[klnglim]

If it was not something you did just before this happened you should check your solder joints, wires. Something is not making connection or is touching something else and shorting out.

Do you have a multimeter. If so you should check every solder joint you have made and make sure it is making proper connection with the board trace. That will rule out any problem solder work. Also check there is no solder particles lying around.

Finally make sure everything is connected up correctly.

Edit: Check especially the circuit which is reading zero, there is a faulty connection there somewhere.

This is disheartening kinglim, I'm sure it is something simple that can be put right!

I m taking off everything, check all the connection now and will replace new resistor and Mosfet tomorrow

 

[klnglim]

If it was not something you did just before this happened you should check your solder joints, wires. Something is not making connection or is touching something else and shorting out.

Do you have a multimeter. If so you should check every solder joint you have made and make sure it is making proper connection with the board trace. That will rule out any problem solder work. Also check there is no solder particles lying around.

Finally make sure everything is connected up correctly.

Edit: Check especially the circuit which is reading zero, there is a faulty connection there somewhere.

This is disheartening kinglim, I'm sure it is something simple that can be put right!

The solder joint check fine, may be Mosfet cause problem?

 

[baronbeehive]

I m taking off everything, check all the connection now and will replace new resistor and Mosfet tomorrow

Great, I had a similar problem, you might need to check that the opamp hasn't shorted out when you put everything back.

Good luck!

 

[baronbeehive]

To draw a load line like this you first measure B+. It's easiest to measure at top of anode resistors (the lead closest to the front of the amp). I measured 155 V as you can see above. There's your point on the Va horisontal axis. Then calculate the maximum plate current. Ohms law: B+ / plate resistor = 155 / 33k = 4.7 mA, there's your point on the mA vertical axis. Then simply draw a line between the two points and there's your load line.

Plate current is set to 1.2 mA through the CCS. Mark that point on the mA axis. Measure or calculate the Va. Remember that Va is the DIFFERENCE between anode and cathode, NOT anode to ground. Same thing with the grid voltage, Vg. It's the difference between grid and cathode. I measured about 114 V between anode and cathode. There's your second point on the Va axis. Now simply draw a line from the 1.2 mA point until you hit the load line, then go straight down. You should then hit the measured Va (these curves are not exact so ballpark is enough)

Yes, I get all this. We start off with a measured B+ from the LD driver stage, We can then vary the anode resistor to arrive at a value for the plate current. What I would like to know is:

1 - Is there an optimum value, or range of values for the plate current?

2 - Is there an optimum value, or range of values for the current through the CCS?

3 - Is there an optimum value, or range of values for the plate voltage?

I've looked at the tube datasheets but don't really see how we relate our values to what is on the datasheets.

I realise that we can set our operating point on the grid which will tell us what the grid voltage is and this should be set at the most linear part of the grid ideally.

Yes! Since this ps is not regulated it drops significantly when more current is pulled. That's why I lowered the last R in the ps to 750R. I would not recommend you pull anymore current then this. But feel free to experiment, nothing bad is going to happen from that, except maybe bad sound, lol.

The last R in the PS is the cathode resistor, or the 3k3 resistor?

Edit: I think you mean the 3k3 resistor. Which resistor is that?

 

[SonicTrance]

1 - Is there an optimum value, or range of values for the plate current?

Just look at the curves in the datasheets. You want to be in the linear part. That is where there is equal amount of space between the grid lines both left and right.

2 - Is there an optimum value, or range of values for the current through the CCS?

Plate current is current through the CCS / 2, since there’s two triodes. Plate current is what you are setting with the CCS.

3 - Is there an optimum value, or range of values for the plate voltage?

Again, look at the curves.

I realise that we can set our operating point on the grid which will tell us what the grid voltage is and this should be set at the most linear part of the grid ideally.

The grid in the LD input stage are always at 0 V, in relation to ground. We’re setting the load line with the anode resistors and we’re setting the operating point with the CCS.

The last R in the PS is the cathode resistor, or the 3k3 resistor?

It’s the 3k3. The cathode resistor is not part of the power supply but the audio circuit. Cathode resistors are also taken out when you install the CCS’s.

 

[bloodhawk]

The pot and biasing circuit are independent.

That roll off, would you describe it as a high frequency imbalance of some sort, seems even more likely to be the pot!

So what do the 2 meters read, also what tubes is he using

I believe the trimpots are for providing a baseline for the biasing.

Edit: Try flipping the gain switches, they're known to be problemmatic as well.

Interesting.

The meters read - 60 on the left and 45 on the right. With 6080's and the stock driver tubes.

This shifts to 55 and 40 if he uses 4 x 5998's.

Tried the gain switches, the bias imbalance stays the same.

The reason im rather skeptical its the pot is because the imbalance wasn't there before when he only swapped out the coupling caps, so im highly leaning towards the Cathode Bypass/Decoupling caps/bypasses.

Any idea on how to measure the values the trimpots are set to without taking the board out? I want to make sure he didnt accidentally nudge either of those. Also what the stock values should be on the trimpots.

 

[baronbeehive]

Interesting.

The meters read - 60 on the left and 45 on the right. With 6080's and the stock driver tubes.

This shifts to 55 and 40 if he uses 4 x 5998's.

Those bias values are out, there's something up here, but I can't make out what. The left meter is reading correctly for 6080's, but for 5998's the reading of 40 is more likely correct. So I'm at a bit of a loss here. I can't quite understand why your voltage readings didn't show up any differences in the circuits.

I'm at my limits here, I'll have to leave it to the others, hope you can sort it bloodhawk!

Edit: you need to be able to tell if there is a defective circuit, and if so which one so that you can concentrate your efforts on that circuit. My guess is a connection fault somewhere.

 

[bloodhawk]

Those bias values are out, there's something up here, but I can't make out what. The left meter is reading correctly for 6080's, but for 5998's the reading of 40 is more likely correct. So I'm at a bit of a loss here. I can't quite understand why your voltage readings didn't show up any differences in the circuits.

I'm at my limits here, I'll have to leave it to the others, hope you can sort it bloodhawk!

Just to confirm the voltages on the 330R's are to be tested against ground right?

 

[baronbeehive]

Just to confirm the voltages on the 330R's are to be tested against ground right?

Yes.

Edit: You could try the power tube pin sockets as well, especially pin1.
I edited my previous post.
Maybe a pic of the inside would help with a close up of the connections.

 

[bloodhawk]

Yes.

Edit: You could try the power tube pin sockets as well, especially pin1.
I edited my previous post.
Maybe a pic of the inside would help with a close up of the connections.

ANNND fixed it !

Its was the .33 uF RIFA Bypass caps. Removed the ones from the left channel, and it instantly went back to normal.Removed it from the right channel and they are almost perfectly matched now. I might swap the ones being used as the decoupling cap bypass as well, with some better film 0.33uF ones.

For now since i have 2 x MK VI+'s at hand with slightly different upgrade components im really interested in doing blind A/B testing to see if i can the difference each component makes. Specially since i already need to rearrange the shoddy placement job my friend did.

One thing i instantly noticed between my amp and his without the .33uF RIFA bypass caps across the coupling caps , is that low end is much richer and not sharper. Which is exactly what i prefer. And i have a feeling @SonicTrance Might also like.
Its has the right amount of texture and smoothness and feel much "fuller". In a way one can say that , instead of being - Oh hey, there was the low end nice. You literally go - HOLY crap the low end FEELS awesome.

 

[bloodhawk]

@baronbeehive I spoke too soon . The bias balanced out for only a few minutes.

I went back and started taking a few voltage readings and this time-

Tube 4 pin 1 to 330R (on the left, with the amp upside down) reads 45V , while all other Tube Pin 1's to their respective 330R 's read 55V.

However from socket / common ground to any of the 330R's the voltage reads at 80V.

I removed all the resistors and checked their values.. they we're exactly on point. Even the capacitors read perfectly fine out of the circuit. So the issue seems to around this region - https://i.imgur.com/Z3kcdKg.jpg

 

[baronbeehive]

@baronbeehive I spoke too soon . The bias balanced out for only a few minutes.

I went back and started taking a few voltage readings and this time-

Tube 4 pin 1 to 330R (on the left, with the amp upside down) reads 45V , while all other Tube Pin 1's to their respective 330R 's read 55V.

However from socket / common ground to any of the 330R's the voltage reads at 80V.

I removed all the resistors and checked their values.. they we're exactly on point. Even the capacitors read perfectly fine out of the circuit. So the issue seems to around this region - https://i.imgur.com/Z3kcdKg.jpg

Correct.

I know Maxx had this problem and it turned out to be a colder solder around his cathode bypass I think. It seems your circuits are OK up to the tube sockets, at least until warm up. I'm not sure of the board trace but you could check the other pins and anyone that is out check the trace and see where it goes.

What mods has he done, has he done any wiring. In any case I would think that if you redo the connections for the modded parts on this circuit you will probably sort it. I very much doubt that it is the tube sockets themselves, but it might be a wire connected to it.

Also check 330R resistors voltage again after warm up.

 

[baronbeehive]

Again, look at the curves.

It’s the 3k3. The cathode resistor is not part of the power supply but the audio circuit. Cathode resistors are also taken out when you install the CCS’s.

Yes right!

I didn't realise you had altered the 3k3 resistor, that's something I must do, which one is it

I will get round to doing some load lines next.

 

[bloodhawk]

Correct.

I know Maxx had this problem and it turned out to be a colder solder around his cathode bypass I think. It seems your circuits are OK up to the tube sockets, at least until warm up. I'm not sure of the board trace but you could check the other pins and anyone that is out check the trace and see where it goes.

What mods has he done, has he done any wiring. In any case I would think that if you redo the connections for the modded parts on this circuit you will probably sort it. I very much doubt that it is the tube sockets themselves, but it might be a wire connected to it.

Also check 330R resistors voltage again after warm up.

So i redid all the solder joints, and tested the amp. Bias looked good. But now there is another issue, where the left channel is only has a buzzing sound lol.

Another thing i tried was to match the Trimpot resistance values to 0.89 (k Ohm?, dont remember exactly), since my friend mentioned that he did actually try to turn the dial thinking he accidentally nudged one (for the left channel) and that was causing the bias issue.

The buzzing in the left channel could actually because of a wire being dislodged or stuck between something under the board which ill confirm later tonight after work. But not knowing how and what to set the trimpots to is somewhat worrying me.

 

[baronbeehive]

So i redid all the solder joints, and tested the amp. Bias looked good. But now there is another issue, where the left channel is only has a buzzing sound lol.

Another thing i tried was to match the Trimpot resistance values to 0.89 (k Ohm?, dont remember exactly), since my friend mentioned that he did actually try to turn the dial thinking he accidentally nudged one (for the left channel) and that was causing the bias issue.

The buzzing in the left channel could actually because of a wire being dislodged or stuck between something under the board which ill confirm later tonight after work. But not knowing how and what to set the trimpots to is somewhat worrying me.

OMG.... bias good, buzz most certainly not good lol. Just when you think you're getting someplace, the amp turns round and bites you.

I'll have to hand you over to someone who knows more, I'm at my limits here. The trimpot issue is not important it only allows you to manually adjust any imbalance.

Got to hand it to you for the work you're putting into someone else's amp!