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  <<< also a good source for 813 metal plate

813 / 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 plate.
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:

filament voltage: 10V
filament current: 5A

max plate voltage: 2kV
dissipated anode power: 125W

- Plate resistance calculation 2000ohm
Amplification factor 7.7x
Amplification factor on 10Kohm load 6.6

GM70 specifications:

filament voltage: 20V
filament current: 3A

max plate voltage: 1.5kV
long-dissipated anode power: 125W
height 185mm, diameter 68mm

- Plate resistance calculation 2300ohm
Amplification factor 8x
Amplification factor on 10Kohm load 6.7

For the 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:

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
resistance in grid circuit: 0.15Mohm

Inter electrode capacitance:
input: 11
±2 pF
output: 1.9±0.3 pF
transfer: 4.5pF

To create a 813 or GM70 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

        Turn ratio: 11K5 / 8ohm
        Size: 135 x 135 x 135 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