Critique for circuit of great value 805 SET?

[protos]

Since there was some interest in the amazing value of hi-end chinese amps I requested the schematics to get an idea of what they are doing.This is the one for the Ming-da 3008-AB which you can see on www.cattylink.com which is an integrated with 300B/805 available with shipping to Europe for less than 1000 Euro.

[hifi]

it costs 1163usd + shipping

/ Mikael

[protos]

Sorry for the quality maybe this is better:

[protos]

It's about 10 JKD to the euro.

[giribaldo]

hi all,
i'm new in this forum, and i see there are a lot interesting discussion here, so i would like to ask someone for a problem...
in the true is not a problem now but it will be for sure in future,
i have a meixing 3008ab is the one with the 805 drived by the 300b.
i'm very happy with this amp i buyed at not high price i wanted to hear how 805 sound and i must to say that this amp sound really really good with my rf7 klipsch, a beautifull bass and high and also very clear mid, for this at this time is 1 of my favorite amps.
The problem is come when i tried to change the original 300b with a new set of electro harmonix... i thought this baby was self bias or something like this, but now i think i was wrong!!!
i changed the tubes turn on the amp and immediately i hear some noise, but i tought was for new tubes, after sometime i look the left 805 become red (the graphite become red) and noise was more...
i turned off immediately the amp, changed the 300b with the original and all was good again.
so i look at the tubes and the original was 47 and 65 mA, the eHarmonix was 69 and 69 mA, this because the electro Harmonix was matched, i think the original was not (47 and 65) i think is for this that only the left 805 become red...
Now the problem is:
i seen under the chassi two trimmer, they are connected to the 300b, i think i will need to make a bias with this trimmer to use two new 300b with different rating from original,
But there is no Manual and no data with the amp to make this operation, and also i don't have the schematic (the first i see is here in this post),
Someone have some idea how to proced to bias this amplifier?
thanks all
Giribaldo

[analog_sa]

Someone have some idea how to proced to bias this amplifier?

As you seem to have discovered already it uses fixed bias. The 300B is the one you bias and the dc coupling ensures the 805 is properly biased too. I haven't a clue of bias points for the 805 and can't read what the diagram says but measuring the original bias accross the 805 cathode resistor and aiming to replicate it seems a good idea.

The biasing network baffles me a bit. It seems derived between the negative voltage at the 300B cathode and a small positive voltage at the srpp cathode. Dunno if i'm seeing right anyway.