Ming Da MC34-B bias and listen

[Richard]

Jack came over Tues and we had a wonderful time playing with the Ming and listening to music.

He brought a new set of Svet 6L6GC and the original Sovtek 5881s in the amp. We checked and matched them into pairs before fitting and firing up to bias. The Svets were a new Quad from Watford and were truly well matched with only 2mA between any of them. I'll buy my next ones there

No data for the Ming so we started from scratch.

Mains was 237V as usual here and HT at the anode was 418V. The Ming has a 10R resistor pin 8 to earth bus on each valve so we checked voltage across it and related it to mA; I=V/R. (Easy to leave the meter clipped on and read, say, .52V as .052A or 52mA as the pot is turned.)

As checked it was 52, 52, 55, 60. So the factory setting was somewhere around there(!) Multiplying this by 418 and working within the 30W plate dis allowed we decided a range of 50mA to 70mA was realistic. (.07A x 418V = 29W) We settled on 60mA with the Svets in place which is 25W plate dis and would allow any reasonable quality 6L6 type.

Checked the amp into 8R,

Sine was very clean and symetrical and clipped cleanly with no crossover distortion visible at all. SW was very good indeed.

Both channels checked almost exactly for sensitivity and output,

20KHz 23W
1KHz 24W
30Hz 21W
20Hz 9W

Response ref 1W 1KHz was -1dB 10Hz to 30KHz (with -3dB at 70KHz)

This is pretty good stuff though it may be presented differently in advertising;

It's fair to say response is -1dB 10Hz to 30KHz which shows the circuit is capable and the amp will work well at lower powers.

An indication perhaps of the power supply and/or transformers though is the power bandwidth which is 9W over the usual hifi 20Hz to 20KHz.

It's more likely the amp will be presented simply as a 20W, or even 24W, amp based on it's 1KHz or 30Hz to 20KHz performance.

This is important to analyse, as performance at low frequencies is expensive and difficult to achieve. £30 transformers may give this sort of performance but extending the power down to 20Hz may well need £130 ones.

All in all a very well built amp which performed well on the bench for its price.

A listen in my system which needs a few watts to get going showed a very similar "voice" to my amps. Quite remarkable how similar on simple stuff and sounded "right" to me. With denser rock the differences were more apparent.

We wondered how to upgrade it. Given the transformers I really only see a couple of areas. The pot is an Alps Blue so I'd try the resistor mod. The HT and bias bridges could be remade with schottkies. I didn't pay much attention at the time but the signal input wiring, I think, was unscreened single core run together and may be causing some muddling of the LR soundstage. The coupling caps looked like plastic and sounded like polyester; they are own brand so I don't know but would leave them. All the other caps are branded Rubycons and the resistors look like metal films. There is no noise at all.

Really it's a very "complete" amp with nothing wasted. It may need to be monoblocced with bigger TXs to get a lot more performance but that would defeat its objective which is great sound for not a lot of money.

Now who should buy one? Well Jack will have a lot of good sound from it but won't learn a lot so it depends on what you want to do. Learning to solder a kit amp and trying different flavours of components would be better for someone who wanted to learn electronics but that's not all of us is it.

Rich