[Amps] 8877 Warm-Up Period

[Jim Barber]

Correction: HEATER.
Brian may be lurking with his trusty rapier in hand...

Jim N7CXI


On Mon, Jan 23, 2023 at 2:17 PM Jim Barber <barberaudio at gmail.com> wrote:

> Interesting.
>
> I have a HV step-start because if I didn't it would trip the breaker. The
> xformer is a Dahl 2800V 1A with a 53 uF
> oil-filled cap and 200K bleeder resistance.The cap is on the large side,
> granted. It takes *around* 500mS to get
> from 0 to ~2KV, where the step-start contactor pulls in with a CLANK.
>
> The filament is running from a 10A transformer connected to a 2A 120V
> variac.
> My self-serving speculation is that the combined resistance of the
> transformers will soften the blow enough so that I
> don't have to add a step-start for the filament. Do I have any proof of
> this? No.
>
> I suppose I could put a 100A shunt in the 5V filament circuit, find my
> differential probe and have the scope sample
> it. Sounds like work, though...
>
> Thanks,
> Jim N7CXI
>
> On Mon, Jan 23, 2023 at 2:02 PM Byron Tatum <bjtatum1 at att.net> wrote:
>
>> Hello All Fellow Amp Builders-    I agree that it is best to allow the
>> 8877 adaquate warm-up time ahead of the application of high voltage, even
>> when in full cut-off (of course). But equally important in my opinion is to
>> have a filament step-start circuit. Initial application of filament voltage
>> is a big stress on the filament. That is one reason why it is stated not to
>> have a filament transformer with more capacity than 10 amps. At cold
>> start-up the filament resistance is extremely low. It is my opinion that
>> reducing this initial shock to the filament will add to tube life. The
>> tubes normally have a long lifetime in continuous commercial service
>> (barring other issues that may occur) partly because the filament is not
>> stressed from daily turn-ons. I always incorporate a filament step-start
>> circuit in my builds.Byron W5FH
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>