>
>Rich said:
>
>>Stabilizing the screen supply with a series pass regulator is not
>>good engineering practice unless a screen supply bleeder is used to
>>protect against reverse screen current.
>
>I didn't make it clear that the 'low impedance' screen supply means low
>impedance both for source and sink - i.e good regulation for both source
>and sink.. For 6146B's, I use zeners - not ideal, but the screen current
>doesn't go very negative - rarely more than 1 or 2mA.
>
** Zeners are plenty good enough at 1/3 of their dissipation rating --
and they are fail-safe. For screen potentials of less than 700v, a zener
string is a good choice.
>>Stabilizing the grid supply in AB1 is of little value since there is
>>neither forward or reverse grid current.
>
>Stabilising has a lot of value if the screen supply is stabilised.
>Regulation is the parameter that has little value. Stabilising both is best
>to get around mains variations.
** However, both of the Ugly Amplifiers on my Web site were sold to
operators who eventually experienced trouble with the screen-regulator
electronics. As a result, the mo'-linear amplifier I am currently
gathering parts for will use a 1500V or so resonant-choke screen supply
with a 30mA bleeder. Hopefully, this will be adequate. If not, I will
add a FET-pass positive floating regulator shown in:
http://www.somis.org/D.a.08.GIF
However, since the speaking screen-I on a 4cx10-kJ is only c, 35mA
average, the regulation shouldn't be all that bad.
-- RE: mains variations -- With an AB1 tetrode, when the mains sag,
the anode-potential sags and ZSAC drops. If the grid bias supply
potential also sags, ZSAC increases - which tends to offset the ZSAC drop.
cheers, Peter.
>
>Definitions:
>'Regulation' is a measure of the variation of output voltage with load.
>'Stabilisation' is a measure of the variation of output voltage with input
>(mains) variation.
>
>
>73
>
>Peter G3RZP
>
>
>
>
>
>
>
>
> "R. Measures"
>
> <r@somis.org> To: "Peter C,"
><peter.chadwick@Zarlink.Com>, "Ian White, G3SEK"
> <g3sek@ifwtech.co.uk>
>
> 05-Jan-2004 01:31 cc: " AMPS"
><amps@contesting.com>, <amps-bounces@contesting.com>
> Subject: Re: [Amps] 3.5 kV
>2A REGULATED Power Supply: Schematic ?
>
>
>
>
>
>
>** Stabilizing the screen supply with a series pass regulator is not
>good engineering practice unless a screen supply bleeder is used to
>protect against reverse screen current. Stabilizing the grid supply in
>AB1 is of little value since there is neither forward or reverse grid
>current.
>
>>I have found problems when using high impedance supplies with 6146s,
>>especially as they get older. Especially bad is the Yaesu 'anti flash
>>over diode' in series with the screen grid - any negative screen current
>>pushes up the screen volts, and the anode current runs away, with
>>disastrous results for the valves.
>>
>>Grid bias and screen supplies should either both be stabilised, or
>>neither. If neither supply is stabilised, variations in supply volts tend
>>to cancel. I prefer to stabilise them both, and keep the supply impedance
>>(at low frequencies, including zero frequency) low. I'm still using the
>>original 6146Bs in my FT102, and after 20 years of hard work, they're
>still
>>fine.
>>
>>73
>>
>>Peter G3RZP
>>
>>
>>_______________________________________________
>>Amps mailing list
>>Amps@contesting.com
>>http://lists.contesting.com/mailman/listinfo/amps
>>
>
>
>
>
>
>
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