Experiments were done using the Pleiades experimental jig.
Summary: The Pleiades version 2 output transformer with more turns goes down in frequency much lower than version 1 and the frequency responce is comparable to the Altec octal base output transformer. Again it was confirmed that disconnecting the already low 3.6V at heaters further increased the gain of the EF183 electron tube on the Pleiades V6.
Referring to the Pleiades V6 schematic. Only about 3.9 volts were used for both the anode and the heater circuit by 3 AA batteries of 1.2V each rechargeable Ni-Mh. The anode to grid resistor used for making the operation possible at such low voltage is Rag=8MΩ. The anode current slightly exceeded 100 micro ampères or 100μA!
It is known that it is a very low noise preamplifier. So initially it was decided to listen to the other extreme of the dynamic range by listening to how it clips or overloads.
So the Sennheiser HD580 headphones were connected directly to the output transformer secondary. The headphones mono bridged are a load of 300Ω/2=150Ω which is a nice load.
The line out of the Sony portable CD player was connected to the grid of the EF183 electron tube through a 22nF blue MKT Philips ? coupling capacitor. The overload characteristics were nice but could have been better. While overloading occurred the Ia meddle was moving to the left just like on a compressing amplifier. At small overload distortion was nice but at extremes it could have been better. More investigation is needed here.
The output transformers are mounted on octal sockets in order to make almost instant change from one to the other by just plugging out and in.
It was decided to leave the battery + always connected to the anode circuit. Thereby turning on or off the preamp by connecting or disconnecting one of the heater leads. It is a joy to hear the sound fade in or out with no click or any noise side effects. It is also a joy to hear the sound change as the cathode warms up. And to reach a temperature point above which the anode current decreases. On such circuits the temperature influence on the bias is important. Higher temperature creates more electron cloud, the more the grid becomes negative internaly as the cathode has become more positive by the more electrons who have left leaving (positive) protons. These effects are very important if we wish to operate the tube with just a few anode volts. This is why an external positive bias resistor is needed to compensate this effect. More of this later on.
(Later addition: When a 330Ω resistor terminated the V6 transformer, decreasing temperature no longer increased gain. It seems the effect happens when termination is high such as 10KΩ. Why? When terminated at 10KΩ maximum gain at 1KHz takes place when Ia drops from 100μA to less than 47μA. Does this have to do with less DC magnetization of the output transformer? Is this an effect of the electron tube or transformer? An explanation may be as follows. When the cathode is further underheated anode current may drop from 100μA to 40μA. At that point the gain is max when the termination is 10KΩ. But the output impedance of the preamp has dropped from 300Ω to 800Ω. So when it is terminated by 300Ω there is a voltage drop that offsets the increase. More investigation is needed here.
The 2 compared Pleiades transformers wound on Magnetec 070 core are:
Version 1, 350 turns: 35 turns, 16H:140mH
Version 2, 670 turns: 70 turns, 80H:800mH
The Altec was used as reference transformer.
It has to be remembered that on the Pleiades V6 not only the electron tube is operating in class A (electrons flowing all the time) but the output transformer too. The transformer is connected directly to the anode so the anode current passes the through it. Advantages are. Expected lower noise as we have a wire of small resistance at the anode and not a carbon resistor. Also most of the Vb voltage makes it to the anode. Also the primary inductance by induction creates the signal which can be double than the 3.9 rail. So the possible output is comparable to a resistor anode load amp with double the supply voltage. Also there may be more quick overload due to the transformer being already magnetized by DC. It may be a bad or a good thing. It depends on where the quiescent point of the magnetisation is. It may be a similar principle to biasing an active device for best performance. Some DC should be a good thing as the material is already turned on by this magnetically , perhaps more sensitive. The BH curve of the material has to be consulted.
It was imediatelly apparent that the version 2 had the bass that was lacking on version 1.
Setup. Signal path:
(The Hook - Hatzinasios) CD - Sony portable CD player - Pleiades V6 connection to grid through 22nF - Sennheiser HD580
Then the Alan Parson's reference test CD was played.
Then an Ηχος & Hi-Fi magazine test CD with sinewaves.
The VU meter of the Realistic disco mixer was used for measuring objective dB levels.
Signal path, setup:
Test sine CD - Sony portable CD player - Pleiades V6 - realistic disco meter - Sennheiser HD580
Note that the input impedance of the Realistic disco mixer is 10KΩ and a broadband increase in mid range is expected as a high load results is some bass and treble frequency responce decrease.
With such a load the Pleiades version 2 output transformer had a -3dB point at 120 Hz which is comparable to the Altec responce. The -3dB point at the high end was 14KHz a bit more that the Altec but there may had been a systematic error long time since the Altec was measured.
The -3 dB point of Pleiades version 1 transformer was 315Hz .
So the version 1 seems suitable for even the low male voice and will be used in the Pleiades V6 prepreamp prototype
The 315Hz 3 dB point is great too if one wants to compensate for the proximity effect by using the output transformer. But it seems more fit to do any compensation if needed (depending on if a mic is directional or not (omni)) using the input transformer primary inductance.
A microphone that will be connected to the Pleiades V6 is the Sennheiser MD211. So the output transformer does not need and perhaps is undesirable to have a reduced low frequency responce.
More measurments may be posted here or in a nearby future euroelectron post:
It was interesting listening to the sound immediately increase when the heater was disconnected. This was when using the version 1 output transformer while listening to music.
This instant effect did not for some reason happen on the version 2 output transformer.
However when doing the frequency responce tests, firstly the version 1 was tried. The fader was adjusted for 0 VU at 1 KHz. After the tests with the version 1 output transformer the heater was disconnected waiting for the electron tube to cool down and the sound to fade out.
Then version 2 was plugged in. As the electron tube heater was warming up the needle passed by 0dB and reached a max of +5dB and then started decreasing again until,it settled for +.5 dB. The fader was moved down so that the needle shows 0 VU. And then it was time to play with temperatures.
As the heater cable was removed it was a joy to see the needle moving to the right again reading +4dB and then moving continuously towards to the left direction until there was no sound.
It is evident that the heater temperature in this circuit of very low Va and 8MΩ anode to grid accelerating resistor the electron tube will have a greater gain at even less than 3.9V at the heaters.
When the V6 hardwired prototype is built the maximum gain will be found by trying to reach the maximum point using a series heater rheostat before connecting a fixed resistor.
Another parameter that can be played with is the value of the anode to grid accelerating resistor. This may be made variable too for experimenting in finding the maximum gain conditions in real conditions ie in practice.
A microphone has not been tested yet with the version 2 output transformer.
References:
On preserving transconductance of electron tubes at a low anode potential - euroelectron blogspot
Operating features of the Audion - E.H Aemostring
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