On Feb 26, 8:54*pm, Patrick Turner wrote:
For those wondering what model Ming Da amps I modified, it was those
shown at
http://www.mei-xing.com/English/cp/2_300845-AB.html
But the tubes shown above were not the same as I found in my sample of
these amps which may be a few years older. The 845 were Shuguang type
845A, 300B were non balloon bottle and 6SN7 were all CBS made in USA.
The data on the Meixing site says they make 75W and have 4 and 6 ohm
load match outlets.
I found PO at clipping at about 85W on the original amps, but found
the matches for loads to be ideal when you use 8 ohms on the "4 ohm"
outlet, 23ohms on the "8 ohm" outlet. Using 6 ohms on the "8 ohm"
outlet woud destroy output tubes.
The only way 4 ohms can safely be used is when plugged between 4 and 8
terminals.
Much better performance can be attained by improving the absurd driver
amp for the 845 which uses 300B with only 5mA for Ia and in SE mode.
I have a set of schematics in neat .gif form for anyone interested.
The work of improving each monbloc involved removing about 30 R&C
parts, and replacing with 25 but with better and simpler circuit.
The Ming Da uses point to point with some strip circuit boards with
brass terminals. These are actually brass stand off spacers but used
as solder points. They all have a nut on a thread to hold them in the
strip boards and many have a lug under the nut to which wires around
the circuit are soldered. Therefore many connections are via solder
lug and bolted connections which of course may one day come loose or
corrode. So I removed all the lugged and bolted connections and made
sure ALL connections are soldered only. Ming Da should know better
than to create amps which have such appalling defects. I found that
adjusting the bias on the 4 bias pots meant you must have the amp
sitting upside down on a block of wood under the power transformers to
stop the weight pressing on the tubes. Then one has to probe around he
ciircuit while twiddling bias pots. The 300B have a shared cathode
heating supply so adjusting bias for the 300B is impossible, and just
total ****ing bull****. So bias adjustment is a completely difficult
and bamboozling process likely to lead owners to breaking tubes or
electrocution. I have never seen worse. Soldering quality is poor with
many parts hanging around in mid air. Nowhere near enough thought was
ever put into parts placement within the chassis. There is no choke
filtering in the anode supply for 845 tubes, just CRC with 117uF, 50
ohms, 117uF. However, there is enough class A power and it is
adequate. Fanatics will find there is plenty of room to place a 5H
choke to replace the R in the CRC filter. Another major improvement is
to arrange the 300B as an LTP and have a CT choke to get dc to the
300B anodes and so there is at least 20mA for Ia which would then have
the 300B working better to make a more detailed accurate sound.
When one looks at the underchassis area of a Ming Da, one is impressed
by the neat tidy wirig but frankly, looks are deceiving because what
they have there is a ****ing big mess.
Its the same with so many Chinese made amps. I rewired a pair of
Quad-40 also recently.
They never quite go the full distance to real quality.
Patrick Turner.
China did produce high quality products years ago. They
were not available outside of the communist countries block,though.
Once again,it was many years ago during Mao period. The junk with
which it has flooded the entire world certainly doesn't create a
positive image for China as an advanced modern civilization...