• 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.

Hi-end 845 with only two stage

yesterday evening I have started some measurement about 6HV5A used like 845 driver.

The schematic was very simple: inductance on anode, Rk = 100ohm, Ck = 470uF, Rg = 75Kohm and output capacitor 2.2uF on 33 or 100Kohm load.

With medium voltage operation (about 600V) these tubes give low sperformances because distortion is high 1.1% at 25Vrms.

The good result are obtain using 980V and 100ohm on catode to keep the power nead the 30w (35w is the maximun allowed), bias about 30mA.

With these value I have got the good 1% at 120Vrms on 33Kohm load.

The freq. response is limited to 50Hz(-3db) with 20H but it will be 10Hz(-3db) using 100H 50mA Sowter 8982.

The voltage amplification is 120/0.7=155x.

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Finally the first test result using 6HV5A to drive 845

6HV5A loaded by 20H (Rdc=70ohm) and Rk=100ohm Ck=220uF
950V 28mA

845 loades by Tango X-10S 10K/8ohm Vg=150v Rg=22Kohm
950V 100mA

10Vrms on 8ohm with 2% thd

Interstage 2.2uF // 0.33uF Audyn-Cap Plus

Very good sound

Very good voltage gain

Power can be increased using 1100V instead of 950v

Soon next test and photos
Hi Andrea,

I am right now in the process of building your 'New 845' with the EL34/845 combo.

This one looks a lot simpler. How does it compare? Can it be driven direct from a CD player

I am just about to finalise the power transformer order. Should I hold off? I would love to see a schematic of your power supply.


This amplifier is in develop phase so some changes will be necessary to ear the final result.

In a first comparation test yesterady evening I like very much the sound of this new amp.

The my 845 amp. use Jensen copper film paper in oil interstage and this new two stage 845 use a MKP cap so a real comparation is not possible.

The my 845 amp. use Valve Art 845 and this new two stage use 845 Shuguang.

So a real comparation at this time is not possible but I will do this in the next weeks.
Interesting design Andrea, I'm working a two stage 211 based amplifier using the 5842/417A. Like yours also fixed bias and a 100H magnequest choke (exo-001) to load the 5842.. I have a pair of GE VT4C that will go in this amplifier.

Have not yet decided on the OPT..

Supplies per my usual practice will probably be regulated - might reconsider that if I end up using a parafeed configuration on the output stage.
audiodesign said:
In not normal use a 5842 to drive the 845 because the 5842 have too low voltage swing and too less voltage gain

Quite well aware of that, but I have 211/VT4C tubes in hand which have much higher mu (11) than the 845.. I was just commenting on the similarities, not suggesting you (or anyone to be clear) even consider the 5842 for this application.

I think even the D3A would be pretty marginal driving the 845, at least in triode connection even with a 1:2 step up IT.

I'll definitely check out what I can get the Lundahl amorphous core opt for in this country - it might be an excellent choice for my amplifier. :D