"We are trying for something that's already found us"
Jim Morrison
This was found sleeping in a drawer since around 2000.
A mic prepreamp with just a 7025 electron tube (1 section so just one triode). One input transformer, one output transformer, one resistor, 2 battery holders, and that's it. All the 3, tube and transformers sticking out at the top of the box.
It sounds nice. As if you are about to take off. Very quiet too although not as quiet as Pleiades V0 but when an ECC82 was inserted it become as quiet but with more bass due to the ECC82's lower output impedance the shunt by the primary inductance of the output transformer having less effect.
How does it work or what is the circuit?
It is perhaps the most conventional circuit.
The Neutrik female chassis XLR pin 1 is connected to chassis.
Pin 2 and 3 are connected to the primary of a Sowter 4935f input transformer.
The secondary is connected to (-) battery and it's other side directly to grid. It was forgotten to connect (-) to chassis but the prepreamp still operates with no noise issue.
Just a cathode resistor from cathode to (-) of 470Ω.
The output transformer is a military Stancor 9A1 38, B-454831, 20KΩ to 600Ω. One side of the hi Z primary is connected to plate or anode and the other side to battery (+). The secondary is connected to pins 2 and 3 of a Neutrik male chassis connector with its pin 1 connected to the chassis pin.
How beautiful.
It is self contained and everything is built in an aluminum diecast box of 4.5in x (3+3/4)in x (1+1/4)in. The was not covered with for example shellac and was just polished but when it is rubbed by a tissue it leaves grey residue.
Power supply is by two 9 volt ordinary 6LR61 batteries. The batteries are independent.
One battery supplies the heater. A 7805 regulator is actually used to keep the electron tube supplied at 5V.
The other battery is the anode supply of 9 volts.
A double battery holder bought from Canford UK is fitted on a big rectangular hole on one side of the chassis box, (the other short side contains the Neutrik XLRs. There are 2 removable drawers one for each battery. These battery holders are possibly made by Bulgin.
The beauty is that the anode battery drawer can be fully inserted until the drawer snap closes and keeping it as is all the time.
The heater battery drawer controls the on or off operation mode. Once the battery drawer is snapped in, the tube cathode gradually warms up and this gives the most smooth turn on operation just like an AKG C12a condenser mic for example. Same for turning off. The is no click sound even when connecting to a preamplifier and having the gain full up.
The idea is that when one of the batteries, the heater one is exhausted, it is possible to continue, the batteries are swaped so the weak one will have no problem at supplying the so small anode current, both batteries giving a second chance.
This Pleiades V(-1) prepreamp was almost made up by chance while trying to find a way to compensate for the so much bass a mic gives if it's is not proximity, Fletcher-Munson and voice effort compensated. The 7025 was found by chance to operate better than other tubes that were around, such as ECC83?.
Under heating was found by chance to improve the sound, hence the 5V heater supply.
Fuses should had been included for each battery holder.
Signal path, setup:
Kitchen room with tiles half way up - male softly singing voice - Beyer chrome M55 500Ω at 1 in - Pleiades V(-1) - Realistic disco mixer - Sennheiser HD580
The sound is very nice with mid presence reasonance as the military transformer is almost unterminated by the higher impedance of the transistor realistic disco mixer input. The sound is very loud.
When a 0 ohms XLR male is connected the hiss almost disappears but there is some that can still be heard.
By taking the 7025 out and connecting it to Pleiades V0 and the same mic with a Canford 400mH input transformer the sound gets thin as there is now a second low cut at the input due to the lower inductance compared to the Sowter. The Pleiades V0 has a biasing resistor of 10MΩ from anode to grid. With its ECC82 substituted back it sounded great with full sound and bass. This seems to be due to the lower impedance of the ECC82 tube letting the similar Admiral output transformer to attenuate the bass less.
Then the 4th permutation. The ECC82 was connected to the Pleiades V(-1). The bass now was excessive and unnatural. This is because we have no low cut on the Sowter trasformer and much less low cut at the output too due to the lower impedance of the electron tube.
So the Pleiades V(-1) pre pre amplifier will be kept as it was freshly designed by almost chance or serendipity with its 7025 electron tube.
Comparing the 2 Pleiades amplifiers with the same conditions, mic, etc, they both sound almost correct from vocal chords to listener's brain. The Pleiades V0 sounds more flat and full. The Pleiades V(-1) sounds more peaky in the midrange. And this is what gives its presence character. The peak Q may be adjusted by a variable resistor across the transformer output. To summarize the peaky effect seems to be coming from the tube high output impedance driving the military primary inductance. Or it may be damped when the Pleiades V(-1) prepreamp is connected to a low impedance preamplifier, one transformer driving the other transformer as when connecting a Neumann U47 microphone with its VF14 prepreamp to a preamp with an input transformer.
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