Re: Happy St Patrick's Day, Boss!

"Eeyore" <rabbitsfriendsandrelations@[EMAIL PROTECTED]
> wrote in message
news:47E16BB3.25D484E6@[EMAIL PROTECTED]
>
>> > Designing with DISCRETE transistors is an art. The possibilities are
>> > endless. The real magic is of course the availability of npn and pnp
>> devices. Sadly
>> > there is no tube equivalent of a pnp.
>>
Iain wrote:
>> Graham.  Tube amps are delightful in their seeming simplicity.
>> I am always amazed how a 6SN7 triode and a handful of
>> components can play such sweet music:-)
>
> That is of course one of their attractions and is similarly very
suitable
> for
> and attractive to the home constructor who can easily 'relate' to the
> simple
> circuitry.

One of the attractions for the homebrew tube builder
is that tube amps are so "tactile", and building gives one
so much pleasure.

>
> I don't know many people who can relate to an IC !

Gosh. Does anyone bother to build SS amplifiers as
hombrew projects?  I certainly wouldn't.  I make the
occasional meter driver with an Op amp.

>
>
> Well .... since no normal speaker is represented meaningfully by a
0.22uF
> load, I'd probably not over worry about that.

Then you would be ignoring what has for years
been regarded as one of the standard tests for
unconditional stability.  No normal speaker is
represented meaningfully by an infinite Z either,
but tube amps are also tested for stability into
open circuit.

>
>> If you think stabilising a tube amp with NFB
>> 15dB is "trivial", I would  be very glad if you
>> would spare a few moments of your time to
>> take us through it, step by step.
>
> Every cathode follower has that kind of NFB !

Indeed. But the cathode follower is not driving 20V
into an OPT that has leakage inductance and is the
source for the NFB loop.
>
> Designing for stability is a very interesting subject though.

It's something I have been interested in for a very
long time.  Patrick has considerable know-how
and experience in this.  He has described many of his
empirical methods hich I have adopted, and am
starting to understand what needs to be done to tame
the beast.

Some square waves pics of  an ongoing  PP amp
project.  I think I can still improve the 5kHz into 220nF.

http://www.kolumbus.fi/iain.churches/Projects/M30SquareWavePics.html

Stability is a problem that brings many a DIY project to
a shuddering halt.  The builder switches the amp on, and
it oscillates wildly.  He switches it off, checks all the
connections. Someone advises him to make sure that
the feedback polarity is correct.After that, he is stuck.
That may be part pf the reason why SET with ZNFB
is so popular with homebrew builders:-)

An old professional amp builder I knew years ago, used to
build Williamsons.  I remember him saying that he  had extreme
difficulties in getting them stable with any OPT other
than the original spec Partridge or the Sowter equivalent.

regards to all
Iain