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]

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]

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]

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.

 

[toufu]

Looks nice. How much did you pay for the pair?

 

[ja2dhc]

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

My amp has been arrived at my home. I described my first impression
into below Blog (sorry in Japanese. please use PC translation into English).

http://ja1cty.at.webry.info/200910/article_9.html

The sonic is more than my expectation.

 

[88man]

Hi ja2dhc, did you ever look into the Ming Da MC-3008-A2 - it's also a an 805 mono block? I am considering buying one.

 

[ja2dhc]

Hi ja2dhc, did you ever look into the Ming Da MC-3008-A2 - it's also a an 805 mono block? I am considering buying one.

Hi, 88man !

I didn't know Ming Da MC-3008-A2 but I visited the Web below to know
what it is.

Pacific Valve & Electric Company Ming Da MC 3008A Mono Blocks

The tubes alignments are as follows:

My amp: 6AC7 > 2A3(or 300B) > 805
Ming Da: 12AX7 > 12AU7 > 300B(or 2A3) > 805

Ming Da uses two stages of triode amplification for voltage
gain while my amp use single pentode (6AC7). I think Ming Da
has more voltage gain than mine.

Maximum output powers is 40W by either of them. 40W is
considerably high output by SET as is given by power drive
into 805 by positive grid driven circuity of 300B/2A3.

 

[sulcorebutia]

Hello ja2dhc, seems to me you have purchased an inexpensive and yet intriguing design 805 monoblocks! You can't beat their price anywhere and glad to know that you like the sonic quality too. Recently, I also purchased a China made 300b intergrated amp with the following config:

ef86 pentode mode -> 12au7 SRPP -> 300b

The sonic quality is so-so and lots of improvement can be achieved. However, I won't complain its quality for this price(less then $400USD) and it allows me to do a lot of modifications

P.S. I will attached the photo later on.