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/ GM70 amplifier
with Bartolucci transfomers
started on June 12 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.
This amplifier without
modifications will allow to use the GM70 and the
813 (FU-13) output tubes.
The 813 is a general-purpose transmitting beam power tube with thoriated-tungsten filament with a maximum plate dissipation of 125 watts.
It is a very linear tube in triode connection with the same amplification factor of the GM70 and a reduced internal resistance so with the same 10Kohm output transformer it is possible to get a better damping factor.
The GM70 and the 813 (FU-13) in a direct comparison with the 845 have the same sound quality but these tubes have the great advantage of a higher voltage gain to allow an simple driver stage.
The GM70 and the 813 with graphite plate are not very good so you should use only copper plate for the GM70 and metal plate for the 813 (FU-13 Shuguang).
I have used a 10Kohm output transformers to get a good damping factor (Rout less than 1.6ohm) and an anode voltage of 1100V to get more than 20w in a pure A1 class operation.
In these last years many DIY are designing amplifiers with transmitter tubes working in A2 class operation.
I never will create an amplifier with these tubes because there are no benefits to use these, only the cost is lower but there is no impact on the total budget because the transformers are the main cost of a vacuum tube amplifier.
Tubes like the 805, 808 and 304TL also if used with strong drivers are less linear of 845, GM70 and 813 so it is necessary use feedback.
External size 50 x 60 cm.
Follows the photo with all these tubes, starting on the
left there is the GM70 with copper plate, 813 RCA and the FU-13 Shuguang with metal
The sockets show has been created by Jakeband, he produce a very good teflon socket with copper terminal at a right price.
813 (FU-13) specifications:
GM70 if you don't like teflon sockets it is possible
convert a 813 ceramic socket increasing the size of only one pin.
Follows the photo of the common vacuum tubes used like driver in power amplifier.
Many of these tubes have a plate resistance
about 2Kohm but only the D3A and 6C45
have an amplification factor greater than 50.
D3A specifications in triode connection:
To create a 813
single ended amplifier with enough sensibility to be driver directly by a CD
player or a phono preamplifier it is necessary have a driver stage with a
voltage gain about 50x or 70x.
Using the pentode D3a in trode connection it is possible build a pure single ended two stages amplifier because it have high gain and low internal resistance.
In this amplifier it is possible to use the Russian 6C45 (instead of D3a) without many changes (disconnect only 2 pin on the socket) but in this case will be necessary 1.5Vrms in input (instead of 1Vrms) to drive this amplifier.
To use the 6C45 it is necessary limit the power supply voltage of the driver reducing the relative secondary of the transformer because this tube have a max anode voltage of 150V.
This driver stage use a cathode bias so it is necessary a very good audio capacitor in parallel to the cathode resistor 100ohm.
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 3 x 220uF 6.3v Sanyo OS-Con (no more available) in parallel to have the 660uF value but you can insert a single 680uF.
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.
This amplifier have enough power (about 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 electrostatic.
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.
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.
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 (about 100db).
The 813 (FU-13) and the GM70 require dc filaments with any kind of loudspeakers due to the higher amplification factor and the trimmer are useless.
Any amplifiers with dc filaments should use a separated transformer for the filaments because using a single transformer the noise generated by the rectification process in input capacitor power supply is amplified to the anodic secondaries.
Follows the operating point choiced on triode curves for the 813, from Frens Mazenier.
Follows the operating point choiced for the GM70.
Turn ratio: 1:1
Lprim. = 48H
Size: 110 x 110 x 125 mm
The price for 2 x INT8C + 2 x 115BN is about 900 euro + tax 20%.
The price for all the kit 2 x INT8C + 2 x 115BN
+ CA422 + CA201 is about 1300 euro + tax 20%.
The environment is:
on 8ohm load with input 0.95Vrms
frequency response at the max power on 8ohm load
The environment is:
on 8ohm load with input 0.972Vrms
I have used 3 diodes in series to create the high voltage diode bridge (3000V).
Bias regulation board
Driver anodic and Filaments power supply