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Re: [Amps] 8877 Warm-Up Period

To: Byron Tatum <>
Subject: Re: [Amps] 8877 Warm-Up Period
From: Jim Barber <>
Date: Mon, 23 Jan 2023 14:17:11 -0800
List-post: <>

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
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...


On Mon, Jan 23, 2023 at 2:02 PM Byron Tatum <> 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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