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845 amplifier with only two stages
started on September, 2005
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.



This amplifier is based on the idea to create an 845 amplifier with only two stages.
An amplifier with only two stages means less components on the signal path so less sound alterations.
This amplifier to be a true two stages should be driven directly by a pre-phono and CD player without any pre-ampl.
It have enough power (near to 25w) and low output impedance (near to 1ohm) to drive any kind of loudspeakers like  full range, low efficiency with 2 or 3 ways and also any electrostatics.
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.


The 845 vacuum tube have low amplification factor and it need high impedance transformers to get a good damping factor.
It is obvious that an input step-up transformer 1:4 or 1:8 can resolve the voltage gain problem but in this case this amplifier should be driven by very low output impedance source near to 300-600ohm because the input capacity of the grid is multiplied two times for the turns ratio so 10pF will become 10pF * 8 * 8 = 640pF.
An impedance near the 300-600 ohm is not common in the hi-end sources like vacuum tube DAC and pre-phono.
Use an inter stage transformer with a high turn ratio like a 1:4 is not possible because it mean drive the grid of the 845 with a very little current and high impedance.
The output should be driver with a medium current also if used in A1 class operation because these will start to get grid current when are near to low grid values.
Now it is obvious that we need a not common vacuum tube to do this project because it is necessary:

  • very high voltage gain (u) > 100
  • the lowest internal resistance Ra < 5000
  • bias current about 20-50mA
  • low distortion high voltage swing > 100Vrms

The main idea to use the 6HV5A to drive a 845 is of ANSA, he maintan an italian forum and a website on
The 6HV5A is a valid choice, probably the unique,  but this vacuum tube work with low distortion only with plate voltage of min. 900v so it is obvious that  is not possible a resistance load because in this case are necessary power supply voltage very too high near to 2000v.
The unique valid choices for load are the anode choke or the inter stage transformer.
With the internal resistance of 6HV5A near to 3500-4500ohm it is necessary a choke or an inter stage transformer with an inductance value no less than 100H.
Flow(-3db) = R / (2 * 3.14 * L) = 4600 / (2 * 3.14 * 100) = 7Hz
It is difficult to find an inter stage transformer with 100H so the choke is a more simple way to resolve the problem.
Using the choke is necessary the inter stage capacitor and this  influence the sonic performances.
Actually I am using MKP caps. because is not possible find Jensen copper film paper in oil with 2.2uF 1200v.
It is necessary change the input stepper attenuator to 10K because otherwise the frequency response will be limited due to the input capacitor of 6HV5A.
This mean that the input impedance of this amplifier will be 10K.
There is an opened forum about this project on: diyaudio.




The driver stage use a cathode bias so it is necessary a very good audio capacitor in parallel to the cathode resistor 100-150ohm.
The value of this capacitor is the result of many test because its influences the low frequency response (lower value can create a positive peak in the low frequency area for a resonance).
I have used 2 x 220uF 6.3v Sanyo OS-Con (no more available) in parallel to have the 440uF value but you can insert a single 390 or 470 uF.
A valid alternative to the OS-Con are the old Black-Gate N/NX/STD or new ELNA Silmic II or the new Aluminium Organic Polymer Capacitors.

The version1 need a good inter stage capacitor and it must support a
minimum of 1200Vdc.
Until now I have identified only one components valid for this purpose the: 2.2uF 1200Dc 
M-CAP® Supreme Silver / Gold & Oil by Mundorf.
In the schematic of the version 1 I have reduced the power supply voltage to 1000v to be in a secure area for these inter stage capacitors but it can be increased to 1100v if you find a component with higher voltage (Va + Vg = 1100 + 160 = 1260V).

About the power supply capacitors I suggest the Jensen electrolytic with a very compact size and a good sound.

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.

The two power supply transformers should be custom made and these could be toroidal, double c-core or r-core type but in any case it is very important to keep the flux low, 20% less than normal (14000 instead of 17000) and 2
mm˛ per ampere.
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 (more than 92db).


Bartolucci 115BN
        Turn ratio: 11K5 / 8ohm
        Size: 135 x 135 x 135 mm

Bartolucci S.65N5
        Turn ratio: 2:1
        Primary Inductance=100H
Size: 110 x 110 x 125 mm
        Special design for high voltage operation


Sowter  8982s
        DC=50mA max


I have used in some measurements the Clio system by Audiomatica with the Transit by M-Audio to use the USB port of my portable pc (instead of the PCI audio board).

The max power is the measurement result increasing the output level until the distortion spectrum decay is perfect but this amplifier will support larger peaks in less linear area.

Follows the distortion spectrum and frequency response at 13Vrms on 8ohm load so 21W with an input of only 0.67Vrms.


The max power is the measurement result increasing the output level until the distortion spectrum decay is perfect but this amplifier will support larger peaks in less linear area.

Follows the distortion spectrum and frequency response at 13Vrms on 8ohm load so 21W with an input of only 1.228Vrms.

Follows the incredible performances of the driver stage composed by the 6HV5A and 2:1 Bartolucci interstage (Vin=1.228Vrms).



The schematic is like the version 2 but the Rk on 6HV5A is 220ohm and the power supply voltage is 1200Vdc.




voltage current power real current note
900 0.4 360 0.1  845
100 0.4 40 0.1  845 vacuum diodes
50 0.4 40 0.1  845 ext. vacuum diodes
170 0.2 34  0.02  bias voltage base for GM70-813
50 0.2 10 0.02
 D3A/5842 extension for GZ34  
  504  total    
11 5 55 3.3   845-GM70-813
6.3 5 31.5 3.6   6HV5A
5 3 15 2   GZ34
5 3 15 2   GZ34
5 3 15 2   GZ34
5 3 15 2   GZ34
  147  total

To get zero noise on any kind of loudspeakers has been used a DC filament for the 845.