Chinese 805 SET monoblock

[ja2dhc]

I ordered a pair of 805 SET monoblock to China. They completed to
build the amps for me as a new model and they sent photos and
schematics which are indicated in my Web below.

http://ja1cty.at.webry.info/200910/article_4.html
http://ja1cty.servehttp.com/805/T10B-3.gif

The schematic employees unique approaches as follows:

*Cathode Choke Driver for 805
*Pentode first stage
*All heaters/Filaments are driven by DC

Their Web(Chinese only) : http://www.bezdz.com/index.htm

I am looking forward to arriving the amps to my home.

[kavermei]

Hi,

that's an interesting use of rectifier bridges.

What is the story behind the choke in the cathode of the 2A3?

[TugaTweaker]

805 tube sounds really good. I diyed Nobu Shishido's 805 set circuit , with the Tango's interstage to ensure a close aproach . EFL Japan has everything .
Good luck .

[ja2dhc]

Originally Posted by kavermei

What is the story behind the choke in the cathode of the 2A3?

Thanks for reading my post.

The attached photo is obtained from TINA simulator to indicate
the grid swing of 805 when the amp outputs 40W.

The minus peak of the signal reaches to -44V although this amp
does not have minus voltage power supply. This is caused by
an inductive load of 2A3.

[kavermei]

Oh, I see it now. Interesting design...
I think, in that case, everything depends on the quality of the choke.

The screenshot from the simulator shows some distortion (the negative peaks are narrower than the positive peaks), do you think this is caused by the 2A3 or by the choke?

[ja2dhc]

Originally Posted by kavermei

The screenshot from the simulator shows some distortion (the negative peaks are narrower than the positive peaks), do you think this is caused by the 2A3 or by the choke?

I don't think this distortion is caused by the choke. This distortion
is mainly generated by the first tube of Chinese 6AC7 pentode.
The low output impedance of 2A3 cathode follower mitigates the
characteristics of the choke. In other words, a cheap choke can be
used in this topology. This is a secret point of this design.

The attached photo shows output waveform when 40W output.
The distortion of the grid as you pointed out is canceled by 805's
distortion and looks symmetrical. This is another secret point of
this deign.

Overall NFB : 3dB.
THD: 8% @ 40W
Dumping Factor : 1.2

[kavermei]

Looks good... enjoy your new amplifiers!

[sorenj07]

Wow, that's a pretty snazzy design. I'm sure it could always be improved, but still. Maybe a slightly different pentode frontend, if it's causing all this distortion?

[vancao]

Front-end penthode may not provide enough gain. In my case, a double triode (12AU7) gets more powerful punch. I'll try the choke to see how the grid will swing. Enjoy great orders.

[ja2dhc]

Originally Posted by vancao

Front-end penthode may not provide enough gain.

Hi, You are right. The sensitivity of the total amp gain which I
simulated by TINA is low as follows:

Input : 1Vrms Output : 1.7W

However the schematic says 2Vrms input > 25W output. I suspect
this difference is caused by wrong value of the load resistor
of 6J4P (Chinese 6AC7) in the schematic. I will check the value
when the amp will arrive at my home.