One of my
clients recently asked me if he could get better music with a
tubed preamp as an alternative to his Sutherland phono amp.
The Sutherland is all solid state using opamps, battery powered, and
cost
several thousand dollars in about 2004.
I said yes, I
could do better with tubes, but I would have to use just the
ONE solid state device in the signal path, a single 2SK369 j-fet as the
input device.
I was then contracted to proove what I said.
My client did
have enough quality elsewhere in his system so that the
benefits of a better phono amp stage would be discernible.
He has a nicely sized room which is well carpeted and furnished to
reduce
reverb and glare from too much treble. His speakers in 2005 were
Vienna Acoustic Mozarts, with Townsend super tweeters, and a custom
built subwoofer which I built. Sub amp is Musical Fidelity A3CR.
The power amps are 22 watt mono bloc SEUL 13E1 that I built in 1997,
upgraded for 2003, and the line
stage
preamp is also one of mine.
The line stage was upgraded in 2005, and its power supply re-built to
power
both line stage and the Rocket phono stage.
His vinyl replay uses a Michel Orb TT with an extensive sand and wire
damping kit from a dude in Denmark, the cartridge is Helicon Lyra.
Cabling is thin wire home brew plaited for interconnects. Speaker
cables
are Nordost. All up, this system is one of the few I am very happy to
spend
hours in front of, and despite my client's pair of good quality silver
disc
players for CD, SACD, DVD, even including a Sony player with
Allen Wright's special upgrade.
We both find that vinyl sounds better, and is more emotionally
engaging,
unless the recording isn't good.
It is remarkable how often they got it right in the studio/venue in the
analog
1970s, and they cannot do much better now when surrounded by digital
gadgets, and even with 96/24bits, and now, hard drives and goodness
knows
what.
Gary's system was extremely listenable.....
Directly
below the Michel TT is a Marantz tuner, then the "Rocket" phono
amp I built.
The line stage preamp and power supply for both line and phono stages
are to
the left of the silver coloured Marantz tuner.
The "Rocket" six tube phono amp, dec 2005. It rocks!
"Rocket"
Phono Sheet A amp schematic, 2005, edited 2011.
"Rocket"
Phono Sheet B regulator schematic, 2005, edited 2011.
The basic
layout is all single ended topology with cascode j-fet + triode
input stage and µ-follower output stage. Passive RIAA is all done
in one
network between stages.
I did at first try a fully balanced differential LTP input stage with
conversion
to unbalanced output in a quasi balanced output stage but the
complexity
was much worse than what you see above and I had more trouble trying
to keep hum and noise low. I have settled for all unbalanced topology
because really, who the hell really needs balanced? This circuit gives
low
noise, wide bandwidth, low distortion, and oddles of music, and the
Rocket's
low output resistance may power any tubed or solid state preamp or
sound
card for recording vinyl onto a CD. What more does anyone want?
The load for
the cartridge can be altered to any value by soldering a chosen
R1 across two points on a board provided within the amp. The gain can
be
altered for either HIGH for MC, or LOW by altering a soldered link.
Without the link, gain at 1kHz = 45dB, with the link, = 57dB, or 12dB
more
for MC.
So if MC rated output is as low as 0.2mV, output = 140mV.
The j-fet
input gate is biased at 0V and the source resistances R3 to R11 are
taken to a shunt regulated negative voltage also held steady by 10,000
uF caps.
The set up allows for a minimum effect of electrolytic bypass caps in
the j-fet
source circuit. No capacitors are needed between the j-fet gate and
input
terminal, as I have on an ealier 10 Tube Preamp.
The Rocket was an evolution after building the 10 tube
preamp.
The cascode
circuit works as follows :- The input signal is applied to the very
high input impedance gate of the j-fet. Any input voltage signal across
the gate
to source, Vgs of the j-fet makes a current change in the j-fet path of
drain
to source. Id = gm x Vgs.
The j-fet 2SK369 has characteristics for 5mA drain current of :-
Gm = 40mA/V, Rd = 80k, and µ = 3,200.
The j-fet is very similar to having say 10 paralleled 6SH7 pentodes
which
have characteristics expressed as Gm, Ra, and amplification factor,
µ.
The pentode grid input resistance is extremely high like the gate input
resistance
so the terms used for vacuum tube may be used to describe a j-fet.
Before explaining the workings of the
Rocket cascode input stage,
everyone should know there is some
basic info on cascode at my
page on 10 Tube Preamp. Go to Sheet 2,
and read the notes which follow.
The specific
operation of the Rocket input stage is as follows:-
The operation is for high gain, with link soldered in. The j-fet source
resistance
comprises the resulting total R formed by R3, 4, 5, 6 , 7 and 8 which
is 43.6 ohms.
The load seen
by the V1 tube varies from a maximum of 23.5k (ohms) at very
low F to about 15k at 1kHz due to the nature of the impedance of
the RIAA
filter which varies with F.
Let us consider the situation at 1 kHz.
V1 tube gain, A, = µ x RL
/ ( RL + Ra ).
A picure says 1,000 words...
And yet a pile of calculations is still necessary for
understanding.....
In this case
V1 paralleled 6DJ8 has Ra = 2.5k approx, and µ = 33,
so A = 33 x 15 / ( 15 +2. 5 ) = 28.28, approx.
( Ra varies considerably with the idle anode current and for 1/2
a 6DJ8,
Ra = 5k approximately so for the paralleled tube it is about 2.5k.
An approximate calculation is all that is needed to explain how the
circuit
works. )
Consider V1
operation. Since the V1 grid is effectively grounded, the V1
voltage gain depends on voltage change at the V1 cathode.
V1 is
operating in "grounded grid" or "common grid mode", which is unusual,
but very effective in some situations where we wish to avoid the Miller
effect.
Driving a triode or any tube by changing cathode voltage also means a
current
change must occur which is unlike a "normal amp" in common cathode mode
where cathode is grounded and the voltage change at grid needs no
current change
because grid input resistance is many meg-ohms.
Bright ppl amoung you will realise that if the anode load of the triode
was a
constant current source, then the Rin at the cathode of a grounded grid
triode
would become infinite because despite voltage change at a "low
impedance"
input port, there would be no current *change*, although the steady
flow of Idc
would remain present, and constant, despite Vac change. This is a bit
difficult
to envisage, but triodes work very well as voltage devices without any
current
change whatever. But in this Rocket case, there is an RL = 15k0, and
current
in the load does occur, and hence the same current change occurs in V1,
and so
thew Rin at the V1 cathode = RLa / Voltage Gain = 15k0 / 28.28 = 530
ohms.
This cathode
Rin resistance is the load seen by the drain of the j-fet.
The whole
j-fet load in which current change occurs is the drain load in addition
to any source load, if there is one in the form of an unbypassed Rs.
In this case, with Rocket set for high gain, the resultant Rs
considering the
resistance network formed by R3,4,5,6,7,8 in the main amp schematic.
The j-fet
voltage gain is the total voltage change between drain and
source divided by the voltage applied between gate and source.
The RL total
= 530ohms drain load plus 44 ohms source load = 574 ohms.
Gain = gm x RL = 0.04A/V x 574 ohms = 22.96.
At this point
we might nominate the signal voltage at the j-fet drain = 0.53Vrms.
Ie, Vd = 530mV. The fet current = 1.0mA. Therefore across the Ra of 44
ohms,
there will be 44mV. The Vd and Vs are oppositely phased, so Vds = 574mV.
J-fet gain = 22.96, so Vgs = 574 / 22.96 = 25.0mV.
The input
signal applied between gate and 0V is what is applied at the RCA socket
input terminal, and is the sum of Vgs + VRs, as the two voltages have
the same
phase, and Vg-0V input must always exceed Vs. So thus Vin = 25 + 44 =
69mV.
The j-fet
therefore has overall gain between its gate and drain = Vd / Vg
= 530mV / 69mV = 7.68.
The effect of the unbypassed Rs is to provide local series current
negative
feedback and this maximum gain of 22.96 to 7.68, which is a gain
reduction
factor of 0.335, which is about 9dB of NFB. It means that the
distortion of the
j-fet will be reduced by this factor.
The THD of the J-fet.
I have
plotted the RL = 530 ohms on the 2SK369 curves with Point Q idle
at Vds = 10Vdc and Id = 5.2mA.
For equal +/-
0.1V Vgs change, the Id changes between 2.1mA to 5.2mA
at idle to 9.3mA max. Total Id change = 9.3 - 2.1 = 7.2mA.
From the
graph, it can be shown that for Vgs change of 0.2Vp-p,
there is Vds change of 3.8Vp-p, so gain = 3.8 / 0.2 = 19.0.
This is less than what I calculated above at 22.96.
Gm = 7.2mA
for Vgs change of 0.2V, or 36mA/V average. In fact I have found
gm to measure more like 40mA/V, but the curves above are from Toshiba
data
and from whatever batch they tested on the day. Curves give a basic
guide and
are not show the exact truth about a device, which may in fact be
better than
curves indicate.
The fraction
of 2H in the output signal may be roughly calculated as
Difference of Id swing + and - / ( 2 x
total Id swing )
= 4.1 - 3.1 / ( 2 x [ 9.3 - 2.1 ] ) = 1.0 / 14.4 = 6.9%.
Ouput voltage max = 3.8Vp-p = 1.35Vrms with 530 ohm load.
If there is
6.9% at 1.35Vrms with no NFB, then for Vd = 7.68mV and no FB
THD = 6.9% x ( 0.00768V / 1.35V = 0.039%. The current NFB will reduce
this to around 0.0129%. If the input cartridge signal increased to
10mV, then
THD should not exceed 0.13%, and it is mostly 2H, with much lower levels
of higher numbered H.
When the amp
is used for MM cartridges with a typical rated output of 5mV
at 1kHz the gain link increases the Rs value from 44 ohms to 259 ohms,
and the amount of current NFB is very much increased, and gain is much
reduced. The J-fet will still produce similar levels of signal as for
the gain
setting to suit MC, but THD will perhaps be 1/4 of the levels with MC.
It can be
proven that the total gain for the j-fet and V1 without current FB
may more simply be calculated as j-fet gm x tube RL.
The maths wizards amoung you will understand that this gain is the
product
of the j-fet gain and triode gain, in this case it is 22.96 x 28.28 for
RLa = 15k0.
This equals 649, if the j-fet load includes the Rs of 44 ohms.
The full equations for the two gains may be written out as a product
and it
will be found the fet gain x tube gain without use of any Rs is most
simply
j-fet Gm x Triode RLa, ie, 0.04 x 15k0, ie, 600.
The remarkable thing is that the tube may be either 12AU7, 6CG7, 6DJ8,
12AT7 or 6AU6 in triode, or 6EJ7 in triode, and the gain will remain at
600.
In this circuit, the tube type used has no influence on the gain, and
gain is
purely determined by the j-fet Gm and the value for Rs. The triode must
be able to operate happily with the intended Ia = Id of the j-fet. It
would
not be possible to use 12AX7 or 12AY7 because these would have to
operate with grid current which would cause much higher distortion.
Now the
overall j-fet plus tube gain may be easily calculated when any
un-bypassed Rs is used for Current NFB.
Gain with Rs FB, A' = Gain A without Rs / (
1 + [ A x ß ] )
where ß = fraction of output fed back to j-fet source = Rs / Tube
RLa.
For Rs = 44 ohms, A' with FB = 600 / ( 1 +
[ 600 x 44/15,000 ] )
= 600 / 2.76 = 217.
For MM
cartridges the generator impedance is far higher, and the typical R
loading for MC of say 470r would very much lower the output of the MM
cartridge, and create H distortion and F response bothers. Therefore
the
MC loading R must be removed when using MM, so I mount my own
cart loading R1 and C on an RCA socket in parallel with RCA cartridge
input, so it may be easily removed when I have to test someone's TT
with MM cart.
The usual MM
cartridge loading is 47k plus C = 200pF to 400pF.
This isn't a
high amount of C and whatever C is strapped across the input
to load the MM cart and tame HF response must augment the Miller C.
Fortunately, the j-fet gain is only around 10 for MC, and much less for
MM
if the Rs is increased to reduce gain for MM carts. Thus the Miller C
is very
low;
lower than a high gain input tube such as a 12AX7 with a gain of say 85.
Using the j-fet as a cascode driving device will always result in
having a low
voltage gain at the first input device and thus have a small Miller
effect.
If you didn't
have the V1 triode in series with the j-fet then the
j-fet gain
will be around 500 because the load of 15k0 is a relatively large
fraction of the
j-fet Rd, 80k. Without current FB, the Miller effect would be
huge.
It will be found that THD with RL = 15k0 is also huge, and because Id
change
is low and Vds change is high, one will still need to use current FB to
reduce
gain to say 200, but THD will be much higher than using the fet for
current
change plus the triode to handle the voltage change much better. The
story
about 2SK369 isn't done yet, and that loads of around 6k0 seemd to give
lowest 2H at about 5Vrms output. It appears the j-fet is similar to
pentodes
which will produce very low 2H at one value of RL, with higher 2H of
opposite
phases either side of this load giving low THD.
In some j-fet
input stages, a pair of j-fets can be used in series cascode.
The gain of the bottom fet is unity, because whatever the drain load of
the top
fet is, source Rin = 1/Gm, or 25 ohms with 2SK369, and the bottom fet
gain
= gm x Rs in = 0.04 x 25 = 1.0. For this reason, cascoded j-fets make
excellent
input stages for RF signals at low levels where Miller C is to be
minimised.
The
"equivalant input noise resistance", ENR, of a j-fet approximately
= 0.7 / gm, where 0.7 is a constant given in reputable literature such
as the
British radio amateur handbooks. Gm is in Amps per Volt.
So ENR for 2SK369 =
0.7 / 0.04 = 17.5 ohms.
The ENR is
considerd an equivalant R in series with a perfect amplifier
which produces no noise, and the noise which is measured in any given
device
is equal to the Johnson random electron noise of a resistance in series
with
the perfect amp. The noise varies in proportion to the square root of
the ENR,
so that if the ENR is increased 4 times, then noise doubles. Or put
another
way, ENR must be reduced by 1/4 to halve the noise.
A triode's
ENR = 2.5 / gm, so if the triode is a 12AX7, ENR = 2.5 /
0.0016
= 1,562 ohms.
The j-fet EINR = 0.0112 times the 12AX7's ENR. The input noise
varies in
proportion to the square root of ENR, so that if the ENR
difference is 0.0112,
then the noise difference = sq.rt. of 0.0112 =
0.105.
Thus we could expect the j-fet to produce 1/10 of the noise of a good
12AX7,
ie, the j-fet will be 20db quieter than the triode.
In practice,
this is exactly what we do achieve by using a high
transconductance
j-fet, and in fact the noise produced by the
j-fet is far more benign than the
triode since the triode noise
includes flicker noise which is low frequency and
this sounds rumbly in a phono amp which has high gain at low F.
The noise
of the j-fet has little flicker so the noise heard from the fet circuit
is a very high pitched hiss,
but it is at a level way below the triode hiss noise.
This has been
my experience with 2SK369. Although the type number
indicates this tiny
T092 package device is a mosfet, it really isn't, and
just as well
because mosfets have a lot more low frequency "popcorn"
noise than
j-fets, so choice of j-fet is quite critical.
2SK147 was made by
Hitachi, but is now not made by anyone.
We rely on 2SK369 which is exactly the same as the 2SK147, but with a
slightly lower power dissipation of only 0.6 watts. In the circuit
above
the j-fet power dissipation at
idle is only about 0.05 watts, so the j-fet won't
fail unless a short occurs between anode and cathode. I bought a batch
of
10 x 2SK369 for $1 each, so one can afford to
replace them.
People say
j-fets are fragile devices, and I would agree, but used in
phone
stages at such low levels they seem to last well, but methinks the cap
between
gate
and 0V, C3 needs to be there to shunt any static discharges.
I have
serviced Audio Research and Conrad
Johnson preamps with
far more complex hybrid circuits than I am using, and these are
difficult
to diagnose when a fault occurs, but my product uses far less
complexity,
is very easy to service.
I also found
that amoung 4 randomly chosen samples of 2SK369, the gm
matching was
within 0.5%. This results in almost identical gains of the two
stereo channels of the
phono amp above and in another amp i built
for myself in 2004, mentioned in pages on 10 Tube Preamp.
There is adjustment for channel gains with 20k linear pot in series
with
R20 130k on Sheet A above. These pots do cause some loss of gain as all
balance adjusting circuits do but the gain loss is only 1dB in mid pot
position
and it is unlikely more than 2 dB balance difference will exist.
I've not much
needed to have balance controls in most amp systems I have
built.
The output
impedance at the anode of V1 is extremely high, and approx
= Ra +
(µ x Rk) where Rk is the impedance looking into the j-fet drain,
perhaps 100k+,
and µ = V1 amplification factor.
Since V1 µ = 33, the effective Ra of the 6DJ8 = 33 x 100k = 3.3
meg ohms.
But we have 23.5 kohms as the dc RL between anode and B+, so
at 5 Hz when the input impedance of the RIAA filter is very high, we
can
say that the Rout from V1 = 23.5k in parallel with 3.3Mohms, or 23.33k.
So Rout from V1 anode = 23.33k with regard to calculations for the
values
in the passive RIAA filter.
If triodes such as 12AT7, 12AU7, 6EJ7, 6AU6 are used instead of 6DJ8,
the effective Ro from V1 will not change substantially so no changes
need
to be made to the passive RIAA filter values once these have been
established.
I found it
extremely difficult to calculate the exact RIAA filter values but I did
get approximate values which I could start with. Then I used a reverse
RIAA
filter between my
signal gene and the preamp, and I adjusted the values to those
I have in the above schematic by
means of series/parallel arrangements of
standard value resistances and
capacitances.
Providing my reverse RIAA filter is correct, which I was told it was,
so to
will the preamp RIAA filter after trimming R&C values. With
persistance,
the reponse was
brought within +/- 0.25dB of dead flat along the band with
the reverse
RIAA filter in place; the preamp output was a flat response using
sine
waves between 10Hz and 20kHz, and a square wave at 1 khz looked
like a very nice
square wave without ripples or sloped
horizontals. With RIAA filters it is impossible to get the sharp square
corners
on square
waves because of the restriction in bandwidth. My RIAA filter
differs slightly to most other folks because it has an
additional R17 22k added
in after the R16 + R18 + C7 3180uS and 318uS filter formed by R39, R18,
and C7.
The R17 + R19 + C8
act to give 75uS and about 3uS time constant filtering,
and the R 17
builds the 75uS filter out from the R19 + C8 to help isolate the
interactive effects. I find this arrangement was easier to get to
within 0.25dB
of the wanted flat response than when ommitting R17 and using just a
lower
value cap from the top of R18 to
0V with a smaller value R in series for the
50kHz or 3.18uS time
constant. Some people use a 3 stage amp with RIAA
3180uS and 318uS
attenuation done after stage 1, then 75uS done after stage 2,
and before buffer stage
3, to get the most non interactive and accurate
RIAA eq, but here I have done it by simply adding some resistance
between
the 318uS and 75uS eq. it is simpler, and the V2 gain tube isn't
subject to
large HF signals;
its input has a flat F response so there is less distortion.
The results
of testing the accuracy of a phono amp will depend on the flat
response
of the signal generator sine waves and its low distortion.
Its output
resistance must be less than 100 ohms lest you get some errors
at above
10kHz. The Reverse RIAA filter has R3 = 3k3 to limit the amount
of
boost to HF above about 50kHz.
Most practical cutting head amps have
this extra HF time constant of
about 3uS.
Rocket Regulation.
In my own 10
tube 2004 preamp I didn't use any B+ regulation.
However in this
amp which I sold I used an emitter follower style basic
regulator at least for the B+
commonly supplied to both channels.
This is in addition to the large value RC filter caps. The emitter
follower with
BU208A plus MJE340 in a darlington pair was a
simple way to achieve good
ripple rejection and LF stability, and
prevent cross channel talk.
The floating 12.6 Vdc applied to the heaters is shunt regulated by the
two
low voltage npn and pnp power transistors. The heater voltage is
also biased
and held steady with a cap to 0V.
V2 Gain.
V2, 12AT7, is
the second gain tube with gain approx = 45. V2 acts with with
the
output follower tube V3 to form what is a µ-follower
configuration with
excellent
linearity. If a sufficiently large 1kHz input signal is applied to the
j-fet
input to generate 10Vrms output from V3, THD = approx
0.2%, so that when
output is only 0.5Vrms, THD falls to below 0.02% ( mainly all 2H ) and
below
the noise
floor. Regardless of what has been said before about THD, that is what
I measured.
The use of
parallel twin triodes for each triode element in the circuit
means that
the one has minimal interaction between channels or different parts of
the circuit
if the actions of only ONE triode exist per tube socket and tube
envelope.
The parallel halves of a twin creates a single triode with same µ
but
Ra = 1/2 of one triode, and double the gm.
I do not believe paralleled triodes give poorer sound
in power amps or preamps,
but believe parallel tubes better the sound.
There are
those who would lock me in a dungeon and throw away the key for
using a
12AT7 for ANYTHING! They like to
stamp on this tube with their
Audio Nazi jack boots. But I find that most precious
prejudices against many
tubes are often just a lot of balderdash, and you can't expect to
get the same
sound from all brands and batches of 12AT7, or any other tube.
If you don't
try different brands of
tubes, you won't know which are the best,
but yet I see no reason to be extremist and try every tube in the
universe.
You may hear
someone say they like Mullards or Siemans NOS, but you
have to try them to see if they were right; 12AT7 is not the most
linear
triode
around, but no worse than 12AU7 or 12BH7 and others but we are only
asking
thse devices to make less than 1Vrms of output and I have used the
tubes in
almost the most linear way of building a
single ended voltage amp, ie,
µ-follower, so lack of linearity is just not a problem, and
anyone will be
pleasantly surprised with the
outcome.
While I rather like Siemans 6CG7, I am not so sure Sieman's 12AT7 have
the same magic, and so perhaps a Mullard 12AT7, or 6201 might sound
better
because the Mullard sound, ( if there is a "Mullard sound" ) tends
to be
be "polite", and kind to harsh recordings, especially of female
screamers.
Because there is no
global loop of NFB around the two gain blocks of this
amp the changes
of tube types that one makes may be able to be heard by
those with
good ears and accurate gear in the rest of their audio chain.
One
interesting tube people could try for V2 is a 12AY7. It has similar
gain
and characteristics to a 6SL7, but is a 9 pin mini tube and made for
lower noise.
However its anode current will be a lot lower than 12AT7
if just plugged
straight into this circuit instead of the 12AT7, and there will be too
little
current in the 6CG7 follower.
But for better operation with 12AY7, try R26 = 22k, R 25 = 1k approx so
a minimum Ia = 2mA, then connect about 100k between V3 cathode and
0V to increase the
idle current in V3 for better operation.
Of course if you can use a 12AY7, or 12AT7 for V2, then why try a
12AX7? It will also work OK with the same slight mods as required by
12AY7.
Maybe gain will be
about 80 though, it could be too much.
I may be also
thrown over a cliff soon when people see that I am no fan of
having
10mA in each 1/2 triode section whenever I use a triode.
Honestly, I
have never heard a big sound improvement when running signal
triodes in preamps at high temperatures due to high anode
dissipation.
The use of high currents in signal triodes does
place the tube operation into
its region of more constant Ra and gm, but when you analyse my
schematic
you will see that anode loads are
high values, and the linearity just does not
suffer because of the operating points. Usually when ppl use say 10mA
in 1/2
a 6CG7 when Ea = 120V, there is also an anode RL bringing DC to the
anode
and its value with B+ = 300V will be 18k0, and only 2 x Ra, and THD is
likely
to be 4 times what I have doing it my way with less Ia and much higher
RLa.
If ppl didn't
want to use a 12AT7, they could use a 6DJ8 with a simple
change of
heater wiring and perhaps a change to the cathode R25 and R26.
The V1 tube
could be either 6DJ8, 6CG7 without any circuit changes.
The V3 6CG7 at
the output could be a 6DJ8.
The tube
choice does affect the sound quality probably more than any other
variable factor such as coupling caps. I don't actually think there
are better
sounding caps than Wima polypropylene, 630v rated.
The topology used for each stage would also have a big effect on
sound.
The V3 output followers work with a lot of local NFB. Replacing brands
of 6CG7 would not seem
to be able to change the sound at all because so
much natural local NFB is being used. All cathode followers have a lot
of
local NFB, since all the output is in series with the applied grid
voltage,
and gain is less than 1.0.
The signal voltage between grid and cathode is
only approximately 1/18 of the output voltage at
the cathode.
So one would expect the NFB to lessen sonic signatures caused by tubes
with a different "voice", a word I hate to use, because in these sorts
of
amps the tubes don't have much voice, you only hear what was
recorded.
There are no global loops of NFB in this phono amp, so the sonic
signatures
of the input j-fet, V1 6DJ8, V2 12AT7 would add together give you what
is heard.
The present j-fet is set up is one of several ways at least, and the V1
grounded grid triode is also set up quite differently to more usual
common
cathode gain blocs.
How
can people form opinions about the sound of a particular type or brand
of tube when their circuit topology can vary so much? So changing one
of
these devices in the chain is only changing the combination; it is
impossible
to attribute all the sound quality to any one device.
Lady
luck plays a part here.
But changing a tube still may make a slight sound change, maybe the
sound
will seem less bright, warmer, whatever, its hard to predict an outcome
without trying a few
tubes.
I usually find that If I place well chosen tubes which test well to
give low
noise, correct gain, low microphony, and low reverse grid current, then
music really flows, and the sound is
good if not extraordinary, and changing
tubes is a bit like opening a bottle of an alternative
very fine wine; the
taste may differ but we can savour what comes from different wineries.
The new owner
of this amp did confirm I'd proved my original point,
that a few tubes
and ONE j-fet could sound better than a bunch of opamps
from America.
The Rocket
was supplied with Wima coupling capacitors initially. After some
months
with Wimas a trial of Auricaps was carried out with the line
level
preamp. One channel of the line stage had Wimas as originally supplied,
one with Auricaps. A mono source of signal from a CD player was used as
a
source and switched from one channel to the other. The mono output
signal
from either channel was then taken to both power amps and a mono
signal
played from both speakers. Both myself or my client could not pick
which
channel had the Auricaps more than 50% of the time and I decided there
was no difference in the sonic
performance using either Wimas or Auricaps.
Please note
that only the coupling caps in the signal path were changed
to
Auricaps. The bypassing of power supply electrolytics and parts of
the circuit prone to RF
oscillations if left unbypassed were bypassed with
polyester capacitors. I believe such rail bypassing
capacitors have absolutely
zero
negative effect on the great sonic character of such tube amps.
I remain quite doubtful
that Auricaps can any beneficial effect and will not
be spending up to instal them in any of my own amps I use
myself.
For power
supply details see Nemo line preamp and power supply.