The Rocket phono amplifier, Dec 2005.

Edited 2011.
The Rocket still goes on rocking!
Contents of this page :-
Description and picture of a rather good system,
Picture of the Rocket.
Schematic Sheet A and B of the Rocket,
Full schematic explanation.
Schematic of reverse RIAA Eq testing circuit.
Picture of chassis underside of Rocket with Wimas and Auricaps
To see the Rocket power supply, see the PSU details
at preamp-nemo-line+psu-2005.htm
___________________________________________________________

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.....
Gary's system....

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.

phono amplifie, dec 2005.
The "Rocket" six tube phono amp, dec 2005. It rocks!

"Rocket" Phono Sheet A amp schematic, 2005, edited 2011.
schem-rocket-ph-amp-sheet-A-2011.gif

"Rocket" Phono Sheet B regulator schematic, 2005, edited 2011.
schem-rocket-ph-regulators-sheet-B-2011.gif

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...
Schem-basic-rocket-input-cascode-2011.GIF
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.
2SK369-Id-Vs-Vds-0V-18V-curves-2011.gif

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.

Now the voltage gain of the J-fet with 9dB current FB has been calculated
above at 7.68. Therefore, with a high output MC cartridge with rated output
at 1khz = 1mV, signal at the drain would be 7.68mV. Distortion is roughly
proportional to output voltage providing there is no clipping of the wave
by device cut off or other factor such as the negative going voltage drain
swing.

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.

There is a simpler way to consider all these concerns about voltage gain :-

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.

There is more to consider with this cascode circuit. The j-fet operates with a
voltage gain of about 10 between input gate and drain. J-fets, like triodes, have
considerable capacitance between the drain and gate, so with any gain there
is Miller input capacitance, but since the MC has such a low "generator impedance"
of less than 20 ohms the Miller C will have a negligible effect on the response
from the cartridge. In fact, to reduce high F harmonics produced by the MC
cart, it may be a benefit to strap a large C across the loading resistance for
the MC cart, perhaps 0.03uF or more. 

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.

Schematic for reverese RIAA filter.

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.

Internal links for the filament heater wiring allows the variation of heater
voltages from 12.6V to 6.3V depending on the tubes chosen, without needing
to alter soldered wires at lugs of tube sockets. 12V tubes need 12V applied to
pins 4 to 5, or 6V between (4+5) and 9 and 6V can only use 6V and it must
be applied to pins 4 to 5.

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.
Rocket underside wimas.

Rocket underside 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.

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