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INTRODUCTION
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.
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TUBES
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:
The main idea to use the 6HV5A to drive a 845 is
of ANSA, he maintan an italian forum and a website on
ansaht.altervista.org
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.
DESIGN OF THE TWO VERSIONS
PASSIVE COMPONENTS
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).
TRANSFORMES AND CHOKE
Bartolucci
115BN
Bartolucci S.65N5 |
|
 Sowter 8982s Inductance=100H DC=50mA max Rdc=310ohm Weight=3.3Kg |
MEASUREMENTS VERSION 1
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.
MEASUREMENTS VERSION 2
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).
MEASUREMENTS VERSION 3 (final)
The schematic is like the version 2 but the Rk on 6HV5A is 220ohm and the power supply voltage is 1200Vdc.
POWER SUPPLY
| 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.
MATERIAL SOURCES
PHOTOS
CHASSIE
