Sennheiser MD211 N with Pleiades V6 (Nuvistor 7586)

A 7586 electron tube is connected to the Pleiades V6 head amp instead of the EF183 for those new experiments.


Firstly the reference Grampian DP4/L was tested with the Pleiades 1:10 transformer in the octal base of the Pleiades experimental V6 jig. The sound was very nice but from memory the EF183 suits the Grampian better as it gives more subjective bandwidth at the particular conditions of 3.9V for anode and heater circuit. There is a few ohm resistor in the heater circuit anyway for further reducing the cathode temperature.


This is the schematic of the Pleiades V6 used, the actual coupling capacitor is 22nF, 6MΩ anode biasing or Pleiades bias resistor and the Nuvistor 7586 instead of EF183:




For connecting the MD211 the input transformer was changed to the Altec Pearless 4722.


A Pleiades filter of 41mH (in fact 82mH//85mH, 2 XLR adapters one after the other) in series with 140Ω is connected across the output of the mic to compensate for voice effort, equal loudness curves etc to listener's brain. It reduces unnatural bass with a gentle slope, ie low cut of much less than 6dB per octave below a high, 700Hz? frequency. See previous posts.


Signal path, setup:


Male speech level singing voice at 2in-6in - Sennheiser MD211 N - Pleiades filter (140Ω,41mH) - Pleiades V6 with Altec 4722 and Nuvistor 7586 - Sony TC-D5 Pro - Sennheiser HD580


The sound was big and natural with great signal to noise ratio.


The gain was high enough making the MD211 at least as sensitive to a condenser mic with its head amplifier. So the Sony TC-D5 Pro level control for full modulation was at around 5-7 out of 10. When using the EF183 it normally needs to be at only 3 which is not necessarily a good thing as it increases a chance of overload of an amplifying stage on the TC-D5.


Then after disconnecting the mic, the hiss noise increased, and then 0Ω XLR male link was connected to the input. Most mid range hiss disappeared but a high frequency hiss was left. Perhaps due to a systematic error, a pin of the female octal socket at the low Z (mic) side of the input transfomers was broken and the experiments had to be continued with a crocodile jumper cable connecting one of the mic terminals.


Then the input transformer was removed. Hiss was observed and when a short jumper link was connected between ground and input capacitor to grid, the noise almost disappeared in an impressive way. This test was done to have a quick evaluation of the Nuvistor 7586 in this circuit.


The anode current is only 20μA and the electron tube sings. It is impressive that before connecting the mic there is some hiss and when it is connected a big sound comes from the headphones. Full ambience can be heard.


At some point the Pleiades bias resistor was reduced to nearly 2MΩ from 6MΩ. The anode current jumped to 60μA. The sound was not better. In fact it was more tight in a bad sense. Like the sound of a transistor amplifier of low output impedance driving a full range speaker as opposed to a higher output impedance transistor or electron tube amplifier giving a more easy, liquid and full sound. (See Nelson Pass first Watt amplifier and Pleiades 2N3053 1 transistor 30mW power amplifier). Neither noise performance was better. When a 2MΩ Pleiades resistor is connected the input impedance of the amplifier must be recused much less than 100KΩ possibly loading the transformer and mic a lot.


So a low anode current of 20μA gave great results including noise performance.