[ARC5] BC-453 mods

[J. Forster]

The Darlington is not because gain is needed. It's to combine the HV of
the bipolar and the high Z of the FET.

-John

=================



> Yes.  The NPN transistor will need a couple of resistors for bias and a
> bypass cap on the base, but it will stretch the operating voltage to
> whatever your B+ preference may be.  It is not hard at all to find NPN's
> rated for 300, 400, or 500V and they are in expensive.  A Darlington is
> probably overkill for this application.
>
> Dennis AE6C
>
> On Sat, Aug 6, 2011 at 2:14 PM, J. Forster <jfor at quikus.com> wrote:
>
>> Yes. Or a Darlington-White. I think it needs a resistor or two.
>>
>> -John
>>
>> ===============
>>
>>
>> > That makes sense.  The JFET would be the input device and the bipolar
>> is
>> > probably a cascode connection (kinda like a grounded grid amp) to
>> stand
>> > off
>> > the higher voltages.
>> >
>> > Dennis AE6C
>> >
>> > On Sat, Aug 6, 2011 at 1:15 PM, J. Forster <jfor at quikus.com> wrote:
>> >
>> >> I've seen two transistor versions, a FET and regular transistor,
>> coupled
>> >> together, used.
>> >>
>> >> -John
>> >>
>> >> ================
>> >>
>> >>
>> >> > FYI.  A JFET is not a triode  It does have a "triode" like region
>> >> prior
>> >> to
>> >> > pinchoff, but beyond that Vds bias point the current saturates like
>> a
>> >> > pentode.  The pinchoff voltage is low enough that it is rather
>> >> impractical
>> >> > ficult to run it as a triode in most cases.  But it's a good sub
>> for a
>> >> > pentode and you don't need any "screen" voltage.  The biggest
>> >> limitation
>> >> > is
>> >> > the max operating voltage.
>> >> >
>> >> > Dennis AE6C
>> >> >
>> >> > On Sat, Aug 6, 2011 at 9:50 AM, Kenneth G. Gordon
>> >> > <kgordon2006 at frontier.com>wrote:
>> >> >
>> >> >> Making the 1st IF regenerative in any of the models is easily
>> done,
>> >> >> and makes a big difference for weak signals....not so much for
>> strong
>> >> >> ones.
>> >> >>
>> >> >> Two pieces of #12 TH or THHN (with insulation still on) soldered
>> to
>> >> >> the grid and plate terminals, and a pot in the cathode is all it
>> >> >> takes.
>> >> >>
>> >> >> The main problem I always had when trying to replace tubes with
>> FETs
>> >> >> is that an FET is basically a triode. For multi-grid tubes, this
>> is
>> >> >> often a real problem.
>> >> >>
>> >> >> For triodes, not so much.
>> >> >>
>> >> >> I have a pair of Tubesters here which I took out of a Heathkit
>> VTVM.
>> >> >> As I remember it, one replaced a 6AL5 and the other a 12AX7.
>> >> >>
>> >> >> vy 73,
>> >> >>
>> >> >> Ken W7EKB
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>>
>>
>