baronbeehive said:

sonictrance said:

 
.........................





Good to hear that you're finally enjoying your MK6 the proper way with mods in place!

Now, go ahead and do the Ra switch mod and 5998/421A bias mod! :wink_face:

Click to expand...

Click to expand...

I have my lowest setting resistors connected to the poles of the switch, from ground side of Ra. The other R's, on B+ side of Ra, are then connected in series to that pole resistor. Can't remember why I did it like that but it doesn't matter. You can also just solder a wire to the poles, from either side of Ra, and then your different "addit Ra" from the other side of Ra to the different positions on the switch.

SMLD MODS: (continued)
 
6AS7G/421A Tube Bias Mod - By SonicTrance.
 
4 x 330ohm resistors connected in parallel to cathode resistors via switch to double current when using 421A's.
 

 
Still plenty of space for more mods .
 
Next, the impedance mod for tube/headphone matching!
 
Quote:

sonictrance said:

I have my lowest setting resistors connected to the poles of the switch, from ground side of Ra. The other R's, on B+ side of Ra, are then connected in series to that pole resistor. Can't remember why I did it like that but it doesn't matter. You can also just solder a wire to the poles, from either side of Ra, and then your different "addit Ra" from the other side of Ra to the different positions on the switch.

Click to expand...

 
Thanks Sonic!
 
You didn't mention if 6AS7G's sound better on their impedance setting, as well as the 421A's?
 
Also the current to 6AS7G's would be double the stock setting if the switch was on the 421A setting, is that OK?
 
Looking forward to try out the mod when I get the full set of 421A's .

baronbeehive said:

  Still plenty of space for more mods

.
 

Click to expand...

Nicely done! Have you measured to see if the cathode resistances are correct for the different settings? If not, please do so before power on.
 

You didn't mention if 6AS7G's sound better on their impedance setting, as well as the 421A's? 

Click to expand...

They do sound better. The Ra switch is not a night and day difference though, but clearly noticeable and an important mod! I noticed most improvement with my LCD-XC's at 20 ohms.
 

Also the current to 6AS7G's would be double the stock setting if the switch was on the 421A setting, is that OK? 

Click to expand...

That's definitely not OK, baron! The amp will overheat very quickly. Very important that you don't use 6AS7/6080's in 5998/421A setting!
 
Quote:

Looking forward to try out the mod when I get the full set of 421A's 

. 

Click to expand...

Looking forward to hear your impressions too!
 
Remember to control measure the different settings on your Ra switch (and bias switch) before power on. Also check for shorts and things like that.
 
Happy modding!
 

sonictrance said:

  Nicely done! Have you measured to see if the cathode resistances are correct for the different settings? If not, please do so before power on.
 
They do sound better. The Ra switch is not a night and day difference though, but clearly noticeable and an important mod! I noticed most improvement with my LCD-XC's at 20 ohms.
 
That's definitely not OK, baron! The amp will overheat very quickly. Very important that you don't use 6AS7/6080's in 5998/421A setting!
 
Looking forward to hear your impressions too!
 
Remember to control measure the different settings on your Ra switch (and bias switch) before power on. Also check for shorts and things like that.
 
Happy modding!

 

Click to expand...

 
Thanks!
 
I have measured the resistances in the different settings and they're good.
 
I thought I would mention that about the doubled current for 6AS7G's because no mention has been made of this, so I think it would be best to have a stock setting for the 330ohms and only change this when using 421A's. Perhaps mention could be made of this as a warning on page 1?
 
As my headphones are only 38ohms and your LCD's are 20ohm it would appear that low impedance headphones are most affected by the impedance mod.

baronbeehive said:

 

sonictrance said:

  Nicely done! Have you measured to see if the cathode resistances are correct for the different settings? If not, please do so before power on.
 
They do sound better. The Ra switch is not a night and day difference though, but clearly noticeable and an important mod! I noticed most improvement with my LCD-XC's at 20 ohms.
 
That's definitely not OK, baron! The amp will overheat very quickly. Very important that you don't use 6AS7/6080's in 5998/421A setting!
 
Looking forward to hear your impressions too!
 
Remember to control measure the different settings on your Ra switch (and bias switch) before power on. Also check for shorts and things like that.
 
Happy modding!

 

Click to expand...

 
Thanks!
 
I have measured the resistances in the different settings and they're good.
 
I thought I would mention that about the doubled current for 6AS7G's because no mention has been made of this, so I think it would be best to have a stock setting for the 330ohms and only change this when using 421A's. Perhaps mention could be made of this as a warning on page 1?
 
As my headphones are only 38ohms and your LCD's are 20ohm it would appear that low impedance headphones are most affected by the impedance mod.

Click to expand...

Yes, it would be good to mention that on first page. It's been a long time since I heard anything from @Redge78 though...
 
I believe your 38 ohm cans will improve greatly with the Ra mod.

BTW how does tube bias mod at the cathodes affect the impedance mod at the anodes? Is there any interaction? Does the change at the cathodes cause a change at the anodes?
 
I've taken some readings and the switch doesn't appear to affect it.

baronbeehive said:

  BTW how does tube bias mod at the cathodes affect the impedance mod at the anodes? Is there any interaction? Does the change at the cathodes cause a change at the anodes?
 
I've taken some readings and the switch doesn't appear to affect it.

Click to expand...

No relation. The cathode resistors only sets the cathode/anode current.
 
You can take measurements once you have a complete set of 421A or 5998's;
 
Measure voltage across cathode resistors, with unit on obviously. 
 
Should be around 10V with 165 ohms resistance which equals about 60mA of current.

maxx134 said:

Swap out the driver tubes for some 6c8g with adapters..
You get even better looks/glow and livelier sound..
Your too happy!
Just kidding, enjoy every level before you consider more, lol

Click to expand...

 
Happiness, short lived unfortunately .
 
I've just had the time to test the amp for the first time with the switch in and I've got a problem again! The right hand meter this time is only showing about 40mA, and I've taken voltage readings at the cathode bypass and the voltage is up on that same right hand circuit compared to the others.
 
All I have done since it was working perfectly was to try to find that annoying piece of wire which involved taking the board out again to no avail. Everything looked OK on the reverse when I put it back after a good old clean with air and isopropyl alcohol. That and the switch was all I did! Maybe something was disturbed in the process?

Hey guys been away.
All stock cable used especially for both heater and PSU to the transistors are brittle and prone to disconnect with even one time opening board so check there...

maxx134 said:

Hey guys been away.
All stock cable used especially for both heater and PSU to the transistors are brittle and prone to disconnect with even one time opening board so check there...

Click to expand...

 
Hi Maxx, yes I was careful to check the wires after what happened last time before putting the board back but it's such a struggle to get it back in you are always wondering if something happened during the process. Sonic said that he used a special screwdriver to pull down the tube sockets and I tried using a tool with a bent end which didn't work well, so I think next time I will have to make sure I have the right tool for the job to avoid forcing it.
 
Anyway I will check everything starting with the board in to see if there is anything obvious, but I'm not in a rush, other things to do. I can keep in touch by PM's if you would rather, or keep posting as I go, whatever!

baronbeehive said:

 

maxx134 said:

Hey guys been away.
All stock cable used especially for both heater and PSU to the transistors are brittle and prone to disconnect with even one time opening board so check there...

Click to expand...

 
Hi Maxx, yes I was careful to check the wires after what happened last time before putting the board back but it's such a struggle to get it back in you are always wondering if something happened during the process. Sonic said that he used a special screwdriver to pull down the tube sockets and I tried using a tool with a bent end which didn't work well, so I think next time I will have to make sure I have the right tool for the job to avoid forcing it.
 
Anyway I will check everything starting with the board in to see if there is anything obvious, but I'm not in a rush, other things to do. I can keep in touch by PM's if you would rather, or keep posting as I go, whatever!

Click to expand...

Nothing special about my screwdriver other than it has insulation on the shaft, but that's not important. I just poke the screwdriver through the center of the socket, where the guide pin goes, and pop the sockets in place one by one. I start with the driver tube sockets. I need to force it in place though, no getting around that as there's virtually no extra clearance at all.

sonictrance said:

  Nothing special about my screwdriver other than it has insulation on the shaft, but that's not important. I just poke the screwdriver through the center of the socket, where the guide pin goes, and pop the sockets in place one by one. I start with the driver tube sockets. I need to force it in place though, no getting around that as there's virtually no extra clearance at all.

Click to expand...

 
Oh I see. I ended up doing much the same thing with an ordinary screwdriver and the force needed is a bit of a concern when pushing against those ceramic sockets. I tried several things one of which was a dentist type tool with a bent end which kept slipping out so I threw that away. I did think of a right angled screwdriver but unless it had a smallish end would not have fitted up the central hole.
 
I haven't had time yet to look at the amp, it can't be much and the resistances and continuity readings for the switch are fine.

Check your tiny gain switch.
Flip them from high to low, or low to high.
Check that they in same position.
I am thinking about abolishing this stupid gain switch, in any position.
Another untouched path to look into..

The gain switch is part of the NFB loop. Nothing good will come of removing it.

OK, thanks, I could flip it to high/low gain to check. I'm working on the house atm so I will get back to the amp later.
 
The amp was working fine after getting it back from the engineer. So what could have happened?
Since then I removed the board to try to find that piece of wire but couldn't find it. The board was a struggle to put back as ever but I made a point of checking the wires on the back first.
Then I fitted the switch and as I said the readings were fine and the switch was out of the way of anything underneath it. The solder joints were good.
Then I fitted the fans, one of the nuts on the inside was close to the top of one of the 4 1200uF 220v power caps, the other nuts cleared everything.
The I cleaned inside with air and isopropyl alcohol.
Then I rearranged the parts inside and fitted cable ties etc so that the parts inside were in their final position.
Finally I put 2 new 3 amp fuses inside.
 
Could that piece of wire be causing a problem, or something happening when putting the board back, or the switch, or the fans, or cleaning it, or the rearrangement inside, or the fuses?
 
My feeling is that maybe rearranging the parts inside might have done something. For example I noticed that the engineer must have had 2 1k resistors handy because he fitted them in series to replace the 2k resistor so possibly the fact that they are pointing upwards might have caused them to come in contact with a wire when I rearranged everything. I feel it must be something like that that has happened, but I will look into it more later.
 
Edit: I meant 0.5 amp fuses.