845 amplifier with Fiat transfomers
started on September  15 ^st , 2009  

BEWARE! This is an expert-only project because the high power supply voltage, about 1KV. Be careful, it can kill you!
Keep always one hand (better if it is the left hand) behind your back.

INTRODUCTION

After 2 years of tests I have develop with Fiat srl and Walter Gentilucci an output transformer with the following specifications:

• good frequency response  18Hz-20KHz at -0.7dB at full power
• very low static resistance (Rdc) near to 80ohm
• same sound quality of Tango and Bartolucci.
• very low cost, about 300 euro  for a pair in the cheap edition.

This project born to test this new output transformers in the best configuration.

It is possible use many tubes as input stage depending by the voltage gain necessary.

The pure single triode 6C45 is the first candidate as alternative to the D3a and no changes are necessary in the schematic.

To use the 6C45 probably is necessary reduce the power supply for the first stage.

UPDATE 2023:

I suggest to use the 6Z9P - 6ZH9P - E180F instead the expensive and rare D3a, see on my last projects.

POWER SUPPLY

TUBES

I suggest this project only to person with some experience with vacuum tubes amplifier because it use very high voltage so it is necessary be care and use only good wire for the internal cabling.
A very good quality stranded tinned copper wire could be buy directly from E-Z-HOOK that carries an extensive line of fine stranded and extra flexible wire.
I have used the model 9506 with WVDC (Working Voltage Direct Current ) of 5000V for all the main power supply connections, the model 9501 for the input signal and the model 9505 for the medium voltage.
All the wires near the tube socket have been cover by Sterling tubes in order to avoid that the high temperature will destroy them.

In the 845 amplifiers it is possible use ac filaments reducing the noise with simple 100ohm 3w trimmer and dc filaments are necessary only with if you have very high efficiency loudspeakers (about 100db).

 

TRANSFORMES

See the introduction for the specifications and here follows a list of cost.

TR main power supply € 130,00
TR toroidal power supply € 60,00
TR output € 300,00 for a pair
Choke 100H 50mA € 140,00 for a pair
Choke 10H 200mA € 70,00 for a pair
Choke 10mH 5A € 30,00 for a pair

MEASUREMENTS

Driver in the test environment.

All the amplifier on 8ohm load in the test environment with D3a input tube and 0.4Vrms signal.

PHOTOS