[baronbeehive]

I recognize this amp! It seems like forever ago now. Brings back memories =) I see that I hadn't completed all the mods yet in this pic. Here's a pic with the Ra mod and CCS tail completed as well....

... good times!

As I said your amp is the most beautiful... hope Maxx didn't hear that lol!
.

 

[baronbeehive]

Here you see several possible 6SL7 load lines, with bias point 1 having a voltage swing of 40v and point 3 having a swing of 100v. Therefore it would seem that point 3 would offer the best in terms of dynamics, but point 3 is also lower down towards the bottom of the graph where the grid lines are starting to curve indicating distortion, whereas point 1 lies on straighter more vertical grid lines indicating no distortion.

It would be interesting to try 2 or 3 extreme points to see what the auditory effects are. I didn't do this but concetrated on the positions where there would be near zero distortion for maximum transparency.

Edit: I see I did try quite a few varied positions after all, I forgot, but there's still a chance for some adventurous person to try more extreme positions just out of curiosity.

 

[baronbeehive]

Well that's a tough question as I'm not sure if I'm aware of all the mods that can be done.
Replacing all the wires with spc sounds too tedious at the moment.

In fact, you've already carried out the main mods, there's also the impedance mod matching output impedance to headphone impedance, which is a minor mod not resulting in much sonic benefit, there's also a power tube mod that allows you to switch between different types of tube, like 6AS7G's and 421A's. I've found since doing all the mods here that the humble common or garden RCA 6AS7G's sound so good that you don't need anything fancier than that.

So the only thing really would be to replace key wires with good quality wire, silver plated copper, and either OCC copper or OCC silver for the 6 output wires from the board to the output XLR jacks and the headphone socket.

 

[baronbeehive]

Those four stacked resistors at the top which are near the power capacitors. Are there any recommendations/mods as to how those should be replaced?

The pair on the right should be 2x8k2 Mills MRA5, making a combined value of 4k1. The board value should read 8k2. You may have a more recent board in which case the value should read 4k1.
The pair on the left should be 2x5k1 Mills MRA5, making a combined value of 2k55. The board values should read 5k1. You may have a more recent board in which case the value should read 2k55.

The old boards had the wrong value which has probably been updated now

What values do you have on the board

These are the important resistors to change as you can see by the fact that they are partially cooked already, but the other resistors around that area would benefit from uprating as well.

 

[CopperFox]

Did you install the larger fans or the same size as stock?

80mm Noctuas, so it is the same size as stock except thicker of course. There would still be room for one or two more on the bottom plate.

We put our white decoupling caps here without any issues:

...they are very close to the hot 330R power resistors, but so long as you have good air flow.

Also you could put in a switch and resistor combo to change from 6SL7's to 6SN7's like here with a switch to change from 6AS7 to 421A power tubes.

We didn't optimise for 6SN7's ecause the higher gain of the 6SL7's is a better match for this amp, which is relatively low in power.

Edit: Sonic's amp is the most beautiful.

Mine looks like this before putting the case extension and bottom plate back on now that I've installed the power tube decoupling caps:

It took some rearranging of other caps to put the middle Wima in its right place.
It would have fit between the four largest electrolytic caps on top of the white 100R 5W resistors as well, but I had some concerns about heat in that particular spot as it's the part of the amp where at least the exterior gets hottest during extended use. The two fans are not blowing directly into that spot even if they were a bit larger. So that's why it would seem reasonable to add one or two more fans.

A 6SL7 / 6SN7 switch would be interesting. What would be or how would one determine the correct resistor value range for 6SN7s?

 

[CopperFox]

I recognize this amp! It seems like forever ago now. Brings back memories =) I see that I hadn't completed all the mods yet in this pic. Here's a pic with the Ra mod and CCS tail completed as well. I think the Ra mod is what @CopperFox is talking about here:

That is where we optimized the white cathode follower output stage for a specific load, using the Cavalli formula Ra = (1/Gm*(1/Ka*(1+Ka)/(1+Kr)*(Rp+Rl)/Rp+Rl/Rp))
I even went so far as to test my own tubes for actual gm, using my heathkit tester, and then calculate the different values to optimize the WCF for different tubes and loads.
I found this chart, this is for "stock" gm though:

Thanks for the chart, I don't remember having I've seen it before. Is the RI in the Cavalli formula the headphone impedance value?

 

[CopperFox]

The pair on the right should be 2x8k2 Mills MRA5, making a combined value of 4k1. The board value should read 8k2. You may have a more recent board in which case the value should read 4k1.
The pair on the left should be 2x5k1 Mills MRA5, making a combined value of 2k55. The board values should read 5k1. You may have a more recent board in which case the value should read 2k55.

The old boards had the wrong value which has probably been updated now

What values do you have on the board

These are the important resistors to change as you can see by the fact that they are partially cooked already, but the other resistors around that area would benefit from uprating as well.

My board says 8K2/2W and 5K1/2W. Thanks for this clarification, thought the board readings could be misleading somehow. How old is my board?

 

[baronbeehive]

My board says 8K2/2W and 5K1/2W. Thanks for this clarification, thought the board readings could be misleading somehow. How old is my board?

There was a mismatch between the actual values of those resistors and the board values. LD confirmed this, and were to put this right which they haven't done for your board obviously. I don't know how old yours is because I don't know if the board still reads the same on the newer amps.

The actual values should be as I said earlier, which involves putting in the 2 resistors in parallel to make the final value of 4k1 for the 8k2 resistors. And go for 5watt resistors here.

Great work!

 

[baronbeehive]

80mm Noctuas, so it is the same size as stock except thicker of course. There would still be room for one or two more on the bottom plate.

The Enermax fan I got a while ago has been superseeded but one like this would tick the box: https://www.enermaxeu.com/products/fans/silent-fans/cluster-adv/
The 120mm one pushes a high volume of air silently which would find it's way throughout the chassis and up the sides to the top, that's what we have done. You would have to bore bigger holes in the bottom plate but if you wanted to fit additional fans you would have to do that anyway.

Better to have an external speed controller and power supply to take this out of the amp, something like this: https://www.amazon.co.uk/Cooling-co...94&sr=1-125&keywords=3+Pin+to+4+Pin+fan+Cable

The fan has LED's which look very pretty!

 

[SonicTrance]

Thanks for the chart, I don't remember having I've seen it before. Is the RI in the Cavalli formula the headphone impedance value?

Yes, Rl is the headphone impedance.

 

[baronbeehive]

A 6SL7 / 6SN7 switch would be interesting. What would be or how would one determine the correct resistor value range for 6SN7s?

You would have to draw the load line for the 6SN7 tube using it's datasheet, and most likely test different values for SQ like I did for the 6SL7, I might get back to you but haven't the time atm.

Keep us posted!
.

 

[baronbeehive]

Thanks to Yaska729 who has said that the revamped LD website is back up.. and here it is: http://www.littledot.net/

 

[Yaska729]

Thanks to Yaska729 who has said that the revamped LD website is back up.. and here it is: http://www.littledot.net/

Not a problem, but honestly it might be easier just go to their new site, if the customer service is needed.

 

[baronbeehive]

Here's 2 suggested load lines for the 6SN7 tube.

Line 1 - outputs 18mA, with a 10K anode and 280R, or 888R cathode
Line 2 - outputs 5mA, with a 35K anode and 666R, or 2K cathode

I think points 1 and 3 would be the most transparent.


@CopperFox you could try these values. If you wanted you could try an intermediate anode of around 22K also.

If you didn't want to be bothered with too many resistors, I would go for a compromise and try around 250R, 500R, 1K and 2K for cathodes, along with the 2 or 3 different anodes, it would still be ear enough.

Guys... look at the stock bias point right in there amongst the curved grid lines. I suppose it's ok because the voltage swing is so small, that it might not get into the distortion areas. But it's pretty weird down so low, and I know that a lot of people preferred the 6SN7, maybe for its possibly more tubey qualities?

It would be interesting to try those more extreme areas of the grid for different sonic qualities.

 

[baronbeehive]

Out of interest I have included on the graph the ideal bias point I found for the 6SL7 tube, (in green). You can see why the amp is better for 6SL7's, at least with the stock bias point. Another factor is that the 6SL7 has an amplification factor of 70 whereas the 6SN7 is 20.

@CopperFox I would ignore the points on line 1 and just go for the points on line 2 or points on an intermediate line given by an anode of around 22k as I said previously. The reason is that the max rating for the 6SN7 is 20mA with a typical operation of 10mA.