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Old 23rd Mar 2008, 9:20 pm   #1
stephen
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Default Taylor 45D grid volts

Hi - i though i would check how accurate the grid volt setting was on my Taylor 45D. Measuring from green grid socket to black cathode socket on top panel i get around 3 volts DC when the grid control is set to 10volts. I measured this twice - once with an avo mk8 then again with an ancient VTVM and got similar figures. Why is this - the valve tester seems fine. I checked the filament volts and they are correct and the voltage selector is in the right setting on the back. Thanks.
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Old 23rd Mar 2008, 10:45 pm   #2
igranic
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Default Re: Taylor 45D grid volts

Stephen

The strange readings are the result of the 45d, in common with its predecessors (45c, 45b), using a half-wave AC supply for Grid voltage, and a full-wave AC supply for Anode voltage. The winding which supplies Grid Voltage is arrange to be in anti-phase to the windings which supply the Anode and Screen.

The following diagram from the manual illustrates the circuit used.

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As you can see, the Grid Voltage supply from the 30V transformer winding is current limited by R20 then shunt rectified by the diodes MR2.

In effect, the valve under test is acting as a grid-controlled rectifier, and as the Anode Voltage swings positive with respect to cathode (and current commences to flow), the grid swings negative to control the anode current. Both the grid volts control, and the meter (which can be set to display either mutual conductance or anode current) are calibrated so as to take acount of the half wave operation. Trying to measure the half-wave grid voltage with a DC voltmeter will inevitably give a misleading reading.
As it is half-wave AC, then 10 volts on the control should result in a DC average reading at the grid of around 3.5 volts less the forward drop of the two diodes, so 3 volts would be a near-enough correct reading.

A detailed description of the theory of operation is given in the user manual.

HTH

Edward

Last edited by igranic; 23rd Mar 2008 at 10:57 pm.
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Old 24th Mar 2008, 6:12 pm   #3
stephen
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Default Re: Taylor 45D grid volts

Thank you Edward - that makes sense. What does the wire wound open slider resistor do that is mounted on the circuit board? Regards Stephen
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Old 24th Mar 2008, 7:13 pm   #4
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Default Re: Taylor 45D grid volts

Stephen,

The wirewound preset (RV19) is the meter calibration control for the Anode Current tests. AFAIK it has no effect on other test functions.

Edward
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Old 24th Mar 2008, 7:36 pm   #5
stephen
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Default Re: Taylor 45D grid volts

I've just been playing with the anode current test and i'm confused. I have a good ECC83 and a poor ECC83. Pluggin gin the good ECC83 and knob set up for 10ma the meter climbs to 1.8 and stabilises before pressing the meter test button then to increases to 4 with button pressed. With the poor ECC83 meter climbs to 0.8 before pressing button then goes up to 1.3 on pressing button. Do you read Ia by taking the before reading from the button pressed reading so these are 2.2 and 0.5? Regards Stephen
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Old 24th Mar 2008, 7:48 pm   #6
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Default Re: Taylor 45D grid volts

Stephen,

Do you have a copy of the Operating Instructions?

Paragraph 17, on page 15, details the methode of measuring anode current.

Edward
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Old 24th Mar 2008, 8:06 pm   #7
stephen
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Default Re: Taylor 45D grid volts

Yes i have the op instructions and i did my best to follow it!
After completing mutual conductance i moved the red test selector knob from gm to 10ma and the meter climbed from zero to 1.8 in case of good valve then rose to 4 in case of good valve when the meter test button is pressed or 0.8 then 1.3 in case of bad valve.
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Old 24th Mar 2008, 9:41 pm   #8
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Default Re: Taylor 45D grid volts

Stephen,

Then the anode current at which the mutual conductance measurement was taken was approximately 4mA and 1.8mA in the good and bad valves respectively. Although the 45d is not as sophisticated as the AVO Valve Characteristic Meters, it is, with care, possible to plot basic valve characteristic curves using it, or even the 45c.

If you are not familiar with valve theory, or electronic theory in general, or would like a better understanding, then I can thoroughly recommend the book "Foundations of Wireless" by M G Scroggie, preferably the 1958 seventh edition, or slightly later. I often refer to my copy in order to refresh my memory of stuff that I learnt thirty or forty years ago. Copies of the book are often available for a few pounds from stallholders at the various BVWS events, amateur radio rallies, radiojumbles, Amazon, etc.

Edward
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