nuvistors

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cressy
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#61

Post by cressy »

yep the tx is wired ok, i didnt touch the wiring when i lifted it out of the other case, and the meter was working fine before ali,
something tripped the rcd and knocked the sockets off when i was testing it, but when i switched the sockets back on they didnt trip when i tested it again.
still baffled :roll: :(
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cressy
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#62

Post by cressy »

aha, sorted it, i had a dry joint on the centre tap, reflowed it and i now get 215v on the rail. checked the voltages on the anodes and discovered that the second stage valve on the left channel want getting anything. perhaps that why the imaging was lopsided. i shall change it
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#63

Post by Mike H »

Never dull this lark is it :D
 
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#64

Post by cressy »

nope :D wonder if the anode resistor is duff or wether thats a dry joint :roll:
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#65

Post by Cressy Snr »

Don't unplug any phonos whilst it is on. Check that it functions without any connections to other equipment.

The earth continuity MUST be sound from the 0V busbar and where the centre tap is grounded
to the mains earth pin of the plug, with the unit isolated from other gear. Test this with the plug disconnected from the mains.
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#66

Post by cressy »

i managed to get one channel working and found a 1/2w 18k resistor in the bottom of the spares box. trouble is, the right channel doesnt work still even with a replacement resistor in :roll:

im getting 87v apiece on the first stage nuvistors anodes, on the 2nd stage ones im getting 90v on the left hand channel and 1v on the right

any ideas on problem 2?
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#67

Post by Mike H »

Power not getting through to #2, right?

Or, summink is shorting anode to ground.

Or, no negative bias, cathode resistor is zero / shorted to ground so valve is full-on ... :?:
 
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#68

Post by Mike H »

When I wrote "right", I meant of course, righthand channel, not "right mush?" and other suchlike blokey jocularities :D
 
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#69

Post by cressy »

well...................................
i need to give my head a rattle for not spotting the problem yesterday, i just looked at it now and in 10 seconds spotted the problem.

the heater pin was bent over at an angle and was touching the anode pin so it was shorting out . pins were bent back into place and rechecked the heater voltages with the rectifier out. these are fine, so shoved the rect back in and there are now the correct voltages where they should be.

wonder if the nuvistor will have blown :roll:

suppose i better plug it into the amp :tool:
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#70

Post by cressy »

nope it hasnt blown :D
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#71

Post by Mike H »

Well done, it often happens something won't show up until next day despite any amount of looking at it first time :D

Quite often I stop for the day after a wiring up session, leaving the final double-checks until the next day before trying to fire it up, for that reason.
 
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#72

Post by cressy »

stupid question, but how do i work out the optimum size for the output cap?

after a conversation with mi old man about the phono and its sound the consensus was that it was bass light. the finger was pointed at the tiny 0.22 uf output cap and the lack of a resistor to earth across the output.

so ive modded it to have a 560k across the output ( biggest i could find in the spares box) and some 3.3uf stc caps id for speaker crossovers.

its now not so bass light, and i also got round to adding the 18meg resistors in parallel with the 300k resistors to make up the required 295k so the correction has been err corrected.

mi old man estimated that it would be rolled off at around 30hz or so with the 3.3 uf in but i dont know how to calculate it in the first place

help would be appreciated in pointing me in the right direction to find out how to work this out.

cheers ant
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#73

Post by pre65 »

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G-Popz THE easy listening connoisseur. (Philip)
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cressy
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#74

Post by cressy »

cheers phil it appears that by accident the 3.3uf cap value gives an f3 db point at 20hz. i didnt want to go any lower than that anyway due to warps. the calculator also told me the f3 point with the 0.22uf cap in was 280hz :roll: no wonder it was bass light.

that site led me on to quite a few pages about corner frequencies and other good stuff so i think i'll do some required reading later. i really need to start printing stuff out to read, i always forget where i find info on't web
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#75

Post by Cressy Snr »

cressy wrote:cheers phil it appears that by accident the 3.3uf cap value gives an f3 db point at 20hz.
Well I estimated about 30Hz so that wasn't a bad guess :)
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