Hi-end 845 with only two stage

[audiodesign]

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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[audiodesign]

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

 

[audiodesign]

Clio measurement

10Vrms on 8ohm

 

[audiodesign]

The 20H inductance is only for this first test.

I am thinking to change this with LL1668/25mA

See actual frequency response

 

[audiodesign]

the LL1668/25mA
have 100H so about 5Hz (3-db)


Andrea Ciuffoli
www.audiodesignguide.com

 

[audiodesign]

test environment

Soon measurement with Lundahl AM output trasf. series

 

[N1ESE]

Good work so far, watching the development of this.

 

[audiodesign]

This is the first schematic

 

[Gluca]

Andrea

did you try, or will you try, a grid choke for the 845? Things get usually better with them

Ciao
Gianluca

 

[Rob11966]

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.

Cheers,

Rob

 

[audiodesign]

The voltage gain is like my 845 amp.

http://www.audiodesignguide.com/New845/New845v2.html
http://www.audiodesignguide.com/New845/New845v3.html

so it can be driver directly by a phono-pre or any CD player

and it does not need a low impedance source like the amplifier with input transformers so it can be driven with normal sources.

 

[Rob11966]

Thanks Andrea,

So how does it sound compared to your "New 845" and if you could build only 1, which would you choose?

I also noted that you initially had problem with noise due to gain and then added the DC rectification - do you have the same issues with this one?

Rob

 

[audiodesign]

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.

 

[audiodesign]

About grid choke for the 845: never tested, to study

 

[inertial]

Hi ,

Just for curiousity, is it this 6HV5A microphonic ? What happens if you
tapping the bulb, can you hear something from the speaker ?

Cheers,
Paolo

 

[kevinkr]

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]

In not normal use a 5842 to drive the 845 because the 5842 have too low voltage swing and too less voltage gain

 

[audiodesign]

to increase the voltage gain you can use an interstage transf. 1:2 like the LL1660/10mA with primaries in parallel

In this case the D3A is teh best choice (see my measurements on http://www.audiodesignguide.com/PF2007/PF2007a.html)

The main problem using a 1:2 interstage transf. is the low output current.

I like very much the Lundahl with amorphous core as OPT.

 

[kevinkr]

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. 😀

 

[aroS3]

D3a - Gm70 option 2stage option

Andrea, you have the irons, you have most of the circuit needs, why don't you try the promising D3a - Gm70 2stage SE solution, D3a can do the trick also with 1:1 interstage...
Regards,
George