Tube Change/Biasing a Stax 006t II

[lupoal]

I add here a question about a doubt... somewere I've read about a "strange" situation inside a 006Tii and, checking inside mine, 006Ts too...
there are those resistors, about 5k, in series to each channel, R+ R- L+ L-, added during product improvments for safety reasons... now about that one connected to R+ it seems it is short-circuited, in praxis that resistor seems to be there to do nothing as, measured, the value is not 5k but 0,3Ohm beacuse of the short
here a couple of pictures about taken at both pcb sides...

in the second picture you can see there are three row connected all to the same point, so the resistoryou can see in background is shorted... imho the row on the right should not be there... anybody has any idea/opinion about that? is there anybody that have a circuit diagram of this ampli son we can read how that resistor should be connected from the project?

 

[kevin gilmore]

you should check the other 3 resistors. if they are all shorted, it was deliberate. if not, definitely a mistake

 

[lupoal]

thks
no, the other three are fine… I had a confirmation from a man living in a very cold country it’s a mistake, I will cut the wrong track

 

[Marcin Gomulkiewicz]

thks
no, the other three are fine… I had a confirmation from a man living in a very cold country it’s a mistake, I will cut the wrong track

Then once you're sure you've cut the right (or in this case wrong) track please post a picture so we could too

 

[lupoal]

ok... if mine will not explode then... you are safe

 

[zeroduke]

Hi! Does anyone know the biasing guide for the SRM-700T after a tube change. Thank you.

 

[Marcin Gomulkiewicz]

ok... if mine will not explode then... you are safe

Have you performed the 'surgery' ?

 

[lupoal]

yes… it works perfect

 

[Marcin Gomulkiewicz]

yes… it works perfect

...and could you please show us, the mere mortals, which track did you cut so we could avoid mistake...?

 

[lupoal]

... the first on the right... that one marked with yellow X

... and I've also changed the trimmer (those horrible single-turn) with a 10-turns... this help when setting the bias... but the real problem is that when you remove the top housing to reach the trimmers and then position it back the temperature inside change and the output voltage too... so I've done a DIY top-housing to... 1) reduce the internal temperature that is good for components life time and... 2) have access with ceramic screw driver to the trimmers keeping the temperature inside the unit more stable... it helps

 

[lupoal]

these are the trimmers... not extremely difficult to do but not very easy too... you need to know how to de-solder and solder properly

and this is the diy cover... well not the very last one which has four holes (normally closed) on the vertical of the trimmers to allow adjustment

I use Sylvania 6SN7 GTB (in the picture are Tungsol) with adapter

 

[Marcin Gomulkiewicz]

... the first on the right... that one marked with yellow X



... and I've also changed the trimmer (those horrible single-turn) with a 10-turns... this help when setting the bias... but the real problem is that when you remove the top housing to reach the trimmers and then position it back the temperature inside change and the output voltage too... so I've done a DIY top-housing to... 1) reduce the internal temperature that is good for components life time and... 2) have access with ceramic screw driver to the trimmers keeping the temperature inside the unit more stable... it helps

Great, thanks!

 

[Marcin Gomulkiewicz]

Done All fine.

BTW. @lupoal what trimpots did you use? I'm thinking of making similar change myself, those original pots are awful...

 

[SolarCetacean]

I just replaced the stock Electro-Harmonix tubes in my 006tS with a pair of NOS RCA Clear Tops. I have to say, I dislike how the right channel TVR1 pot is right next to the volume pot and under a tangle of wires. It's a much bigger hassle to adjust it than the other three trim pots. The new tubes will probably need a few hundred more hours of burn in, as they've been powered on for only about 4 hours so far, but I think the bass is a little more prominent with them. Maybe a bit more dynamics overall, but I'm not too sure of that yet. This 006tS has the CCS mod applied, so tube changes probably make less of a difference than with a stock amp, but the replacement tubes are certainly not any worse than the stock ones.

 

[lupoal]

... in the meantime I kept racking my brains about that transformer screwed onto the pcb, the same pcb on which are the valves that we know would be better not to undergo vibrations that certainly go to diminish the quaintness of the sound (as also on non-valve electronics)...

I did a first experiment by putting three cones made of synthetic/ceramic material (these are the ones used in metal cleaning machines...cost a few cents each) getting some benefits, especially in the low register, and this confirmed that even in this case the transformer vibrates (as they all ALWAYS do) and good it does not do to the sound...

one solution would be to elastically decouple the transformer from the pcb, which is not difficult but not immediate and, above all, requires the purchase of excessive amounts of material that would then remain in my house for life...

drumming with my fingers, I realized that the bottom sheet metal of the cabinet resonates like the skin of a drum ... and that sheet metal is directly coupled to the pcb through hexagonal studs ... and in the basement I had leftovers from an experiment I did a while back ... so ... I applied some STP Gold, Dynamat Extreme equivalent, to the inside of the bottom sheet...

the result?
I can't express an absolute opinion that mod should be compared in parallel with a non-modded machine... and I can't rely completely on the acoustic memory which we all know to be fleeting... but, without falling into easy enthusiasm, I found... slightly wider and deeper sound image... improved intelligibility of low registers (percussion in classical orchestral music)... better ability to control full orchestral sounds than the image that no longer tends to collapse slightly...no noticed negatives such as drier sound with loss of harmonics or others

in summary... an easy, cheap, effective, easily reversible modification... I'm happy and I'll keep it in place

p.s.: I'm using Sylvania 6SN7GTB that are taller and heavier then the standard tubes... but imho sounds much better... so maybe that's also why vibration damping gives such effective results