On Jul 11, 1:00*am, John Byrns wrote:
In article ,
*Patrick Turner wrote:
But in PP amps the B+ can be +500V and as we all know the anode swing
can be to +/-1,500V, and thus threaten to arc. There's a simple answer
which is well know and been around ever since Si diodes began to get
good reverse voltage ratimgs, and that's to connect diodes from each
end of the OPT to 0V with diode anode connected to tube anodes. I like
to use 3 x 1N7007 in series with 1M strapped across each diode to
equalise reverese voltages, and when one end of the OPT tries to rise
to more than 2 x +B, the other end tries to go below 0V so diodes
conduct, and thus the OPT voltage swing is CLAMPED. Voltage clamping
with diodes has been around for ages.
There is a schematic example at *
http://www.turneraudio.com.au/100w-monobloc2-2004.htm
The amp also has Zobel networks across each 1/2 primary of the OPT to
load the OPT at HF.
There seems to be an inconsistency here, between what you said above, "connect
diodes from each end of the OPT to 0V with diode anode connected to tube
anodes", and the way the "schematic example" shows the diodes wired?
Gee, I thought you slept through most days. But you are correct and
the schematic IS RIGHT, and what I said here in the post WAS WRONG.
Readers take note !
But the diodes work excellently to limit or clamp the Va swings to no
more than +/- B+ at each anode.
Now the same scheme would be impossible to implement on an SE amp
because there's only one phase of voltage and nothing much to stop Ea
rising enormously when no load is present and the tube cuts off when
in pentode or beam tetrode mode where grid current usually has no
limiting effect on Ea swings and voltage gain approaches gm x Ra, or
perhaps 130 for an EL34 or 170 for KT88.
But one could rig a voltage doubler supply to create a rail at 2 x B+,
and run SI diodes with their cathodes taken to tube anodes, and diode
anodes taken to 2 x B+, say +800V, so that when and not if Ea rises
that far the diodes conduct, and Ea is clipped. There will be a
negligible rise in the B+ rail as a a result.
Diodes may be also placed between tube anode and 0V to prevent Ea ever
going negative.
I find that SE amps don't tend to exhibit the excessive Ea problems as
much as PP pentode/tetrode amps, and despite building and selling
quite a few SE amps I've never seen a problem with arcing tubes or OPT
insulations.
The other way to clamp output voltage and thus clamp the anode swing
voltages in a tube amp with OPT is to use two networks of 5W rated
Zener diodes in series with a normal 1N5404 3 amp Si diode between the
OPT secondary outlet and 0V so that if the Vo exceeds the rated Vo for
maximum rated power by say +3dB, then the zeners+diodes conduct to
clamp the voltage. Such clamping of outpur voltage is done routinely
on many SS amps where a diode such as 1N5404 is taken to each + and -
rail so that if the Vo ever exceeds the rail voltages then the diodes
conduct. Back emfs generated in speakers and inductiove loads are thus
prevented from flow *backwards* through output SS devices which will
destroy themselves instantly is the Ve-b back flow V exceeds about
7Vpk. The same sort of diode
clamping is used routinely in SS amps with mosfet OP devices to limit
the Vg-s, and thus not exceed a rated Vg-s which would punctuture gate
insulation which is very thin indeed.
Voltage clamping is thus very easy for anyone to achieve.
Patrick Turner.
--
Regards,
John Byrns
Surf my web pages at, *http://fmamradios.com/