Another MoFo

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pre65
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#31 Re: Another MoFo

Post by pre65 »

I've got out my Thurlby 1503 DMM, the 9v power is now supplied by a wall wart, so it's not portable.

Objective is to measure the DCR of the MOTs, so to start I measure a known resistor @ 22R and the meter says 21.9R.

So measuring the MOT I get 2.2R, which is about what I was expecting.

Next step could be to pass a known 3v voltage through the winding ( using a 1.5A bench power supply) to measure the voltage drop, then measure the current flowing (approx 1.4A ?) , then use Ohms law to verify the resistance.
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#32 Re: Another MoFo

Post by pre65 »

Interesting result.

I tested at 3V and 6V and got a calculation of 4R.

I checked the PSU voltage with the Thurlby, OK.

And I re checked the current with a cheapo DMM, OK.

EDIT. IGNORE THE ABOVE, I COCKED UP WITH THE WAY I DID IT. :oops: :oops: :oops:
Last edited by pre65 on Thu Jun 25, 2020 12:26 pm, edited 1 time in total.
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#33 Re: Another MoFo

Post by pre65 »

I'm was confused,but I'm not now. :lol:

I set the power supply for 6 volts and connect to one MOT.

I connect the meter to the same two terminals on the MOT, and it reads 3.36V

Double checked with cheapo DMM.

So it is 2.29R

3.36V @ 1.47A
Last edited by pre65 on Thu Jun 25, 2020 12:28 pm, edited 1 time in total.
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#34 Re: Another MoFo

Post by Nick »

The meter may be getting confused about measuring a inductor.

Sorry, I have to correct this. Its like nails on a blackboard

"Next step could be to apply 3v across the winding ( using a 1.5A bench power supply) then measure the current flowing (approx 1.4A ?) , then use Ohms law to verify the resistance."
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#35 Re: Another MoFo

Post by Nick »

I connect the meter to the same two terminals on the MOT, and it reads 3.36V
The supply may be current limiting.
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#36 Re: Another MoFo

Post by ed »

Paul Barker wrote: Thu Jun 25, 2020 9:40 am I had a quick look at the circuit, well the one my browser found, I’m not in the least bit interested in even beginning to read a threda which exists so extensive because of Willy waving one upmanship by the proud ignorant people lost in the woods.
?
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#37 Re: Another MoFo

Post by pre65 »

3V across inductor = 1.47A

2V across inductor = 0.98A

= 2.04R
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#38 Re: Another MoFo

Post by pre65 »

Do organic film Polyester capacitors degrade with age, or use ?

I have two MPP335K2F, which the internet says are 3.3uf @ 400v. they were removed from a populated PCB someone sent me years ago, but they would do as the MoFo input cap. 2.2uf was specified but it seems 3.3uf or 4.7uf film caps are acceptable.

The output cap is 6800uf, and I can use one of my soup tin size Kemet PEH169QU4680QU2 caps, 6800uf @ 160v. They are very low ESR.Won't fit on the PCB though. :wink:

What size film cap would one use to bypass a 6800uf electrolytic ? Is it 10% of the value ?
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#39 Re: Another MoFo

Post by Nick »

Do organic film Polyester capacitors degrade with age, or use
Shouldn't do. depends on temperature they saw being de-soldered.

Or you could spend £1

https://uk.rs-online.com/web/p/polyeste ... s/6224757/
They are very low ESR.Won't fit on the PCB though.
The flying leads will probably add more ESR than using a part that will fit on the PCB. Or you could use a £1 part with the same ESR that will fit the PCB.

https://uk.rs-online.com/web/p/aluminiu ... s/3150300/
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#40 Re: Another MoFo

Post by Paul Barker »

“ I've just ordered a couple of pairs of transformers from pieter for future projects (he usually takes a while to make them ) so I have all valve based plans“

Ah yes the long wait, know it well.

Thanks for the offer.
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#41 Re: Another MoFo

Post by pre65 »

Nick wrote: Thu Jun 25, 2020 2:15 pm
Do organic film Polyester capacitors degrade with age, or use
Shouldn't do. depends on temperature they saw being de-soldered.

Or you could spend £1
Searching through a box of arcol resistors I came across a bag of capacitors, obviously wrongly filed.

Amongst them were two Mundorf M cap EVO aluminium oil capacitors. Someone on World Designs had bought them new, but not liked the sound and removed them. (without waiting for them to run in) Bob Orbell offered them for nowt, and as there were no takers I snaffled them.

So, I now have two 2.2uf 450v 85C half decent input capacitors (£8:11 from HFC). :D
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#42 Re: Another MoFo

Post by Paul Barker »

I normally go with first sound if I use coupling caps. Maybe they would have maybe got better results. Hope they come good but yore amp may do better with them than his.
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#43 Re: Another MoFo

Post by Ray P »

steve s wrote: Thu Jun 25, 2020 7:58 am
Nick wrote: Wed Jun 24, 2020 9:33 pm

The problem is you often end up with design by democracy, where the ones who have the strongest (but possibly wrong) opinion get heard and the ones that know what to do but don't care if anyone else knows it gets ignored.
Wise words nick..I see that on all forums,

It's usually a surprise when you hear those with strong opinions kit.
Of course, it doesn't happen here...
Sorry, I couldn't resist!
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#44 Re: Another MoFo

Post by pre65 »

I've now ordere the resistors and 6800uf cap (Nichicon FW) from our friend at HiFi Collective.

So, by early next week I should have all the hardware to hand, although I'm still not totally sure about the heatsinks.

With the power supply, is there any benefit in a regulated supply ?
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#45 Re: Another MoFo

Post by Nick »

With the power supply, is there any benefit in a regulated supply ?
For the output stage of a power amp probably not. Quiet and low impedance would be my choice.
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