[baronbeehive]

  Hi Baron!
 
As you know, I had a short in my amp. It started with a burnt anode resistor leading to the switch, then a short while switching.
 
It turns out that the positive rail took the hit. With a broken transistor, zener diode and resistor. Even though only the positive rail was faulty I changed most psu parts on both rails, including the filter caps. When I pulled out one of the old filter caps I must have broken the internal connection on the board somehow. The solder pads were fine and after removing the cap I tested the trace (which goes under the cap) and it worked fine, but it didn't work from the other side of board (where you solder), hence the wire connection.
 
Here it is, right after the bridge rectifier:

That's pretty weird, my problem pads all came from pulling out from the other side to the solder joint, except for one new component which I put in with the leg at a slight angle which caught the edge of the pad. But maybe the you damaged it without any obvious problem, could be a small tear in the covering. That goes to show how careful we should be given that the amps have gone well for years from the factory so the quality is there in the components.
 
Good to know everythings fine with the help of the other guys.
 
I think I'm right to put off the big switch on until I've had mine tested properly, still trying to book it in though

.

 

[Mogos]

Sonic I knew you could do it 

.
Guys it is time to add to the official instruction a recomendation to change the transistors in the mkvi. I am not a specialist but it looks like there is a demand for more current from the super moding parts.

 

[baronbeehive]

  Sonic I knew you could do it 

.
Guys it is time to add to the official instruction a recomendation to change the transistors in the mkvi. I am not a specialist but it looks like there is a demand for more current from the super moding parts.

 
I was wondering about this too, but Maxx and Redge haven't had problems as far as I know?
 
It will be interesting to see what happens to mine when it's up and running!

 

[SonicTrance]

  Sonic I knew you could do it 

.
Guys it is time to add to the official instruction a recomendation to change the transistors in the mkvi. I am not a specialist but it looks like there is a demand for more current from the super moding parts.

Thanks Mogos!
 
I was almost ready to give up a few times, but like Maxx and Redge told me, it's been a fast learning period! I have a much greater understanding of how the psu works now.
 
In hindsight there was only three components that I HAD to change. One transistor and the zener and resistor below it. But I ended up changing, well, a lot more, lol. I don't regret that though, as my amp now have much tougher components in psu area.
 
It feels so good to listen to my amp again, lol

Though, I've not installed the switch again. I just don't trust it. I've measured it countless times for shorts and there's nothing, but still don't trust it, lol.
I think I'm going to rewire the switch from scratch and then install it again. 

 

[baronbeehive]

Sounds like you're just a little bit paranoid about that switch lol.
 
I wish I was confident enough to test mine myself but I feel better having it done by a professional, I would be starting from a knowledge base of zero! I will try to find out from him what he thinks about those transistors etc., he is a well qualified expert/teacher and has built his own valve amp. I would like to involve him but I think he is too busy for that. That's why I'm having to wait to book it in.

 

[SonicTrance]

  Sounds like you're just a little bit paranoid about that switch lol.
 
I wish I was confident enough to test mine myself but I feel better having it done by a professional, I would be starting from a knowledge base of zero! I will try to find out from him what he thinks about those transistors etc., he is a well qualified expert/teacher and has built his own valve amp. I would like to involve him but I think he is too busy for that. That's why I'm having to wait to book it in.

Probably a little paranoid, yes! 

 But I'm still re-wiring it. It took me one month to troubleshoot and fix the amp and I'm not letting the switch ruin it, lol.
 
That sounds like the perfect guy to go over your amp!

 

[Maxx134]

The pad on either side of the cap most likely WAS NOT DAMAGED in any way...!!!

Sonic did nothing wrong except being too ambitious. .

In taking the cap out,.there is simply NO WAY to solder that cap from the top component side,
Which is where the board trace is(!),
which leads to the rest of the PSU...

Soldering from the bottom side only *edit*
Is not enough.

 

[Maxx134]

Sounds like you're just a little bit paranoid about that switch lol.

I wish I was confident enough to test mine myself but I feel better having it done by a professional, I would be starting from a knowledge base of zero! I will try to find out from him what he thinks about those transistors etc., he is a well qualified expert/teacher and has built his own valve amp. I would like to involve him but I think he is too busy for that. That's why I'm having to wait to book it in.

Simply put, the transistors are outdated,
Yet fully capable under normal conditions.

But yes, a short problem will likely cause this area to go first.

Yet the PSU design was already stated by a top tube designer, to be a "highly musical" design.

Probably due to how it uses the caps and transistors.

 

[SonicTrance]

The pad on either side of the cap most likely WAS NOT DAMAGED in any way...!!!

Sonic did nothing wrong except being too ambitious. .

In taking the cap out,.there is simply NO WAY to solder that cap from the top component side,
Which is where the board trace is(!),
which leads to the rest of the PSU...

Soldering from the bottom side does nothing.

Yes, no damage to pads!
 
I guess the connection "pad" on the cap has to make contact with pad on the board:
 

Mine clearly didn't, even after two attempts. Works just as well with the direct wire connection underneath anyway.

 

 

[Bruch]

Stupid question alert!
 
I'm paranoid about damaging the board.
 
Therefore what would be the effect of piggy-backing the new caps onto the existing caps - soldering new legs onto existing legs? I figure that there is much less chance of causing any irreparable damage or shorts this way.
 
Also ... could I crush the WIMA driver-stage decoupling caps in the way that Baron did and use those legs as the termination points for new caps?

 

[coinmaster]

Sorry I've been busy and my email hasn't been updating me on new posts.
 

Without the .68 ohm resistors and meters connected I fired the amp up and after second relay kicked the four 2K resistors by the gain switches started to smoke!

This is because the opamps were driving your grids positive which is why I told you to take the opamps out while doing this. You could have destroyed your tube or a few other things. The 2k resistors were acting as plate resistors with the grid being a pseudo anode.
Check the 2k resistors and make sure they still measure 2k. They will have an effect on how your amp sounds.
 

-36V diode: -39,5V (a little high, I'm replacing this diode)

Don't expect it to help much, I've tried 1% vishay diodes and a few others and it still goes that high. I'm not quite sure why.
In either case you're still within the operating range of the opamp and the trimmer takes some of it off anyway.
 

They are in the exact same locations of your r
.68ohm resistors. .

Are you sure those aren't really weird resistors or ferrite beads?
 

Stupid question alert!
 
I'm paranoid about damaging the board.
 
Therefore what would be the effect of piggy-backing the new caps onto the existing caps - soldering new legs onto existing legs? I figure that there is much less chance of causing any irreparable damage or shorts this way.
 
Also ... could I crush the WIMA driver-stage decoupling caps in the way that Baron did and use those legs as the termination points for new caps?

What caps are we talking about?
If you aren't careful you can cause oscillation from the wire inductance by stacking caps. It's pretty difficult to damage the board unless you are rough with it, personally I would just solder them out, as long as you aren't taking things out like the PSU caps there's not a lot of risk of board damage. If yo want to take the lazy approach you can stack the caps but don't blame me if they ring

 

[baronbeehive]

  Stupid question alert!
 
I'm paranoid about damaging the board.
 
Therefore what would be the effect of piggy-backing the new caps onto the existing caps - soldering new legs onto existing legs? I figure that there is much less chance of causing any irreparable damage or shorts this way.
 
Also ... could I crush the WIMA driver-stage decoupling caps in the way that Baron did and use those legs as the termination points for new caps?
 

 
Yes, this was my worry, but you won't get anwhere if you worry about it too much. If your soldering iron is good you should be able to get a good solder melt with a max dwell time of 2-3 seconds and providing the solder melts the component should come out no problem. My iron wasn't really up to the job and so I probably overheated some components which caused my problem.
 
Next time I really mean to get a continuous vacuum desolderer. If you watch a video of this it really makes light work of desoldering, and you don't need that 3rd hand I mentioned lol. This video is definately recommended: https://www.youtube.com/watch?v=h_RiRL-flgs
 
You can reuse the legs, or new wire but it's tricky and much better to put the new component in the proper place by itself.

 

[SonicTrance]

  Sorry I've been busy and my email hasn't been updating me on new posts.
 

Without the .68 ohm resistors and meters connected I fired the amp up and after second relay kicked the four 2K resistors by the gain switches started to smoke!

This is because the opamps were driving your grids positive which is why I told you to take the opamps out while doing this. You could have destroyed your tube or a few other things. The 2k resistors were acting as plate resistors with the grid being a pseudo anode.
Check the 2k resistors and make sure they still measure 2k. They will have an effect on how your amp sounds.
 

-36V diode: -39,5V (a little high, I'm replacing this diode)

Don't expect it to help much, I've tried 1% vishay diodes and a few others and it still goes that high. I'm not quite sure why.
In either case you're still within the operating range of the opamp and the trimmer takes some of it off anyway.

If you knew that was going to happen you should have said so from the beginning. This is what you wrote: "In fact, you should take the opamps out of their sockets for now since the zeners may not be functioning properly and the opamps are expensive to replace" I took that as a recommendation, not a necessity.
 
I think when you give advice in a thread like this you need to be very clear and write all facts. If not, then clearly state that you're not sure what's going to happen. I get that it's mostly trial and error for all of us, but when advice are given it should be clear. My 2 cent.
 
Anyway, I've already replaced the 2K's and the amp is working fine. And yes, you're right about the 36v zener. I've replaced it but that didn't change much. It now reads about 38,5v. Still close enough I think. 

 

[coinmaster]

Quote: If you knew that was going to happen you should have said so from the beginning. This is what you wrote: "In fact, you should take the opamps out of their sockets for now since the zeners may not be functioning properly and the opamps are expensive to replace" I took that as a recommendation, not a necessity.

 

I then spent a paragraph explaining to you how you should really listen to me and take them out.
Sure I didn't go into all the possible reasons of why but I didn't think I really had to since I told you the opamps themselves could be destroyed. At the very least you were willing to risk destroying $60 in opamps so don't look at me if you decided not to listen.
I'll admit that I forgot to tell you sooner, It slipped my mind, I haven't dealt with troubleshooting a stock MK6 since mogos was having his issues.

 

[SonicTrance]

Destroyed caused of malfunctioning zeners you said, and as I tested the diodes and they were fine I didn't take the opamps out. But hey, no real harm done anyway as opamps are good! Only had to replace the 2K's.
 
 
So, let me ask you this coin;
 
What do you think about biasing the 5998/421A's with half of stock cathode resistor value? 165 ohms.
 
I've measured the cathode voltage using 6AS7's and 421A's and the voltage is about half using 421A's. I get about 21v over the cathode resistor using 6AS7 and 11v using 421A's. Meters show 60mA with 6AS7 and 30-32mA with 421A.
 
So, would the lower 165 ohms double the current on the 421A? Which would make for much better sound and power as the tubes are more properly biased? Would the opamps take damage? Or maybe compensate the loss of cathode resistance? Would the cathode voltage possibly half as well?
 
Tell me your thoughts please.