nuvistors

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Mike H
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#31

Post by Mike H »

Dave the bass wrote:Congratulations to all the Cresswells :-)

DTB
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Ali Tait
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#32

Post by Ali Tait »

Seconded!
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Greg
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#33

Post by Greg »

Yes, from me as well!
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cressy
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#34

Post by cressy »

having finished the 6s4s amp finally and it not having exploded, caught fire or done anything it shouldnt, thoughts have now again turned to this phono stage.

now, the question is, how to implement it. the cheap ebay one is to put it simply, not up to scratch. i thought about replacing its components with nice quality ones but then thought whats the point.

i was thinking about the best way to go about it might be to build it in modules, i.e a left channel, right channel and psu all separate to keep hum to a minimum, with the sowter suts in each channels 'box'. but then came back to the problem of hum pickup via the psu umbilicals. i dont really have space to do a resaonably large box and was worried about the proximity of the power supply to the circuit in this case.

any suggestions on layout? 3 box 2 box or single chassis?

cheers ant
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Nick
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#35

Post by Nick »

Two box with final power supply smoothing/regulation in the pre box.
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Mike H
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#36

Post by Mike H »

I zink zo too


 
 
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cressy
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#37

Post by cressy »

well i finally got the nuvistor phono stage built, i built the circuit up on veroboard, used one of mikes psu boards for the psu caps and shoved it all in the case i made for that 6sn7 preamp. gain is abit better than the angle audio phono ive been using and the sound is on a different level altogether. there is very little hum and abit of hiss, both of which are inaudible where i sit to listen.

i ended up using the 2ds4 nuvistors paul gave me at owston instead of the 6ds4s id intended on. the heaters are supposed to be 2.1v but have 2.5v on them as i had a 2.5v heater tx intended for a 2a3. not sure what that'll do in the long run though.

theres a dry joint in the power supply somewhere i need to chase down though as after about 20 mins there was a horrid noise on both channels simultaneously. switching it off quickly thinking something had burnt out, i couldnt see anything scorched or feel anything hot, and after switching it back on once it'd cooled down it worked perfectly again.....

the biggest problem was finding the bases for the nuvistors, only place i could find them ws langrex. there was a guy selling them on ebay but theve disappeared now.

the 300k resistor at the front needs an 18 meg in parallel with in to make the required 295k but i havent got any physically small enough to add to the board so if anyone has any please let me know.

cheers to andrew and mike for the circuit and to paul for the 2ds4s.

took a few years but got threre in the end :D
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cressy
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#38

Post by cressy »

i seem to have cured the dry joint issue by reflowing all the solder joints on the psu cap board i got off mike. i also added a 150k bleed resistor as i noticed id missed it for some reason :roll:

it does sound better after this so the problem must have been on that board.

out of interest, what sort of components should i try out on the board if i want to put in some nice ones? all the resistors are maplins, the 10 and 100 nf caps are some tropical fish jobbies, the 820pf is a lemco and the 47 is a ceramic disc cap iirc. i got the lot off jh components rather than looking for anything special.

cheers guys
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cressy
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#39

Post by cressy »

a question from the back of the class, i want to reconfigure the power supply for the phono from cap input to choke input. the reason being that the psu puts out 340v out after the last cap, so to drop the voltage to a workable figure theres a 5k resistor in series that drops it to 222v. this gives about 100v at the anode of the nuvistor which is a little down on the 112v from the original circuit, but is fine.
however this resistor gets bloody hot and there are intermittent bursts of noise from the damned thing. the 5k unfortunately is right next to the last cap and i think its heating it up too much. the resistor cant be moved and neither can the cap board.

if i reconfigure the psu to choke input the volts will drop sufficiently to dispense with the 5k, but reading on the subject of chokes i note that at lower currents the inductance can drop significantly. the choke is a 6h 200ma one but the phono runs at around 20ma so i suppose the inductance will end up negligable and the volts will shoot back up. i remember reading that adding resistance will keep the choke regulating by making it draw at least the minimum amount of current it needs to keep it regulating.
can anyone point me in the direction of some suitable reading to get a handle on this?
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Nick
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#40

Post by Nick »

but reading on the subject of chokes i note that at lower currents the inductance can drop significantly. the choke is a 6h 200ma one but the phono runs at around 20ma so i suppose the inductance will end up negligable and the volts will shoot back up.
Not sure where you read that, but its wrong. The reason the voltage shoots up when the current draw is less than the current required for choke input, the critical current, is because the DC current in the choke is less than the AC current in the choke, so the low point of the AC current hits the zero current point, so current flow is not constant, so its not choke input. Nothing to do with the inductance of the choke varying.

What may be the source of the misinformation is that "swinging chokes" were used that were designed to have much higher inductance at low current, so at low current the system remains in choke input.

Simple rule of thumb to calculate the critical inductance, divide the input voltage by the inductance to give the current in ma. So with 250v (assuming you get 340v with cap input) and 6H, the minimum current is 42ma. So after the choke the voltage will be 225v, to pull 42ma a 5k resistor will do the job. But it will be dissapating 10w, so you may not see any improvement in heat output. It depends on how much current the phono stage draws, as you can subtract that from the 42ma. But remember that before that current is drawn (valves heat up) the current will be less than critical, and the voltage will be back to the 350v, so caps and everything else has to be happy with that startup voltage.

Also remember the current the choke has to pass is not just the DC current that is output from the supply, it is also shorting the AC to ground via the following cap, so it has to also handle the AC current given by the supplid voltage across its reactance at 100Hz
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cressy
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#41

Post by cressy »

cheers nick, i can understand that explanation :D so as it is at the moment there isnt really anything to be gained in terms of heat output by reconfiguring in this way. a more reasonable solution would be to find a way to move the 5k dropper to somewhere other than smack next to the caps it is heating up.

looking through my box of oddments i found a 6k8 aluminium clad resistor and some 5w 15k resistors. those in parallel would give 4k7 rather than 5k. close enough, and would run cooler than the string of 1k 3w ones thats in there at the moment. i could mount these on the top plate inside the transformer cover out of the way of the rest of the psu.

worth trying before rebuilding the power supply
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cressy
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#42

Post by cressy »

i found a 50w 4k7 resistor kicking around and used that instead. shifted it right to the other side if the case and coupled it up with flying leads. it didnt miss a beat through the 2 lps i played since, wheras it was barely getting through 1 side before making stupid noises.

the 4k7 increased the supply volts sligtly to 230vdc so abit closer to the specced 247v. im not going to bother with any more messing for now unless theres another problem. the resistor still gets too hot to touch though so i might see if i can get a decent heatsink.

i might be able to get a decent impression of its sound too now :roll:
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#43

Post by colin.hepburn »

Mike H wrote:
Dave the bass wrote:Congratulations to all the Cresswells :-)

DTB
Ditto!! Image

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Yep Congratulations...
Colin
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cressy
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#44

Post by cressy »

changing out the 5k dropper for the 50w 4k7 seems to have done the trick, last night i played about half a dozen lps and it didnt do anything stupid. i am still concerned about the heat, but it seems to have settled down ok. i think i'll build a case specifically for it as the ventilation isnt great with the preamp case.

onto my impressions of its sound, the treble really is brilliant, much much better than the ss one i was using before, as is the mid. bass isnt much better than the ss one though. a little more texture to it but its a bit thin at the bottom. imaging isnt as good as the ss one, it doesnt really extend beyond the speakers but is solid enough.

however for me as soon as a cymbal is struck the weaknesses dissolve away.

its undoubtedly coloured, but not so you are bothered by it. probably down to component choice rather than anything else

im quite pleased with it :D

i think i'll have to come up with some casework thats abit special for it.
thanks again to andrew, mike and everyone else who helped design it and to paul for the nuvistors themselves.
cheers guys
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#45

Post by Andrew »

The imaging should be helped by making sure the components in the RIAA are matched, in fact its better they are a bit off the required value but by the same amount rather than either side of the ideal which would mean a larger difference between them.

You could also trying matching he nuvistors, if you have a few to play with.

Andrew
Last edited by Andrew on Tue Sep 06, 2011 10:40 am, edited 1 time in total.
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