[colliedoggy]

I have just bought a mk viii se.
 
I suspect there may be a minor fault on the circuit which I want to deal with as soon as possible.
 
It fires up fine and the bias currents on the left and right meters are 56mA and 58mA respectively.
 
However, after a few hours of use the left meter reading drops significantly. After about five hours I turn it off for five or ten minutes as the left meter is getting too close to zero for my comfort. After a rest, everything fires up fine again with nearly matching meter readings to start with, before the left one gradually drops again over a few hours.
 
I have swapped all the valves from left to right channel and the behaviour doesn't change.
 
What is going on here please?
 
(I am very much looking forward to the mods thread. I'm not going to spend as much as other people here have, but I will definitely be making the most cost effective mods at some point in the future.)
 
Many thanks!

 

[SonicTrance]

I have just bought a mk viii se.

I suspect there may be a minor fault on the circuit which I want to deal with as soon as possible.

It fires up fine and the bias currents on the left and right meters are 56mA and 58mA respectively.

However, after a few hours of use the left meter reading drops significantly. After about five hours I turn it off for five or ten minutes as the left meter is getting too close to zero for my comfort. After a rest, everything fires up fine again with nearly matching meter readings to start with, before the left one gradually drops again over a few hours.

I have swapped all the valves from left to right channel and the behaviour doesn't change.

What is going on here please?

(I am very much looking forward to the mods thread. I'm not going to spend as much as other people here have, but I will definitely be making the most cost effective mods at some point in the future.)

Many thanks!

If you just got it I would contact Little Dot. If there's nothing wrong with your tubes the meter readings should stay the same.

 

[colliedoggy]

I bought it secondhand.
 
The only change I have made was to replace the output sockets with two 3 pin XLR sockets to match my Double Helix balanced cable.

 

[SonicTrance]

Ok. Did the left meter reading fail before you made those changes?

 

[colliedoggy]

I didn't run it for that long before changing the XLR sockets. I just listened for a few minutes single ended. I had to make that change to listen balanced with my cable.

 

[Maxx134]

I bought it secondhand.

The only change I have made was to replace the output sockets with two 3 pin XLR sockets to match my Double Helix balanced cable.

Ugg I guess if it was fine before, you need check your soldering work and pin layout.
Hope your neg wire is not soldered to ground...


At first, your symptoms described are like a failing tube..

So my question is,
is it the same meter that goes down,
or the other meter once you swap everything over (all 3) to other side..?

 

[colliedoggy]

The ground pin on the 3 pin  XLR sockets is not connected to anything. The ground wire inside the case is connected only to the case. I am certain it is wired correctly.
 
Everything is fine for several hours. It is just that after about 5 hours the left needle drops significantly.
 
I first tried changing the 12AT7 valves for different valves. No change in behaviour.
 
Then I tried swapping the 12AT7 valves from one channel to thte other. No change in behaviour.
 
Then I tried swapping the 6N30P valves on the left channel to the right channel and the ones on the right to left. No change in behaviour.
 
Now I understand from post 2145 in this thread by tll that the meter is to monitor the bias current for the +ve signal power tube, not the -ve signal power tube.
 
That being the case, it can't be due to a valve failing, can it?

 

[SonicTrance]

  The ground pin on the 3 pin  XLR sockets is not connected to anything. The ground wire inside the case is connected only to the case. I am certain it is wired correctly.
 
Everything is fine for several hours. It is just that after about 5 hours the left needle drops significantly.
 
I first tried changing the 12AT7 valves for different valves. No change in behaviour.
 
Then I tried swapping the 12AT7 valves from one channel to thte other. No change in behaviour.
 
Then I tried swapping the 6N30P valves on the left channel to the right channel and the ones on the right to left. No change in behaviour.
 
Now I understand from post 2145 in this thread by tll that the meter is to monitor the bias current for the +ve signal power tube, not the -ve signal power tube.
 
That being the case, it can't be due to a valve failing, can it?

I'm not sure this is correct. I had a failing power tube in my mk6 and it didn't matter which socket it was plugged into, all four sockets gave me bad meter readings. If it was in the left channel, the left meter reading were off. Right channel, right meter reading were off.

 

[Maxx134]

The meters show the powertube bias for that chanel,
But you say it the same meter even when you swap tubes...

So unless you have 2 bad power tubes,
Looks like either your meter may have bad connection, or a circuit problem.

 

[Redge78]

However, after a few hours of use the left meter reading drops significantly. After about five hours I turn it off for five or ten minutes as the left meter is getting too close to zero for my comfort. After a rest, everything fires up fine again with nearly matching meter readings to start with, before the left one gradually drops again over a few hours.

OK, you need first to check the temp of your amp.
I strongly suspect that the heat is building up slowly, up to a point where something is failing.
 
Maxx and I have a MK8 and both of us have added some fan to cool the inside.
The MK8 was designed from the MK6, and strangely the 2 fans of the MK6 were replaced by some useless grill on the MK8
 
With a proper ventilation, the amp should never be warmer than your hand, even after 5H operation (while my amp was burning hot before the fan was installed).
 
 

(I am very much looking forward to the mods thread. I'm not going to spend as much as other people here have, but I will definitely be making the most cost effective mods at some point in the future.)

Well, you don't want to pay to much attention to the rich kids around  

You can have the "entry level" pack of the major mods for $150, and some of them are in the $20/$30 range. And it will already be a tremendous improvement over stock.  
 

 

[colliedoggy]

Thanks for that: my first mod will be fan cooling! Did you power your fan from the unit's power supply, or use an additional external source?
 
That said, when I do sit down to make mods, I will attempt to track down the problem component and replace it and the corresponding part on the other channel.
 
On the one hand I want to start modding now, but on the other I don't want to stop listening to it! 

 

[coinmaster]

On the one hand I want to start modding now, but on the other I don't want to stop listening to it!

Ugh, I know that pain more than you do! Not only has modding stopped me from listening, but I've had a damaged ear since january which I keep happening to reinjure. Now I gotta let my caps burn in for about 20 days before I really listen to it.
 

 Did you power your fan from the unit's power supply, or use an additional external source?

Don't use the internal supply. It dirties up the power. Use an external 12v molex plug or something.

 

[Redge78]

  Ugh, I know that pain more than you do! Not only has modding stopped me from listening, but I've had a damaged ear since january which I keep happening to reinjure. Now I gotta let my caps burn in for about 20 days before I really listen to it.
 
Don't use the internal supply. It dirties up the power. Use an external 12v molex plug or something.

Same here. Fan powered by the PC.
 
Anyway, you don't have any plug/connector for a fan on the MK8 board. 

 

[tll]

from my experience with the LD6+, so not necessary relevant to your LD8

1. The meter monitors the loading current in two power tubes only, those which are for +ve signal current amplification
2. The loading current in the power tube depends on
- the power tube itself
- plat voltages at pin #2 and #5
- the values of the loading resistors (330R), a faulty R here? (use a DM measure the DC voltage before and after these loading resistors can confirm the problem is from the voltage supplied to power tubes)
- power supply caps
- MJ1502/1501 regulator under the black heat sink


most likely it is due to heat build up causing some components failing

 

[SonicTrance]

from my experience with the LD6+, so not necessary relevant to your LD8

1. The meter monitors the loading current in two power tubes only, those which are for +ve signal current amplification
2. The loading current in the power tube depends on
- the power tube itself
- plat voltages at pin #2 and #5
- the values of the loading resistors (330R), a faulty R here? (use a DM measure the DC voltage before and after these loading resistors can confirm the problem is from the voltage supplied to power tubes)
- power supply caps
- MJ1502/1501 regulator under the black heat sink


most likely it is due to heat build up causing some components failing

Please read my post above, I also got the mk6+. If a tube is going bad the meter for that channel will show bad readings. It doesn't matter which socket the tube is plugged into.