GB> Re: Tubes in Series was (Re: [Heathkit] Question on 12AU7)LONG

[wolfbob]

I remember the 117 volt tubes. One was a rectifier like the 
117W4 and there was a 117V6. The heaters in these tubes 
would really unwind when they turned on. They came on very 
bright, like white hot and then as the resistance increased 
they would end up at the usual orange. During this warmup, 
you could look through the top of the glass tube and into 
the cathode sleeve and see the heater unwinding about three 
turns. Kept a 9 yearold entranced as I remember.

WBob


----- Original Message ----- 
From: "Mike McCarty" <Mike.McCarty at sbcglobal.net>
To: "jeremy-ca" <km1h at jeremy.mv.com>
Cc: <glowbugs at piobaire.mines.uidaho.edu>; "'Ian'" 
<ianwebb5 at comcast.net>; <heathkit at mailman.qth.net>
Sent: Sunday, February 03, 2008 11:36 PM
Subject: Re: GB> Re: Tubes in Series was (Re: [Heathkit] 
Question on 12AU7)LONG


> jeremy-ca wrote:
>
> [...]
>
>> I believe it was one of the Riders publications that had 
>> the info on designing AC-DC strings using tubes with 
>> different current ratings. I'll try and find it as I 
>> rummage around the book shelves.
>
> It's very easy. One needs the total current flowing in the 
> string
> to be that which the tube with the highest current demand 
> needs.
> One subtracts the current needed by each of the other 
> tubes, and
> puts a resistor in parallel which absorbs the excess 
> current, given
> the voltage the tube needs. One can put a resistor in 
> parallel with
> several tubes' heaters if they all need the same current.
>
> For example, suppose we have a set with a series string of 
> 8 tubes,
> each needing 12.6 V on the heaters, and we run it off of 
> 120 V.
> Suppose that each of the tubes needs 150 mA, except one 
> needs
> 200 mA. How do we do the design?
>
> Ok, 8 x 12.6 is 100.8 V, so we need to drop an additional 
> 19.2 V
> at 200 mA, so we need a "ballast" resistor of 96 ohms. 
> BTW,
> true ballasts are made of material which increases in 
> resistance
> like that of the heater material, to match things. They 
> are not
> just resistors.
>
> Anyway, now we've got our ballast designed. All the tubes 
> but one
> need 150 mA, so we need to pass another 50 mA around each 
> tube.
> Since the heaters have 12.6 V across them, we need another
> 12.6 V / 50 mA = 252 ohms in parallel with each of those 
> tubes.
>
> If they can all be put on one side of the "hungry" tube, 
> then
> we can also just use one 1764 ohm resistor in parallel 
> with
> all of them.
>
> Controlled warm up time is more important for rectifiers 
> than
> the other tubes. It's important not to apply plate voltage 
> from
> a fast warm up rectifier to tubes which have cold 
> cathodes.
>
> Mike
> -- 
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>