Ming-Da MC34-B

[saratoga]

and how many tubes did you buy?
how much did you spend?
i would rather buy PRIMALUNA INTEGRATED AMP

 

[tailskid]

I have, what I believe to be a rebadged Mingda mc34 b integrated amp, its now called an M-Star. Was listening the other day and it just went dead. No power light, nothing. What gives? Bad power supply maybe? Any feed back would be greatly appreciated.

 

[DanTana]

I would get a voltmeter and start checking the voltages on the PS outputs. Careful I believe a couple are around 500v-550v. The rest I believe are 12v. If those are good then maybe your bridge rectifier might of died.

 

[Inventor]

I did fix the MingDa MC34B amplifier, and now it sounds wonderful. The Amp suffered a severe power supply breakdown, fortunately it did not take the iron with it. The AC input fuse did blow.

The bridge rectifier was found shorted: both AC inputs were shorted to ground AND the DC output.

The large DC filtering capacitor was shorted.

Two ground wires in the signal path were not soldered to the grounding lugs. A very slight left-channel hum cleared up after soldering these.

One of the right channel output tubes had very low gain. The other one blew violently. A new matched quad of 6L6's were installed and biased. The left tube with the best gain (testing with my B&K Precision 747) had a bias resistor burned open.

The bias resistors were very low grade units. The open resistor crumbled easily, evidencing it had run hot before failing. All four were replaced with high grade 25 Watt ceramic wirewounds.

The bias pots were replaced with Bourns units. Bias was carefully equalized. After playing for an hour, bias drifted slightly (I use 4 meters at once) so I re-biased and it settled down nicely. I have 171 hours on those tubes now, with no bias drift.

Every solder joint was heated up, and the slightest touch of Kester 60/40 microwave-grade non-corrosive solder was added.

Thanks for the schematic and the good advice.

Jon

 

[gs1971]

Dan,

I'm wondering if you would be kind enough to talk me through how to set the bias on the Ming-Da? I bought mine secondhand and only got the following photo with it....no instructions. Although I have done a bit of DIY before I have never had a go at a valve amp.

My amp is the same as pictured apart from the cathode resistors that are square ceramic 5w 10ohm.

TIA,

Paul.

I

That's a decent picture to go by. Use your DMM and place the negative on "B", then put the postive on either "A" or "C", then you'll adjust the pot "D" if your probes are on "A" and "B", you'll adjust pot "E" if your probes are on "B" and "C". At first move the pots very slightly so you can get which way increases and decreases the voltage. You didn't say which tubes you were using so I'd use 25mv for 6L6 and 30mv for EL34. Your probes will be on either side of the resistor when your adjusting the voltage, you'll have to go back and forth many times until it steadies. One side effects the other so it will take a good 10-15 minutes probably, maybe longer. First you may want to do a quick check on the voltage to see if any of the tubes are way out of spec, and do those first. Also, I'd give the amp a good 10-15 minutes at least to warm up, so if everything is good, just leave it on until then and recheck. Also, I use a 8 ohm 20 watt resistor across the speaker outputs. You can get those at Radio Shack, that will protect it in case of oscillation.

Hi Dan,
This is A 13-year-late reply from 2018
I have just bought an old MC-34B recent, it's my first tube amplifier but I am enjoy with its performance! I googled a lot about MC-34B and was so excited with this post.
But there are still some photos missing so I couldn't see it here.

Could you show me the diagram or photos on how to bias the voltage and how to wire to triode mode?
Not sure if you are still post here, thanks in advance if anyone found my question and reply.