Tube Rolling in Little Dot Mk II Pre-amp

[haisaikat]

Starting a new thread to provide updates on tube rolling for my newly acquired Little Dot Mk II pre-amp. The previous thread of acquiring this can be referenced at https://www.hifivision.com/threads/pre-amp-suggestions-needed.72243/page-3

I have mainly followed the following BIBLE https://www.head-fi.org/threads/little-dot-tube-amps-vacuum-tube-rolling-guide.563884/ posted elsewhere for getting the comprehensive idea of various tubes and their possible effect on sound signature.

The LD I acquired came with 6H6PI power tubes and Mullard M8100 driver tubes instead of the default 6J1 tubes which the previous owner had upgraded.

From the above tube rolling guide I came to understand that Mullard M8161 is another great option for the LD and hence ordered the same from ebay, the link to the item was https://www.ebay.com/itm/310391381072 . As its evident from last post of my previous preamp thread that I received in turn a pair of tubes that had CV4015 kqdd/k written on it with nowhere the name Mullard. After several message exchanges with the ebay seller it was concluded these are a separate category of Military Spec Mullards and this is the way they manufactured the M8161 for military. Unfortunately since these were internationally shipped I decided to keep them and go ahead with the rolling.

I will provide subsequent pics and steps of the rolling shortly.

 

[haisaikat]

There is a online version of the Little Dot MK II user manual available at https://www.dropbox.com/s/5pmrnv1twu42y9i/Little Dot MK II Reference Guide.pdf?dl=0 which is the main guide for the tube rolling steps. Except from the manual is given below



So per above post the jumper pins have to be altered in connection to allow from 8100 to 8161.

So I opened the bottom set of 4 screws on front and rear faces ( 2 screws each side) of course after taking out the 4 tubes from top



This allows taking out the bottom cover easily and after that belown circuit is exposed.


Ideally this is upside down. Note the ones circled are the red gain switches presently set at 3db only by default and is apt for most high impedance amplifiers. No changes there. But it's the other jumpers that we need to alter position, as per manual by default the jumper is set towards the EF 95 label and we need to set it towards the EF92 label in order to use M8161. Quite easy job as it seems.

This is the final state after setting the jumpers to EF92 option.


Now I am going to screw it all together. And here is the final state with new tubes placed in, listening impressions to come later. For a Chinese one seems well documented and organised.


For reference here are old/existing tubes, tucking away for now, the black bottom guards came with the new ones.

 

[haisaikat]

Update on First impression after the roll

The bass and highs with details are instantly better, the punch that was subdued by M8100 seems to be resurrected by M8161. Yet the bass is controlled. Instantly its evident that this is the Bass I like. On the highs, there is certainly additional details that M8100 subdued earlier, result being guitar and similar string equipments benefit from additional sharpness and detail. But somewhere I feel there is a tinge of brightness in the signature that is heard in female voices and high tones. Male voice is good as before, not much noticable change. It may be due to not reaching break in or may be because I have become too much used to listening to the smoothening effect of the M8100 that anything above seems to be bright. I am presently playing web radio for 2-3 days a weeks at 1-2 hours a day trying for break in faster. Will post another impression in some time.

Question, can anyone suggest if for breakin of preamp tubes I need to keep the power amp on? Also what is the typical break in period for these kind of tubes?

 

[haisaikat]

Further update

After around 20-30 hours of play the vocal of M8161 became from shrill to smoother and the bass remained better. However the Vocal and treble was not as smooth as the that of the M8100. I was in a dilemma on whether to keep the M8161 or to switch back to M8100 since on one hand the bass in M8161 was to die for but on the other hand it seems that I have become too used to listening to smooth vocals of the M8100.

Just in this time a mishap happened when i had temporarily switched back to the M8100 for comparison, my 20 month old son broke one of the M8161s.

Since I had already tried the M8161 I wanted to try something else this time, so have now ordered a matched pair of Voshkod 1983 make 6ZH1P-EV tubes from ebay, they are cheaper too https://www.ebay.com/itm/162523290941 . They are expected to arrive in a month's time. Will keep this thread updated.