Phil Allison wrote:
"Patrick Turner"
I've just updated the page at
http://www.turneraudio.com.au/quad2powerampmods.html
I have re-drawn the original 1955 schematic and
included some more notes about Quad ESL57 impedance curves,
and the disastrous effects of having an arcing ESL57 midrange-treble
panel.
** Note this ESL57 schematic:
http://www.quadesl.com/pdf/quad_schematic.pdf
1. The two bass panels are fed via a series string of 6 x 180 kohms
resistors - so even in the unlikely event of a shorted bass panel the
secondary impedance is still over 1 Mohm. This equates to 12 ohms on the
primary side as the turns ratio is close to 300:1.
2. Similarly the two mid panels are fed via 2 x 150 kohms, with one bypassed
by a 560pF cap - so if one of these panels shorts, the impedance is 300
kohms at low frequencies on the secondary side falling to 150 kohms at high
frequencies. This translates to 30 ohms on the primary falling to 15 ohms
at HF side since the ratio is now 100:1.
3. The treble panel is fed via two 560 pF capacitors in parallel - so a
shorted treble panel has virtually no effect on the impedance at low
frequencies. The impedance of a 1120 pF cap is 140 kohms at 1000 Hz,
falling above that. This translates to 14 ohms on the primary side - or
the same load as about a 1 uF cap.
So, a shorted panel in a Quad ESL57 will *not* harm the driving amplifier -
no matter which one it is.
That ESL57 schematic is informative thankyou. I have it already, and
there are other
"versions" of the same speaker with varied R & C used between the panels
and HV step up tranny.
** However:
The very low impedance load situation described in PTs page must therefore
have involved an early ESL57 ( pre 1967 ) when treble panels were directly
fed from the transformer secondary.
The schematic I traced of the wire up of the HV tranny has only one cap,
and is different to
what must be later speaker versions; indeed the two ESL57 I have here
must be "earlies",
and probably quite old, and which may have suffered in unwise hands
prior to their recent purchase,
ie, the guy bought them with serious faults from someone else who was
happy to part with
the bothers...
In this case, a shorted treble panel
will result in the primary impedance becoming about 1 ohm right across the
audio band.
This is what I noticed, except the short is at a voltage input threshold
above 2.7Vrms,
and at a volt input the Z of the crookie is very similar to its mate
which appears healthy.
When fed from a Quad 2 amplifier, this fault would result in a large volume
reduction ( circa 20dB) as well as, of course, no mid or treble output at
all !!!
But the short is a dead short at low voltages....
One would have to be * deaf or semi-unconscious * not to notice something
was VERY wrong and discontinue usage of the system on that side.
But the amps cope at low voltages.
The owner MUST have had the amps cranked well up to excite the crookie
to
make the amp see the bad load; I noticed the amp behaved like an over
loaded amp
with clipping behaviour after 3Vrms, and raised cathode dc bias
voltages, as it vainly
struggled with the low load, and suffered Pda which became excessive,
one tube thermalled out,
470 ohm Rk fuses open, Ek shoots up, exceeding 63V Ck rating, Ck shorts,
owner notices something wrong then!
Damage had probably been done previously to the other amp which may have
been used with the crook speaker.
It seemed so because one KT66 grid was +9Vdc after being on for an hour,
even with separate Rk resistors to better regulate the bias. It was on
the verge of failing,
and left any longer, may have caused real damage.
Did Peter Walker have shares in the MOV GEC company making KT66?
He would have done well due to all the unecessary early failures of all
those KT66....
But I suspect the owner who just bought all this kit was saddled with
ancient KT66
on their last legs, just about to blow; buying old tube amps is often
like buying
a used car, the tyres are crap, the radiator is full of sawdust to stop
water leaking out,
the diff is full of banana skins to keep the noise down, and so on.
Invariably, you buy another dude's problem he hasn't the money to fix.
Even with one output tube pulled out of a QuadII amp, many peole
wouldn't notice anything wrong
since that single remaining tube beavers away as an SE class A amp
producing up to about 4.5Vrms output into a normal 8 ohm load
but with much extra 2H.
BTW:
Quad advised owners that all ESL57s be modified with the addition of two
560 pF caps and 270 kohms ( as shown on the schematic) back in 1967 -
this coincided with the release of the 303 amplifier.
No such 270k R exist on these speakers.
I know what you mean and may have to alter the speakers to suit the
later precautionery approach.
I tested the response of the good ESL57 against my own which have a
flatter response than the '57.
This was done using speakers side by side, 3.5M away from the mic set at
ear height.
Same amp, levels, and pink noise source and mic were used.
My own speakers are very slightly less sensitive, ie, less SPL /W/M,
and this took some calculation because of Z differences.
But for the same voltage input, '57 was 4dB down on the Seas drive units
in mine.
The '57 has a brightness peak between 2k and 7k of +5dB max, and
explains the slightly more detailed
sound whch could be awful if the music source was toppy. The '57 bass
had a big peak at 85Hz
which rolled off more than 18dB/octave so that bass was -3dB below
average overall level at 44Hz,
wheras mine go down to at least 22Hz even using my limited mic, so
utterly no need for a sub-woofer.
Subjectively, the '57 are very nice, with probably enough bass for
anyone without
a sub unless you need to hear modern crap with deliberate bass factored
in.
My own speakers have zero detectable box artifacts due to good woodwork,
and sounded smoother, simply
because the treble response is flatter. To good old blokes whose hearing
is down -6dB at 7kHz, not uncommon, ( mine isn't though ) and whose
tinnitus obstructs anything over 10kHz
then ESL57 would be just fine, with their wives none the wiser since
they seem to have more
tolerance of response differences, and sometimes put up with attrocious
systems.
This test of ESL57 is of only ONE ancient sample. I don't have another
20 speakers to compare to get an average
response which could be said to be The Response of The Quad '57.
The fact is, speakers vary with age, and one would expect differences
after 50 years.
But that a 50 year old speaker picked at random can sound so good
compared to state of the art
domes and cones is a credit to Peter Walker.
I am told that upgraded or repaired panels with better diaphragm
materials will give better sound.
Patrick Turner.
........ Phil