After a first series of listen test with some my friends I can assert: the GM70 copper plate is valid like the 845.
Sure I have tested also the normal GM70 graphite but in this case the sound is a little bit metallic and not very pleasant near to the same result get using the Svetlana SV811-3 and the SV572-3.
The main advantages of the GM70 is the higher  amplification factor (8 instead of 5).

Using the 845 it is necessary use a strong driver stage with a very high voltage amplification, low output impedance and it should be able to supply a high current.
In the case of 845 more than two stages are necessary to get a good result because medium power tubes have low amplification factor.
Another tube easy to driver like the GM70 is the 211 but it have a high plate resistance so it is necessary use very high impedance transformers near to 20Kohm to get a good damping factor.

To create a GM70 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 it is possible build a pure single ended two stages amplifier because it have high gain and low internal resistance.
The D3a it is easy to find on the market and the performances in triode connection are fantastic.
In my PF2007, in my new cheap 2A3 PSE and in my new headphone amplifier  it is possible see some measurements results.
The D3A is a Siemens product and I consider good only the Jan Philips, Siemens, GE and Mullard low power vacuum tubes because these give a very neutral sound (only the Mullard is a little bit smooth if compared to the others).
These same vacuum tubes brands are used in the Audionote kits and in their Hi-end products.
In this amplifier it is possible 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 and we must limit the power supply voltage.
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 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.

The best choice in a two stage amplifier is use an interstage transformer for these  reasons:

• minimize the distortion because the vacuum tube is not loaded by the resistor
• prevent any possible alteration of the sound because any capacitor insert a sonic characteristic
• maximum current transfer because all the bias voltage can be used to drive the final stage

In all my projects I prefer to limit the power but get a lower distortion and lower output impedance so in this project I have chosen  a 10Kohm load for the GM70.
This amplifier have enough power (about 20w) 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 of 18w is the measurement result increasing the output level until the distortion spectrum decay is perfect but  will be supported 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 mm² per ampere.
It is possible use vacuum tube rectifiers but I am not sure these are better because increasing the resistor after the diodes bridge it is possible get a similar result (a little bit smooth sound).
There are many configurations possible for the high voltage power supply (see some examples) including the Hybrid Graetz Bridge concept after a Fulvio Chiappetta paper published by the Italian Magazine.
He tested and comparated this topology versus a pure tube vacuum state power supply design with a 300B output tube amplifier and there was no difference both while listening and measuring the output noise spectrum.
Using vacuum tube rectifiers it is necessary decrease the input capacitors (after diodes) to decrease the current peak so I suggest to use 3 x 100uF instead of 3 x 220uF.
If you think to try vacuum tube rectifiers in the future remember to add a pair of secondaries at 50v and 100v to add in series to the 900v  to compensate the voltage lost on the tube rectifiers.

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 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.
I have used the free software PSU Designer II to simulate the dc filaments power supply to check two different choke:

• Hammond 159ZJ with 10mH and 0.16ohm
• Batolucci  S17 with 0.1H (100mH) 0.64ohm

In the first case we get 45mVpp of noise and in the second case with 0.1H we have 5mVpp.
It is possible calculate the max output noise generated by this filament power supply with:   0.045 * 8 / sqrt(10000 / 8) = 11mVpp (-48dB)
The anodic power supply simulated give a ripple of 80mVpp so on output: 0.080 / sqrt(10000 / 8) = 2.3mVpp (-62dB)
Using high efficiency loudspeakers near to 100dB could be necessary use the 0.1H but in the most of conditions the 10mH will be enough.
In the first measurements I have used ac filaments and it is visible the high 100Hz noise near to -25db but using the dc filaments show in the power supply schematic the noise at 100Hz is reduced to -57dB (see here the difference), so this result is better than calculation probably because I have connected the dc with the virtual center tap.
As show in the simulation could be necessary add a 20w resistor (0.22-0.47ohm) before the input capacitor 10000uF to adjust the dc output value to 20v.
To get some contribution and to promote this design I have an opened forum about this GM70 amplifier.

TUBES

	GM70 specifications: filament voltage: 20V filament current: 3A max plate voltage: 1.5kV long-dissipated anode power: 125W max minute-dissipated anode power: 150W Interelectrode capacity: grid-anode 12pF grid-cathode 8pF anode-cathode 4pF height 185mm, diameter 68mm - Plate resistance calculation 2300ohm - Amplification factor 8x - Amplification factor on 10Kohm load 6.7 To create a valid GM70 socket 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: filament voltage: 6.3V filament current: 315mA Max plate voltage:  200v Max plate dissipation: 4.2w Max cathode current: 33mA Plate resistance: 1.9Kohm Amplification factor: : 77 Max resistance in grid circuit: 0.5Mohm	6C45 specifications: filament voltage: 6.3V filament current: 440mA Max plate voltage: 150v <<<<< it is lower than D3a Max plate dissipation: 7.8w Max cathode current: 52mA Plate resistance: 1.2Kohm Amplification factor: 52 ±16 <<<<< selection is necessary Max resistance in grid circuit: 0.15Mohm Inter electrode capacitance: input: 11±2 pF output: 1.9±0.3 pF transfer: 4.5pF

Follows the operating point choiced for the first test.

TRANSFORMES

Follows the comparison test using all Tango, all Bartolucci and a mix

I will start soon a valid sonic performance test with some friends but as first impression the all Tango combination seem to be very smooth and the Bartolucci are more neutral with more details.

	Bartolucci INT8C         Turn ratio: 1:1         Lprim. = 48H         DC=25-30mA         Size: 110 x 110 x 125 mm 115BN         Turn ratio: 11K5 / 8ohm         DC=90mA         Size: 135 x 135 x 135 mm
		Tango X-10S        Turn ratio: 10K / 4,8,16ohm         DC = 70mA NC-20F        Turn ratio: 1:1        Inductance:  70H at 20mA        DC = 30mA        Rprim. = 410ohm        Rsec. = 410ohm        Size: 83 x 78 x 104 mm        Weight: 1.9Kg

The price for the 2 x Tango NC20F is $800 and for 2 x Tango X-10S is $1500 so the total for two channel is about 2300$.

The price for 2 x INT8C + 2 x 115BN is about 900 euro + tax 20% for this version without pins.

The common environment is:

• D3A in triode connection with 18-20mA bias at 170-180v
• GM70 Copper with 80mA 1100v
• input 0.972Vrms
• output about 18Wrms on 8ohm load

ALL TANGO

• thd 1.31% at 1KHz
• thd 1.54% at 100Hz

ALL BARTOLUCCI

• thd 1.25% at 1KHz
• thd 1.39% at 100Hz

MIX TANGO-BARTOLUCCI

• thd 1.34% at 1KHz
• thd 146% at 100Hz

POWER SUPPLY

With the GM70 I suggest dc filaments to skip 50Hz noise (all the previous measurements are with ac filaments).

For the D3A it is possible use ac filaments.

MATERIAL SOURCES

• Bartolucci transformers
• Tango
  in europe contact:  Acoustic Dimension
• Lundahl
• D3A and GM70 copper see on eBay
• DACT and other parts contact Audiokit
   

PHOTOS

I have used 3 diodes in serie to create the high voltage diode bridge (3000V).

Soft start board

Bias board (low power trimmer not used)

CHASSIE