Frequency response measurements with 2 different output transformers on the Pleiades V6 mic prepreamplifier

Referring to the Pleiades V6 schematic readily found on euroelectron by Google search.


The output transformer version 2 has double the turns on both windings compared to version 1.


They are both 10:1 but version 2 has roughly 4 times the inductance on both terminals.


The core for both is Magnetec 070 Nanoperm nanocrystalinne tape wound core.


Transformer version 1
350:35 turns:turns
16H:140mH


Transformer version 2
671:70 turns:turns
80H:800mH


Setup signal path:


Sinewave test CD - Sony portable CD Walkman line out aproximately 300Ω - (22nF coupling capacitor) - Pleiades V6 - (variable load resistor) - realistic disco mixer at mic in which has an input impedance of 10KΩ


Firstly the termination was 10KΩ, ie only the realistic Disco mixer mic input.


Output transformer version 1


63Hz   -18dB
80Hz   -15dB
125Hz  -9dB
160Hz  -7dB
250Hz  -4dB
314Hz  -3dB
1KHz    0dB
...
...
...
11KHz   -3dB


Output transformer version 2, load 10KΩ,  330Ω

63Hz   -7dB         -5dB
80Hz   -6dB          -4dB
125Hz  -3dB         -2dB
160Hz  -2dB         -1dB
250Hz  -1dB         -0.5dB
315Hz  -0.5dB        0dB
500Hz   0dB           0dB
1KHz    0dB           0dB
5KHz    -1dB
8KHz   -1.5dB         -1dB
10KHz  -2dB           -2.5dB
16KHz  -4.5dB        -5dB
20KHz -6.5dB         -6dB


Battery supply voltage is 3.9V. This underheats the cathode and works great. The anode to grid electron accelerating resistor is 8MΩ.


The actual value of the 330Ω terminated case is 6dB below the 10KΩ case indicating a 330Ω output impedance at 1KHz. Also the increase in gain by about 4 dB when heaters are disconnected and the cathode starts to further cool down was not observed on the 330 ohm termination case. Why? Is this a electron tube or output trasformer effect? An explanation is as follows. When the tube cathode temperature is further decreased the anode current droos from 100μA to 40μA. The output impedance (after the output transformer) increases from 300Ω to 800Ω. Therefore when the termination load is 300Ω there is more voltage drop which offsets the increase in gain. Further investigation is needed here.