• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

input sensitivity of Bartola GM70SE

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#2
6SF5 (at 250V):
Rp: 66000 Ohm
transconductance: 1500 umhos
Theoretical gain (some resistance paralleled by Rp, so it would be lesser): 66000/1500=44

GM70 grid swing is 220Vpp, about 78VRMS.
With 5% headroom it will be about 82VRMS.
82/44=1.86V RMS
 
#4
1661169313477.png

Here's the circuit.

You could see an "Av=40dB" besides the Gyrator PCB, which has already declared that the 6SF5 stage has an amplification factor of 100.

A "Gyrator" or "Mu Follower" will provide a Constant Current load to a triode, which will maximize the amplification factor of the tube to 100% of the "μ" parameter. The correct "Rp" calculating the amplification factor here should be ∞ Ohms. Using Gm and Rp calculating Av doesn't work here.

Bias of GM70 is -110Volts. Assume GM70 works in Class A1. You need 220Volts Peak to Peak from the Source Follower PCB.

So exactly, the input sensitivity of the circuit is 2.2Volts Peak to Peak, which is about 0.778Vrms sine wave. Slightly adding headrooms is reasonable, since the voltage gain of the Source Follower could be a little bit less than 1.

I think euro forget the Gyrator. 😵
1661170934541.png
 
#8
Referring to Andrea Ciuffoli 2 stages 845 project, he has wired three 500v caps in series to reach 1500v rating.

the amp is operating in 1200v for both driver and output sections

I know a fellow built a power supply for his GM70 and operating in 1200v as well. And he used the Solen 51uf 1500v cap for a decade without any hassle
 
#11
I have acquired a stack of 1000 volt 200 uF MKP caps and put them in series and parallel for my GM70 build. This should last basically forever but stacked electrolytics should also work just fine. Another project to look at for inspiration would be "De Tsaar" from Triodedick/audiocreative.
 

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#12
gideon1990 said:
I have acquired a stack of 1000 volt 200 uF MKP caps and put them in series and parallel for my GM70 build. This should last basically forever but stacked electrolytics should also work just fine. Another project to look at for inspiration would be "De Tsaar" from Triodedick/audiocreative.
Click to expand...
Solen is film cap and should last forever. the first cap would be PIO type

Regarding driver stage would be PIO and ASC X386S
 
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