Amp initial testing.

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Cressy Snr
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#16

Post by Cressy Snr »

I could also add that if there is a short across the mains side, then that might explain why the heater TXs are reading negligible resistance. They would be shorted out by the wiring fault so the current from your meter would be bypassing the heater TXs completely. So you would not be measuring what you thought you were measuring.

Steve
Last edited by Cressy Snr on Mon Sep 17, 2007 11:03 pm, edited 1 time in total.
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andrew Ivimey
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#17

Post by andrew Ivimey »

Blimey Philip, I'm sorry to hear this. How about starting from scratch again. Forget the variac or go back and test it on the known, briefly just in case it is Kaput i.e on the turntable.

Disconnect everything from a mains transformer, switch that on and measure the voltages. Use any 6 volt heater valve and see that enough current is going through it to shine it up. Those mains trafos are hefty buggers and could take a bit of abuse.

If this and the voltages check then the Tx should be fine. Check the other one.

Back in circuit, only wire up the heaters and check for 'shine up'. Once that is okay, you must rigorously check the circuit for short circuits. There is only so many mistakes you can make and I guess there is one quite obvious one.

This is the sad and depressing part of these bloody valve amplifiers, but when you find the mistake and after a good laugh at yourself and a cup of tea, it feels dead good to do it right. I know this feeling (appropriate smiley).
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Nick
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#18

Post by Nick »

I could also add that if there is a short across the mains side, then that might explain why the heater TXs are reading negligible resistance. They would be shorted out by the wiring fault so the current from your meter would be bypassing the heater TXs completely. So you would not be measuring what you thought you were measuring.
Hm, it doesn;t work like that, if the primary are shorted, it shuldn't make any difference to the DC resistance of the secondaries. It certainly will to the AC behavour but not what the DC of the meter is using to test.

If the meter zero is 0.4 ohm, it may well be that the meter isn't able to resolve the DC resistance of the 6.3v secondary, which may only be 20 or so turns.
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Cressy Snr
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#19

Post by Cressy Snr »

Nick wrote:
I could also add that if there is a short across the mains side, then that might explain why the heater TXs are reading negligible resistance. They would be shorted out by the wiring fault so the current from your meter would be bypassing the heater TXs completely. So you would not be measuring what you thought you were measuring.
Hm, it doesn;t work like that, if the primary are shorted, it shuldn't make any difference to the DC resistance of the secondaries. It certainly will to the AC behavour but not what the DC of the meter is using to test.

If the meter zero is 0.4 ohm, it may well be that the meter isn't able to resolve the DC resistance of the 6.3v secondary, which may only be 20 or so turns.
Yes I didn't read it properly. I thought he was measuring the primaries of the heater txs. Silly me. :oops:
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andrew Ivimey
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#20

Post by andrew Ivimey »

That's why going back to the beginning and actually using a valve for the heaters before going on to other parts of the circuit may be reassuring.

Given that your meter can still let you down.

good luck

How about hopping into bed and getting up tomorrow, have a really good breakfast and then get back into it.

I've just read this - I really didn't mean to sound patronising argumentative or simplistic.

In such a situation I'd go back to the beginning, even checking the mains wire.

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ed
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#21

Post by ed »

andrew Ivimey wrote:Blimey Philip, I'm sorry to hear this. How about starting from scratch again. Forget the variac or go back and test it on the known, briefly just in case it is Kaput i.e on the turntable.
If the variac is producing 240v as Philip has indicated then this doesn't sound like a wise move...

just a thought
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pre65
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#22

Post by pre65 »

Hi Gents-its getting late but i have found on one amp i had the white/yellow joined wires for the centre tap to cock,and being earthed i suppose was shorting out one winding-DOH !

I will check t'other one in the morning,i hope in a way its the same problem.

I am most thankful for the suggestions given,and will report back in the morning.

I did so want to get these working for the Fest.
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andrew Ivimey
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#23

Post by andrew Ivimey »

well testing the previously 'known' might be.

perhaps I should have stressed 'briefly'.

BUT if the meter clearly shows 240volts on the variac no matter where the dial is (and the local mains voltage is 240volts) then it does rather look like Philip has a new doorstop and I then agree heartily with your caveat.
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Dave the bass
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#24

Post by Dave the bass »

:idea: This is a torroidal TX. They have a big inrush current I seem to remember from college days, it's not that is it thats causing the variac to burn up?

Also, its very important on a Torroidal not to have any earth links that go from top to bottom of the windings over the outside, I think it's summink to do with the construction of them and they see an external earth as a shorted turn or summink, just an idea.

I'd forget about the variac and just power the TX up with the pri's in series away from the amp and see what the results are. I'd wnt to prove the TX is OK or not before looking any further, break the fault down into sections and prove each is OK independantly. Just IMHO n all that.

I got your e-mail Philip, I'll measure when i get home tonight. DHTskis back home in anticipation of WF3.

Good luck with the fault finding, sorry to hear you're having a beast of a time.

DTB
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pre65
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#25

Post by pre65 »

Hi-well refreshed from a nights sleep i've gone back to the amp with my brayne engaged and have found,on one amp

I HAD BOTH THE PRIMARYS AND THE 360V SECONDARYS WRONGLY CONNECTED.

What a dick !

I feel such a fool,i thought i understood transformers,well i do now !!

So now with a sort of working variac i have power on the heater taps.

Now a question-is it bad practice to parallel heater windings ?


Mr Duncehead
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andrew Ivimey
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#26

Post by andrew Ivimey »

Surely (and I know your name's not Shirley - no it doesn't work like in the film) you parallel heater windings only if you need the extra current.

Good news about the 'obvious mistake'. Be cunning! be careful!
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Dave the bass
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#27

Post by Dave the bass »

Wahey. Congrats on finding that fault Philip, self made are always the hardest to spot/find.

On DHTSki I was advised to insulate the spare heater winding, but then I didn't need the extra current capacity.

DTB
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Nick
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#28

Post by Nick »

Well done :-)
Now a question-is it bad practice to parallel heater windings ?
No, not entirly. it depends (as always), there is no reason not to do so if you need the extra current (as Dave says), but if you do it when you don;t need the current, the lower load may mean your expecetd 6.3v will be higher than you want/expect.
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pre65
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#29

Post by pre65 »

Hi-on the second amp i ONLY had the 360V secondarys wired up wrong !
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andrew Ivimey
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#30

Post by andrew Ivimey »

howls of derisive, Philip!

Go and wash your head in a bucket of water.

Really, keep it up, you are getting there! :D
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