cathode you don't need a filament choke, in 
>>> the HF
>>> region.
>>
>> What about when running one on 6-meters?
>>
>> Roger (K8RI)
>>
>>>
>>> Bob  W6TR
>>> ----- Original Message ----- 
>>> From: "Rick Stealey" <rstealey at hotmail.com>
>>> To: <amps at contesting.com>
>>> Sent: Thursday, November 15, 2007 9:26 AM
>>> Subject: [Amps] 8877 Filament choke
>>>
>>>
>>>>
>>>> Info in the ARRL handbook seems to indicate that it might be better to 
>>>> not
>>>> use a filament choke on an 8877 amplifier, that it might cause more
>>>> problems than it solves.
>>>> I wonder what you guys who have homebrewed 8877s think?
>>>>
>>>> Rick  K2XT
>>>>
>>>>
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