This is an electron tube prepreamp where both direct heater and anode are supplied by just a 1.2V AAA battery.
It is important for safety to use a fuse whenever we use batteries.
The electron tube is a CV2269 electrometer tube. The idea of experimenting with electrometer tubes came from a discussion with disc cutting lathe specialist Sean Davies.
The battery is connected to the two terminals of the filament cathode.
At the negative terminal of the AAA battery holder nothing else is connected.
At the positive terminal 3 components are connected:
One side of anode resistor RL=27KΩ, (it's other side to anode).
The 50MΩ grid electron accelerating resistor, (it's other side is connected to grid and one terminal of the secondary, pin 5 of the Freed input transformer). The transformer is the Freed TF1RX10YY SC-B-105074, Freed no.33116. 150Ω or 600Ω to 330KΩ. Inductances ratio 40mH to 250H.
The positive terminal is also connected to the XLR chassis pin.
Now the other terminal of the input transformer secondary, pin 6, is connected through a 39nF capacitor to the chassis pin. Pin 7 which is the transformer chassis was not connected. The 2 low impedance primary windings were connected in parallel. A very low inductor of 4.7mH was also connected in parallel to get as much treble from the mic as possible. This Pleiades filter may be too much for most applications, other values may be tried. The military input transformer with a very high (1:60?) turn ratio was used. It tended to sound much more bassy that its input inductance would suggest and this partly explains the use of the very low inductance Pleiades filter.
A 1μF coupling capacitor is connected to the anode. It's other side to the female RCA output connector.
The chassis Neutrik female XLR connector pin is also connected to pin 1 following the great advice from the Jensen transformers website.
The V1 sounds nice and natural. It was original made to connect an MD421 mic, modern type, to the Tascam porta 01 or 02 at their high impedance mic inputs.
The song Tezcatlipoca on YouTube renewablemusic channel was produced. The MD421 used may had voice coil issues as now when used it is very thin the first seconds it is used and then the bass comes in as it seems the voice coil unjams itself from the magnetic air gap. The singer was using an acoustic cavity near her voice and mic to produce the reasonant mid frequency effects. The make singer was using no acoustic cavity.
The prepreamp could be more hiss quiet. And there is a lot of microphonics from the electron type. Once gently taped it can produce a nice sounding high pitch sine wave that lasts for more than 10 seconds. Obviously this is an electrometer tube and for technical reasons the filament is supported in the most delicate way.
More experiments are needed with other grid accelerating resistors, anode resistors or output transformer. Experiments so far have not proved fruitful. Other specimens of CV2269 have given much lower anode current.
If memory is correct the V1 operates at an anode current of only a few μA.
It is a nice sounding electron tube prepreamp that is made in a very snell box and operates with only an AAA 1.2V battery for a long time.
When overdriving it with some other Pleiades V series prepreamp the sound was interesting and if memory is correct there was the negative bias in reading effect giving automatic gain control as on the VF14 on Neumann U47 at high levels. See also nearby and other euroelectron posts.
It was a revelation connecting the grid return resistor by chance from the negative terminal of the filament to the positive and watch the anode current increasing 3x. Then by chance the anode supply battery was removed and when it's place was short circuited the amplifier started to operate! This is how the idea came of not using a anode supply battery and leaving all voltage supply to the 1.2V filament battery.
No comments:
Post a Comment