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Yaqin MC-5881A amplifier improvements

Yaqin MC-5881A

I bought one of these off ebay, and as soon as it comes, I'll trace the circuit, measure the voltages etc. and ask you knowledgeable chums for advice what components to change/improve. Not a major rebuild, just obvious mistakes if any and similar.

Anyone that already has the circuit, perhaps? This thing is also known as Yaqin (or Yaoin) MC-6L6 and Markhill MC-6L6.

MC5881a_mov.gif
 
Does anyone have experience of calming these things down a bit? Mine runs very hot - both transformers and valves get to scorching temperatures.

I don't need 23w per channel and would be happy to drop some output in favour of a cooler office.

I'm also interested in other people's experience of dealing with the hum these things can make. Is it likely to be the chinese valves, the transformers or just the design?

It appears well put together internally but it's a shame I haven't tracked down a circuit for it yet.

Thanks,

Dan
 
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I know nothing about the amp (well, no more than it has Greek letters in its logo:xeye: ), but that is what I would do in your case in order to relax the heat, so to preserve the amp's longevity and even avoid fire hazard if it got forgotten long unattended:

Firstly, see with a multimeter, the AC voltage on the heaters. There is a thick yellow twisted cable pair feeding all the valves as I can see on the linked pics, so probe there. If it is significantly more than 6.3 VAC there is an over-healthy transformer secondary at work when in British mains. It can be tackled with small resistors.

Secondly, see for the resistors that are connected from cathode to ground, one for each output valve. They maybe the gold metal box ones on the pic. I don't see any bias pots so maybe its auto-bias. In any case measure the DC voltage drop on those between cathode (pin8) and GND, and write down their resistance too. Then we can calculate the working bias. If its more than 45mA, better back it off. If there are trim-pots that I cant't see its easy, if its auto-bias, the resistors are going to be in the hundreds of Ohm range, and you can up the resistance by changing cathode resistors.

I post a 5881 octal base guide, so you can easily locate the cathode resistors. If the resistors are 1-10 Ohm there must be trim-pots somewhere.

As for hum, a little bridge rectifier and a couple of caps are going to help if its AC heaters on the small valves too as I suspect.
 

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It can even be a ground loop rooted in the layout or you may need to lift the ground to chassis (if it has one) with a 10 Ohm sandcast resistor. Have a first look so we get a general idea of how the amp is implemented, one step at a time.
 
just a bit of two-cent advice. if there's at all any room in the chassis, PLEASE consider putting a smallish choke into the B+ supply. it's almost shameful to tap the B+ right off the filter capacitor as there are scandalous amounts of PSR spreading to your whole amp. Even a lowly Hamond 159T (2.5H 300mA 43 ohms) ought to make a significant improvement. Splitting B+ and using one choke for each channel might make even more improvement, like two 159Q's at 7H/150mA/100 ohms ought to be very good. In addition, the little bit of voltage drop from the chokes might well help the amp to run cooler. Do you have any pictures of the inside of the amp? How much free space is there inside?
 
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Bingo! Both suspicions confirmed. The heater voltage is devouring your amp and the bias is too high. To bring back the heaters to normal, you are going to get 2X 0.33 Ohm 10W resistors and wire them in parallel.
Leave a little space between them. Then insert the combo resistor in the start of the heater wire feed as it comes from the mains transformer, in series with one phase (one lifted wire, resistor in series). Take care so the resistor combo is off the PCB and has all possible space around it without touching cabling. This measure is going to relax the heaters to nominal 6.3V AC or about. Measure to confirm. Then see again if the 35.4V DC drop on the 500Ohm combos on PCB has dropped due to normal heating, and how much is it. As it is now, it indicates 70mA per tube. So to recap, drop the heaters to normal, feel if it has cooled acceptably, check the DC voltage drop, and post findings. We will ***** again.

Regards
 
It actually looks like there might be room for a choke or two if you get creative. They're between $10 and 20, and Hammond lists the dimensions. I'd pick one or two up and see if you can fit them in - it'd really really be worth it. Even the smaller ones should do something. check out the 156R at this site:

http://www.hammondmfg.com/153.htm

That's very small. Surely it could work? $10 a pop from Angela.
 
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Hello. Very good result on the heaters. Calculation proved spot on. They get so hot (your 5W resistors) because they get to dissipate almost 7W as a whole. That is why I specified 2X10W. In general, for no long term problems, a resistor must have a rating 3X the actual dissipation and over. So 20W total will be OK.

Your output tubes still consume 70mA each by the reading you gave. How is the heat now? Did it back off a tad? Will let you know how to bias lower if you tell me that the heat is still unacceptable.

No, the valves alone cannot hum. DHT valves can be a PITA to run humless, not 5881s. And there are dead quite SE DHT amps out there too. There are ways to tackle everything. Noise is related to PSU, layout, and grounding issues mainly. Your amp has 40dB gain that pushes such issues also. Tell me, is there a cable from the PCB attached to chassis? Is there a cable from the mains earth 3rd prong attached to chassis? What is the sensitivity of your loudspeakers?
 
Hello. Very good result on the heaters. Calculation proved spot on. They get so hot (your 5W resistors) because they get to dissipate almost 7W as a whole. That is why I specified 2X10W. In general, for no long term problems, a resistor must have a rating 3X the actual dissipation and over. So 20W total will be OK.


Yep - sorted - thanks!

Your output tubes still consume 70mA each by the reading you gave. How is the heat now? Did it back off a tad? Will let you know how to bias lower if you tell me that the heat is still unacceptable.

They are cooler but I think that is still too high. Do you have any advice on lowering it overall (other than running off a variac!)?

Tell me, is there a cable from the PCB attached to chassis? Is there a cable from the mains earth 3rd prong attached to chassis? What is the sensitivity of your loudspeakers?

The earth is wired directly to chassis from the IEC plug. It is also wired in with a load of other earths (eg gnd out for speakers).

I don't know the sensitivity of the current speakers (goodmans double maxim) but I have tried it on various ones including the not sensitive (insensitive?) Kef Cresta 1 and they all hum.

Thanks,

Dan
 
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Lift one 500 Ohm from each pair. This will get the bias down from 70ma to 40 mA. Right now each tube dissipates 26W (overspec). You will measure around 40VDC on each 500Ohm left in circuit, and feel the heat backing off seriously enough. Do that and let me know how many cables go from PCB to chassis and from where off the circuit gnd points.
 
Lift one 500 Ohm from each pair. This will get the bias down from 70ma to 40 mA. Right now each tube dissipates 26W (overspec). You will measure around 40VDC on each 500Ohm left in circuit, and feel the heat backing off seriously enough.

OK, I've done that and get 41.7Vdc down to 3.9mV on one side and 40.8Vdc down to 4.1mV on the other.

Any idea where this voltage inconsistency comes from as the transformers appear to deliver an even supply. Is it enough to be an issue?

let me know how many cables go from PCB to chassis and from where off the circuit gnd points.

Ummm... Well none go from PCB to chassis directly. They are linked at the point the mains comes in. They are all twisted together in a big lump on the PCB. There appears to be another unused take off on the opposite (volume side) of the PCB.

If you look at the picture here you can see the twisted gnd points behind the two electrolytic caps in the middle and the unused gnd point just in front of the plastic socket/plug arrangent at the top middle.

Thanks,

Dan