Amp/Preamp Asylum: REVIEW: EICO ST-84 Amplifier (Tube) by Richard CWY

Model:	ST-84
Category:	Amplifier (Tube)
Suggested Retail Price:	$200
Description:	EICO ST-84 pre-amp
Manufacturer URL:	EICO
Model Picture:	View

Review by Richard CWY ( A ) on November 24, 2002 at 00:09:35
IP Address: 210.3.46.52

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for the ST-84

**EICO ST-84 - stereo pre-amp user review report**
EICO pre-amp 'ST-84' is perhaps the least known pre-amp to EICO fans
than its older brother 'HF-85'. My ST-84 arrived last Friday and
spent the week end checking, cleaning, retubing, and now playing with
it. Wonderful 'modern' tube sound from this mid/late(?) 1960s' gear..!
Brief: If you can find one in reasonably condition and price, don't
hesitate but grasp it!
*Detailed report*
Appearance
ST-84 looks like ST-70 or ST-40 in cosmetic design that blended
some 'solid state' taste in the faceplate (the reason for it being
not noticed by tube lovers?) and the use of aluminium in its
faceplate and side chassis panels, which stays longer from oxidation
than the 'HF' series which use steel and copper in the faceplate.
But the coarse surface of aluminium factplate usually habours dirts
and shows blemish marks that need careful cleaning. That suggests a
high chance of restoring the comestic to 'like new' condition, the
first reason to grasp it!
Circuit
ST-84 uses the same circuit design as in HF-85 except the following
variations found in the line amp (sorry vinyl fans, the phono amp has
been ignored in this report):
a) A 'scratch' and a 'rumble' filters were added to the sonic path -
Don't be scared away because they are absolutely by-passed once they
are switched off.
b) One more capacitor (0.47uF) coupling between line-amp and the
bass/treble tonal control circuit - that makes the negative feedback
loop of the line-amp more 'independent' from the tonal control
circuit that is then fed to the final stage, the buffer-amp.
c) Removal of one capacitor (0.25uF) in the buffer-amp's output lead -
that makes the output signal more 'direct' to the power amp - could
be RISKY IF the first stage tube of the power amp is bad.
d) A 75V potential is applied to the 6.3V AC heater supply - this was
a newer concept to EICO engineers as such a circuit was NOT found in
other 'HF' series pre-amps (HF-85 & HF-61), and NOT in the pre-amp
section of HF-81. But the HF-65 has this circuit design (therefore,
HF-65 was designed well after HF-61 and HF-85 in the 'HF' series?).
Some said that a positive potential at the tube heater can reduce the
potential difference between the plate (anode) and the heater (NOT
the cathode) and therefore the tube at the buffer-amp stage can
comfortably work at high plate voltage to yield 'smoother
performance'. My audition may tell how true it could be. It is
probaby the 2nd reason to grasp it. :)
e) Use of a 'PC' (abbrivation for 'printed circuit'?) in place of a
pair of resistor/capacitor at the cathode of the buffer-amp half
12AX7 - that helps cutting cost at that time in the defence against
solid-state audio (?) and uniform the value of these components (e.g.
the pots) for more balanced performance of both channels. (See
the 'PC' soldered under the chassis to compare with HF-85's photo.)
Components
All I can say is that the components found in ST-84 are of the same
grade as the HF-85. (Again, see the photos to compare.) However, I
found the pots and switches are smoother to operate and in the case
of pots, the variation of value (e.g. tonal and volume contol pots)
is much smoother and linear than in HF-85. I believe that the
manufacturing of these components was more mature in technology in
the decade between HF-85 (in 1950s?) and ST-84 (in 1960s?) just
before the overall surrender of 'tubes' to 'solid-state' audio, the
3rd reason to grasp ST-84!
In addition, tube audio products manufactured at the ending era of
tubes are bound to be 'kept' than 'used' because of consumers'
changing interest to solid-state audio. So, I can bet that many of
high-end tube products may still remain at their 'like new' condition
electrically and functionally, the 4th reason to grasp ST-84!
Checking
I found voltage at various points normal except at the plate voltage
of the fist half 12AX7 at the phono amp - only 18V! That may be the
reason why the seller said "AS TESTED WITH CD PLAYER: UNIT WORKS
EXCELLENT. HOWEVER, WHEN WE HOOKED UP TURNTABLE THERE WAS A RUMBLING
ON ONE CHANNEL, ...SO THIS UNIT NEEDS SOME ATTENTION" - honest
reflection!
Reasons? I found this voltage value was slowly recovering to 30 or
40V in 1 or 2 hours. It steadily stays at 55V after 10 hours
running. I believe it was because of the reforming of the
electrolytic filter capacitors, particularly the axial (orange 'dry'
electrolytic under the chassis, see photo). Now, both channels
remain dead quiet even 'input' is switched to 'phono' and some signal
in the phono jacks. I believe all the electrolytics have now revived
after many years' storage.
All other components checked well within tolerance. They are
just 'like new' in all aspects.
Retubing
All the 12AX7 (5 nos.) and EZ90/6X4 (1 no.) tubes are stock and
fairly 'new' in comestic and functional condition. This observation
partly supports my proposition above. However, except the 6X4, all
the 5 'EICO by Mullard' 12AX7 tubes are the sonically less desirable
short-plated version (all with etched codes: B2C2 - suggesting 1962
made in UK Blackburn?). As I don't use vinyl discs, I keep the 2
stock tubes (one for each channel) at the phono stage but replaced
the line-amp tube with 1 smooth plate Telefunken 12AX7 (in each
channel). The buffer-amp is important for the sounding quality. I
replaced it with 1 ribbed plate Telefunken 12AX7 at this stage (each
channel uses one half 12AX7).
(Note: I sometimes found the more desirable early versions "EICO by
Mullard" 12AX7 tubes in 'HF' series pre-amps - long-plated, D getter,
double supports, etc.. The more ugry the amp looks, the higher
chance that it had been neglected and early version "EICO by Mullard"
tubes remained. They can be your 'treasure island'. But it is a
gamble. Some might well have been dead.)
Audition
I used my EICO HF-60 monoblocs and Chartwell LS3/5A speakers setup.
Source from a Xindak DAC and Denon CD transport. The reference pre-
amp used is EICO HF-85. Before I did the comparision, I retubed my HF-
85 with 2x Telefunken smooth plate and 1x ribbed plate 12AX7 tubes as
I have done in ST-84. All pre-amps and monoblocs remained powered on
all the time for 4 hour audition session at my flat (me alone). The
same pieces of music and vocal were played with HF-85 first and then
with ST-84. Particular sections of the music were re-played with
either when in doubt.
I found ST-84 sounds like modern-made tube pre-amps, with very high
definition, plenty of air, wide soundstaging.. The slight 'driness'
in the high range noticed from HF-85 did not appear in ST-84. I guess
this is the magic of applying 75V potentials to the tube heaters.
Can anybody correct me and enlighten me more on the effect?
Like many EICO fans, my HF-85 is more for keeping than audition for 2
reasons: (a) the functions and facilities (volume pot, input and
output jacks, etc) are fairly outdated and inconvenient to use, and
(b) it has the characteristic 'vintage pre-amp' sounding, i.e. lack
of high resolution in high range as in comparison with modern-made tube pre-amps e.g. Jadis JP-200 (expensive! original one costs over HK$150,000).
When I compare ST-84 with my homebrew Jadis JP-200 clone (which have
pleased many of my audiophile friends), I found ST-84 is perhaps on
par with it - has high definition as modern-made tube pre-amps plus
the warmth and wide soundstaging of tube sound. Superb modern tube
pre-amp sounding - the 5th and most important reason for grasping ST-
84!
My conclusion is therefore: Grasp the EICO ST-84 if you come across
one at reasonable price. (Mine was USD $127.50 only). Its sounding
could worth more than 10 times of its ending bid!
Richard
Note: Photos are the EICO Tube-Amp Fans' Club:
http://groups.yahoo.com/group/eico/

Product Weakness: None at all for such a price and superb sounding!
Product Strengths: Yet cheap in eBay (if found!) - below USD $200 ending bid

Associated Equipment for this Review:
Amplifier:	EICO HF-60
Preamplifier (or None if Integrated):	EICO HF-85, ST-84
Sources (CDP/Turntable):	NA
Speakers:	Chartwell LS3/5A monitors
Cables/Interconnects:	DIY/NA
Music Used (Genre/Selections):	Female vocal, piano, concerto, etc
Room Size (LxWxH):	16' x 10' x 9'
Room Comments/Treatments:	The most I can affort to use at home.
Time Period/Length of Audition:	4 - 6 hours
Other (Power Conditioner etc.):	NA
Type of Audition/Review:	Home Audition

Product Weakness:	None at all for such a price and superb sounding!
Product Strengths:	Yet cheap in eBay (if found!) - below USD $200 ending bid