On Nov 4, 4:49*am, Ian Jackson
wrote:
In message
,
Engineer writes





On Nov 3, 6:12 pm, Ian Jackson
wrote:
In message
,
Engineer writes

(snip)

But what does the invariable negative feedback (from the OPT secondary
to an earlier amplifier stage) do to the output impedance?
--
Ian

Ian, it reduces the source output impedance of the amplifier as a
voltage generator (increasing the damping factor) but does not affect
the speaker impedance to be used. *The speaker impedance determines
the O/P tube plate (anode) load via the OPT ratio.
Cheers,
Roger

But the original question was "What the easiest ways to measure the
output impedance of a push pull tube amplifier?" It didn't mention
loudspeakers.
--
Ian- Hide quoted text -

- Show quoted text -

Henry mentioned a miltary radio with a high output impedance (but I
don't know any with P-P output...) I have a BC348 and the OPT (from a
single 6K6 O/P tube) is designed for 'phones only, both "high" and
"low" impedance. The "low" impedance 'phone O/P is 300 ohms so using
a regular LS is not possible. I put a typical radio OPT in this radio
(I left the original 'phones OPT in place so it could be restored to
original sometime in the next 100 years!) The new OPT and the nominal
8 ohms LS used puts a reasonably correct plate load on the 6K6, about
7K. Volume is quite good - I'd guess about 2 watts output (but not
measured.)
There is no NFB in the original radio but there is a little bit now as
I routed the Rk decoupling cap to the top of the OPT secondary as a
token NFB (properly phased.) Of course, invariably there is a lot of
NFB on P-P audio amplifiers and, as noted above, it reduces the true
output impedance of the amplifier as a voltage source, but it does NOT
change the correct LS impedance as the latter defines the O/P tubes'
plate-to-plate load.
I don't know why Henry first asked about a "PP amplifier then "morphed
to a "military radio" but it doesn't matter - it's all the same
physics!
Cheers,
Roger