On May 18, 7:54*pm, mike s wrote:
I have a couple of output transformers with separate feedback windings, and was hoping that one might be a suitable replacement for the failed transformer in the *RCA 82 C4 Monitor Amplifier. *Schematic herehttp://www.waltzingbear.com/Schematics/RCA/BA-4C.htm
However it uses a high impedance winding *(1/6th of primary anode-anode), whereas the transformers I have both use low impedance windings, about 1:100. *I have a schematic for a British circuit by P J Baxandall *- last on this page *http://mike.wepoco.com/Home/retro-ge...-wire...*which places the feedback winding in series with the input pentode cathode, rather than the high impedance potential divider used in the RCA circuit. *Baxandall claimed to be using a transformer design patented by the BBC (Mayo, Tanner, Ellis). *I've seen the same arrangement used in a Marconi push-pull amplifier.
I reckon I'm going to have to adopt the Baxandall arrangement, *but would be interested to learn what others think of these circuits. *Presumably it was amplifiers like this that inspired the rather quirky Quad 2 extra feedback to the output valves.http://www.drtube.com/schematics/quad/quad-22..gif
Michael
You won't easily find any "drop in" new replacement OPT which could
be used in the the circuits by RCA, Peter Baxandall, or Peter Walker
of Quad.
All these old circuits have serious shortcomings which are only made
worse when someone tries to use an unsuitable OPT at home and they do
not understand how NOT to build an oscillator while trying to build an
amplifier.
You say you "have a couple of OPT" without saying just exactly what
their specification might be. To know what you are doing, you need to
know every single bit of information which describes each tranny
fully. You say your OPT in your RCA monitot amp has failed, so we
assume it has an open primary or shorted turns.
The Hammond 1650P OPT with 6k6 to 4, 8, 16 ohms will be OK with 6L6,
807, 1622 etc, but you will need to addapt the Hammond OPT to the RCA
circuit you already seem to have.
The RCA circuit has NFB loop with R24, 24k, and R8, 2k7, and we don't
know how much voltage is generated at the NFB winding across OPT
terminals 1 to 3. The lazy dumb ****wits at RCA ommitted to provide
us with a more clearly drawn schematic with all the working signal
voltages for all electrodes and transformer windings. My guess is that
OPT terminal 3 produces a NFB voltage of about 50Vrms to be able to
supply a high enough FB voltage at V3 6SN7 cathode, maybe 5Vrms, so
that the amount of NFB is at least 12dB.
A normal OPT with no FB winding and just speaker secondaries could be
used in the RCA circuit but you'd have to re-arrange the FB network to
V3 cathode with R24 being a lower value. This will affect the way V3
cathode is biased; if R24 is made smaller it reduces the total value
of Rk. Fso if R24 = 4k7, the R8 may need to be increased to 3k3 to
give the equivalent of what is in the schematic, ie, 2k7 // 24k for
the dc cathode current, ie, about 2k2.
I'd never ever try Baxandall's circuit, and I'm no great fan of Quad,
or ANY circuit which employs a "paraphase" input pair or uses the
output of one triode to drive another as done on the RCA circuit. It
is always better to employ a LONG TAIL PAIR as in many of my amplifier
schematics which you may inspect at my website at
http://www.turneraudio.com.au
I've always used NFB applied to a cathode of SE triode input stage
ahead of an LTP driver stage and this is better because the distortion
of the input tube is included in the FB and thus reduced along with
all following stages in the loop. The input tube works at low signal
levels so second order products are minimal. The Williamson, Leak and
Radford circuits emboby such principles as I do, but to make the amp
unconditionally stable regardless of load reactance will challenge
your abilities sorely unless you have a full understanding of what you
are doing.
Patrick Turner.