Marantz 9 Long Tail Phase Splitter / Pictures

Long tail phase splitter (also known as Cathode Coupled Phase Splitter) is widely used in push pull tube amp driver stage, it is more convenient and simple but the output voltage has about 10% difference between V1 and V2, to use a larger value resistor as V2 plate resistor is a norm in long tail driver for output voltage balance. The practice is adjusting a variable pot while watching a oscilloscope for a best sine wave form.

Please see picture:


Please see the equivalent circuit


From the equivalent circuit :

image001        <-F1

image003        <- F2


image005     <- F3

image007     <- F4

Let image009     <- F5

Letimage011     <- F6


image013    <- F7

image016     <- F8

From F3 and F7

image018     <- F9

From F4 and F8

image020   <- F10

From the equivalent circuit then:-

image022  <- F11

From F9,F10,F11

image024 <- F12

image026  <- F13

image028 <- F14

image030  <- F15

image032  <- F16

From the equivalent circuit Then :

image034 <- F17

From F9, F10, F17

image036  <- F18

From F16, F18

image038 <- F19

image040   <- F20

image042 <- F21

From F20, F21

image044   <- F22

From F16, F22

image046  <- F23

By current ratio in Rk path and R2+Ra path

image048  <- F24

By current ratio in Rk path and R1+Ra path

image050   <- F25

Then the out put currents are:

image052  <- F26

image054 <- F27

From F3, F4, F25 anf F26

image056   <- F28

From F22, F23, F28

image058  <- F29


image060  <- F30

Since triode 1 output voltage = Iout1R1 so that From F30

image062  <- F31

image064  <- F32

And the voltage gain of triode 1 is equal to output
voltage devided by input voltage Vi so that :

image066  <- F32A

From F3, F4, F24, F27

image068  <- F33

From F22, F23, F33

image070  <- F34

image072  <- F35

Then :

image074   <- F36

And the voltage gain of triode 2 is equal to output
voltage devided by input voltage Vi so that :

image076  <- F36A

F32A and F36A are the fomula for gains of tube V1 and tube V2.
By compare F32A and F36A we found that the voltge gain is a bid different between tridoe 1 and triode 2 in case of R1=R2, this is the reason why a variable resistor is connected in series with R2 for adjustment to achieve balance output voltage. Actually by calculation the value of R1 and R2 can be fixed instead of adjusting the additional variable resistor by oscilloscope waveform monitor.

Please note that   Rx   Ry   RAo   RBo   and   (F)   are group of resistance
and constants please refer to  F1,  F2,  F5,  F6  and  F21 respectively.

Please see Marantz 9 phase splitter circuit diagram


If the adjusting pot is set at 4K and 6K each side so that R1 will
be 46.4+4=50.4K while R2 will be 46.4+6 = 52.4k, we have:

R1 = 50.4K

R2 = 52.4K

Rk = 38.3K

Ra = 2.64K

µ = 33

By F32A V₁ gain = 15.7

By F36A V2 gain = 15.7

This is in balance and the adjusting pot can be replaced with one 4K and one 6K fixed resistor if you desire but I don’t suppose anybody would. This calculation could be a guide line for adjustment, if the balance point is too far away from that point (4K, 6K), the 6DJ8 should be replaced.

One more usable application for long tail splitter is direct drive stage in Push pull amplifier, as the plate potential to ground of both tube is a bit different and this will ruin the direct drive to power tube. A small modification can get the static balance of both tube plate voltage to suit the power tube drive as the DC out put voltage is also the negative bias of the power tube. The job is simple that a resistor equal to the difference of both plate resistor worked with a filter capacitor can resume the DC balance please see picture:


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