[Maxx134]

Since this is a balanced amp,
You should use the balanced output to gain the benefits of full power and zero hum.

If you have hum on balanced oit Itwould be surprised.

Sometimes my single ended output is nice and quiet and sometimes it has a very low level hum.
It depends o. How solid the board to ground I Assuming.

The balanced output was always quiet.
Right now it's dead quite at full volume.

 

[Maxx134]

Your list looks very good!
Although I would not buy the driver stage resistos now.
This weekend I will compare my settings with SonicTrance to decide on settings..
The consensus is that lowering theses values placed the tube in a more linear part of its bias curve,
And the impact on sound was rather HUGE(!)...

Anyways, for the "Solen Fastcap 47uf 400vdc",
Just pull out the data's sheet for it,
and there is a chart with the dimensions based on size.

Also, if a bit too large, no worries, as you can extend the bottom.

I am looking at extending it only about an inch.

 

[nuport]

I am using balanced but get hum when more than 50% volume, even when the input is disconnected. I bought this used on ebay, maybe I should return it and get a new one to mod...

 

[nuport]

Your list looks very good!
Although I would not buy the driver stage resistos now.
This weekend I will compare my settings with SonicTrance to decide on settings..
The consensus is that lowering theses values placed the tube in a more linear part of its bias curve,
And the impact on sound was rather HUGE(!)...

Anyways, for the "Solen Fastcap 47uf 400vdc",
Just pull out the data's sheet for it,
and there is a chart with the dimensions based on size.

Also, if a bit too large, no worries, as you can extend the bottom.

I am looking at extending it only about an inch.

Ok thanks for the tips, I will pass on the driver stage resistors for now. Too bad I don't live in NYC anymore, would love to compare notes

 

[baronbeehive]

  The first thing you need to do is measure the bias voltage on the grid, which would be located anywhere between the bottom half of the coupling capacitor and the 300 ohm resistor to the tube. Not sure where those are specifically as I don't have my board anymore. Or use this as a reference http://wooaudio.com/docs/tube_data/6AS7GA(6080).pdf The voltage should be  around -20 to -25V under proper biasing conditions assuming you are using a 6080 or similar.
 
Measure the grid of both tubes as the bias meter only shows the total average current of the channel by splitting current with the .68 ohm resistor for 1/2 current measurement through two WCFs.
The tube with a lower negative voltage will be the problem tube. Also you should measure the cathode/anode voltage on the tube side of the 330 ohm resistors. They should read in the range of +/- 70-80v with stock biases. I don't know which mods you implemented so.
 
The input stage won't have anything to do with it as it is not DC coupled with the output stage so the 1.5k resistor would not cause it and the WCF cap would not cause it either because it just transmits the AC signal from the plate to the bottom grid, unless there is a short between the bottom grid and the cathode, that would be a problem.

 
Using the 6AS7G data sheet pins 1 and 4 are connected to the grid so this is where I take a voltage measurement? I took voltage readings already from the cathode bypass caps which are now all reading 24-25v since I replaced the faulty tube so if the cathode bypass caps are connected to the grid then the readings are good. If not I will have to locate it on the board from the traces from the pins1 & 4?
 
The voltage measurement on the tube side of the 330ohm resistors, surely I have already done that with the readings from the cathode bypass caps which are in parallel to the 4 330ohm resistors on the reverse side? As I said they read 24-25v not 70-80v that you say? That leaves the anode 330 ohm resistor for that tube which I haven't measured yet?
 
Am I missing something? Probably!
 

Coins also correct that driver stage has nothing to do with it.

The problem is very localized.

My guess it has to do only with the connections around by the socket in which has the burnt resistor.. each dual triode tube is it's own WCF circuit.

The other tubes values can be compared but problem should be that one tube circuit including its WCF cap. unless the two stacked Cathode bypass caps are crossed.

Maybe a bad solder or a component wire touch, or pushed too far in and touch the chassis.

So the yes I agree with Coinmaster to check each powerube bias by reading their voltages to compare the left good side against the right side.

 
The cathode bypass caps are not crossed. I will check the connections of the components in the relevant circuit but what am I looking for is it voltage differences between adjacent circuits, and left/right channels?

 

[baronbeehive]

Sorry for the duplicate post which didn't display properly the first time.

 

[coinmaster]

 As I said they read 24-25v not 70-80v that you say? 

I was referring to ground reference not voltage across resistors. 25v seems right. As for the grids just measure the bottom end of the coupling cap. From ground.

 

[baronbeehive]

  I was referring to ground reference not voltage across resistors. 25v seems right. As for the grids just measure the bottom end of the coupling cap. From ground.

 
I've no idea how to do that. I just watched a video that warned against connecting a live circuit to any metal to measure voltage which is what I was going to do. I've measured all 330 ohm resistors in situ and they all read the same for resistance so overall the 4 circuits match. I just have to measure the 4 resistors on the component side for voltage which should show differences according to the resistor.

 

[coinmaster]

Just touch the black probe to the chassis or stick it in one of the big holes in the board where the big psu caps are then measure with the red probe.

 

[baronbeehive]

  Just touch the black probe to the chassis or stick it in one of the big holes in the board where the big psu caps are then measure with the red probe.

 
Outside left 104v, inside left 102v, inside right 111v, outside right 112v. The same resistor is smoking, quite alarming!
 
I can't see any leads touching and from my pics of the reverse of the board everything looks good with nothing apparently touching the chassis.
 
This is soooo annoying..... all I want to do is listen to it, but then you guys have had your problems as well. Just have to be patient!

 

[Mogos]

baronbeehive did you change the PSU transistors or you are using the stock ones?

 

[coinmaster]

This is soooo annoying..... all I want to do is listen to it, but then you guys have had your problems as well. Just have to be patient!

It's been a year for me. The first 6 months of withdrawals sucks hard. It's difficult to justify the cost of a new mk6 when I can build one but building one has its own annoyances. I haven't decided what to do yet. At the moment I'm just waiting on some source follower pcbs I ordered to greatly reduce the complexity of the output stage so I can experiment with input stages. In the end I will need to have an mk6 though cuz that is so far the best amp I have ever heard so I need it for reference.
 
 

Outside left 104v, inside left 102v, inside right 111v, outside right 112v. The same resistor is smoking, quite alarming!

Need more specifics, left/right of what, where. Also you need to measure grids,the bottom end of your couple cap that connects to the output tubes or preferably directly on the grid pin itsef, use the link I gave toy before to see where it is located on the pins. Unless you mean the grids are at 100+v then u have a serious problem.

 

[baronbeehive]

  baronbeehive did you change the PSU transistors or you are using the stock ones?

 
 
Stock.
 

  It's been a year for me. The first 6 months of withdrawals sucks hard. It's difficult to justify the cost of a new mk6 when I can build one but building one has its own annoyances. I haven't decided what to do yet. At the moment I'm just waiting on some source follower pcbs I ordered to greatly reduce the complexity of the output stage so I can experiment with input stages. In the end I will need to have an mk6 though cuz that is so far the best amp I have ever heard so I need it for reference.
 
 
Need more specifics, left/right of what, where. Also you need to measure grids,the bottom end of your couple cap that connects to the output tubes or preferably directly on the grid pin itsef, use the link I gave toy before to see where it is located on the pins. Unless you mean the grids are at 100+v then u have a serious problem.

 
Looks like you have to have a mkvi lol! Interesting though to see what you can come up with, I wait with interest, but, yes the wait is hard, I finished the mods back in march!
 
Those were the measurements you said to take from bottom of coupling caps to ground from left (good) to right side (bad) meter readings. I can confirm that if necessary by taking readings from the grid pin 1 or 4 which are connected to the grid according to the data sheet but looks like the right side is affected by some fault manifesting in the burnt resistor, just have to find what.

 

[coinmaster]

Are you waiting for the relays to kick in before you measure? Which resistor is burnt? In any case the voltages seem more in line with B+ then grid. Either you are measuring something wrong or you got a major wiring issue.

 

[baronbeehive]

  Are you waiting for the relays to kick in before you measure? Which resistor is burnt? In any case the voltages seem more in line with B+ then grid. Either you are measuring something wrong or you got a major wiring issue.

 
Yes, after they come on.
 
Middle/right resistor.
 
I will measure from the grid pin to ground tomorrow to check.
 
The left meter reading is normal at 60mA for 6AS7G tubes.