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Old 13th Nov 2022, 8:09 pm   #1
6SN7WGTB
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Default Precision Attenuator in Valve mVM

I am restoring a Marconi TF2600 'Sensitive Valve Voltmeter'.

It has a step attenuator which is 0dB - 50dB in 10dB increments. Picture below.

(Thee is a valve buffer stage between the input and this attenuator).

It would appear that the resistors are hand wound with nichrome wire on a former, possibly individually to the unit. The values do indeed give a VERY precise attenuator (maximum error is 0.03dB at 'worst' step).

One was found to be broken pretty much at solder joint between fine wire and thick lead in wire. Repaired albeit a tiny bit shorter!

The unit is specified as 1% of FSD 50Hz - 500kHz, and then up to 5% of FSD at up to 5MHz.

Two questions:
1. Why could I not replace these with low temp coefficient, tight tolerance metal film resistors? (I can obtain values which will result in at least the same if not tighter accuracy nowadays).
2. Wouldn't the latter be better suited as opposed to being wire-wound?
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Old 15th Nov 2022, 12:59 am   #2
karesz*
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Default Re: Precision Attenuator in Valve mVM

Richard,
They are usualy bifilar wounded Manganin-wire resistors.
If you wish replacements-would need special "RF-types" to find, but they are typical only up to ca 500 Ohm to find...
K.
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Old 15th Nov 2022, 7:38 am   #3
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Default Re: Precision Attenuator in Valve mVM

I think you already know Richard, they are non inductive. ( The origional horse and cart versions ).

I like new metal film resistors too, VERY easy to "dial in perfect " strategy.

IF you just want it for pure DC, new reistors will be fine.
Bit like a crystal set!!. GREAT at long waves ( never seen or heard them in Australia ) and for AM. so up to 1 1/2 megs or so, bees knees.
I wooden trust it about 1.5 megs with inductive modern MF types.

karesz* I think has it in one.

Joe

Last edited by joebog1; 15th Nov 2022 at 7:38 am. Reason: spello's
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Old 15th Nov 2022, 9:36 am   #4
6SN7WGTB
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Default Re: Precision Attenuator in Valve mVM

I shouldn't probably have said nichrome as that would be a poor choice thinking about it.

Ah, didn't realise Manganin was so low in temp coeff - looking it up it's 1.5E-05 Ω/Ω/degC (15ppm/degC), although where I find production Manganin resistors, they seem to quote 20-40ppm/degC.

However the wires are either bare, or completely tinned - which didn't immediately suggest Manganin?

The MF ones I was considering are 50ppm, so not so good, although probably not too material here.

I'm more interested in the frequency/inductive issues as I'd like to keep the instrument within spec.

Also I didn't intuitively expect a wire wound on a former to be low inductance.
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Old 15th Nov 2022, 9:23 pm   #5
Ed_Dinning
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Default Re: Precision Attenuator in Valve mVM

Hi Richard, I may have some Manganin or similar resistance wire, silk covered I think.
If you have some idea of the diameter / ohms/foot figures I might be able to find you some

Ed
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Old 16th Nov 2022, 1:28 pm   #6
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Default Re: Precision Attenuator in Valve mVM

Quote:
Originally Posted by Ed_Dinning View Post
Hi Richard, I may have some Manganin or similar resistance wire, silk covered I think.
If you have some idea of the diameter / ohms/foot figures I might be able to find you some

Ed
Thank you for the offer Ed - most thoughtful.

However, having now measured, tested and calibrated, all is well so may live with it as is.I suspect I might end up chasing phantom accuracy with an analogue meter.

Take a look at the attached table - this compares the schematic values with as written on padders, actual measured and best available match in MF.

IF the hand-wound pads were EXACTLY the resistances they are supposed to be, the accuracy would be quite impressive...
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Old 17th Nov 2022, 9:41 am   #7
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Default Re: Precision Attenuator in Valve mVM

Hello Richard,
wich kind of resistance meter did you applied, than for real comparison you must have a practical precision of 0,03-0,01% with good contacting (ie Kelvin), but bridges have usual pecision only at 1%...
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Old 17th Nov 2022, 9:50 am   #8
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Default Re: Precision Attenuator in Valve mVM

Agilent 34410A with Kelvin, calibration certificate for 4W was worst at 100R - 0.0007%.
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Old 18th Nov 2022, 12:15 am   #9
karesz*
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Default Re: Precision Attenuator in Valve mVM

Quote:
Originally Posted by 6SN7WGTB View Post
Agilent 34410A with Kelvin, calibration certificate for 4W was worst at 100R - 0.0007%.
Your meter is a fine thing and really good enough for these problem, but I can see accuacy on 100R & 1Kohm ranges only at 0,007% on hes datasheet, than 0,0007% is for Tc pro Celsius....
https://www.keysight.com/us/en/asset.../5989-3738.pdf
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Old 18th Nov 2022, 8:49 am   #10
6SN7WGTB
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Default Re: Precision Attenuator in Valve mVM

Quote:
Originally Posted by karesz* View Post
Quote:
Originally Posted by 6SN7WGTB View Post
Agilent 34410A with Kelvin, calibration certificate for 4W was worst at 100R - 0.0007%.
Your meter is a fine thing and really good enough for these problem, but I can see accuacy on 100R & 1Kohm ranges only at 0,007% on hes datasheet, than 0,0007% is for Tc pro Celsius....
https://www.keysight.com/us/en/asset.../5989-3738.pdf
I was referring to the actual calibration certificate I have for the meter, not the manufacturer's specification.
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