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Old 20th Oct 2020, 2:00 pm   #21
paulsherwin
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Default Re: Taylor 45C Grid Volt Calibration

Thread reopened as requested.
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Old 20th Oct 2020, 7:08 pm   #22
aviv_zl
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Default Re: Taylor 45C Grid Volt Calibration

Thank for re-opening the thread.

I'm in the middle of testing and calibration process of my Taylor 45D. The principles of operation are similar to those of the 45C and I could find many similarities to what I face with my 45D.
Unfortunately, there is no formal testing/calibration procedure to follow. I will open a dedicated thread to discuss my other findings and to get some tips.

With respect to the specific topic of grid voltage as presented here- Both Taylor 45C and 45D apply the grid voltage in a similar manner:
A dedicated grid winding in the main transformer is used to generate the grid voltage. The positive part of the sine (via series resistor) is clipped off by a stopper diode(s). Then the negative half-wave rectified wave goes through a voltage divider- a graduated linear potentiometer which sets the grid voltage. Then, via an additional resistor to the grid pin.

Below I attached the simplified schematics of the 45C and the 45D which tell the same story.

It is quite obvious that the voltage applied to the grid is not a regulated voltage. However, I would expect that the grid current will be low enough to avoid change of the grid voltage when the anode current changes. This turns out not to be the case…

The Taylor 45D has a calibrated current scale for emission testing. When you depress the test button, a series resistor is bypassed and the shunted panel meter is connected to the cathode. I observed there is a significant voltage change in the grid voltage (~0.9V for low grid voltages and more for higher voltages) depending on the anode current. This seems quite weird to me, as the tester does not actually “knows” which tube is tested and therefore cannot “adjust the reading” based on the drop in the grid voltage (the tube selector just switches socket pins to the right connections).

I wonder what is the idea behind this design and whether something is wrong with my tester. Any thoughts are welcome.

Cheers,
Aviv.
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Old 20th Oct 2020, 7:47 pm   #23
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Default Re: Taylor 45C Grid Volt Calibration

Looking at the picture the upper circuit diagram shows "Mutual conductance" and the bottom one "Anode current measurement" - are you shure that you are comparing the correct circuit diagrams?
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Old 20th Oct 2020, 8:21 pm   #24
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Default Re: Taylor 45C Grid Volt Calibration

They have very different ideas, the 45c is ten years older than the 45D
The Anode current measurement is an addition to the 45D
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Old 20th Oct 2020, 8:22 pm   #25
aviv_zl
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Default Re: Taylor 45C Grid Volt Calibration

Hi Martin,

The circuits for anode current and mutual conductance are quite similar. The grid voltage circuit is the same for both tests. Just for the completness, I'm adding the mutual conductance diagram of the 45D.
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Old 20th Oct 2020, 8:25 pm   #26
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Default Re: Taylor 45C Grid Volt Calibration

And the grid winding is 30volt in the 45D and 20 in the 45C
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Old 21st Oct 2020, 9:28 am   #27
aviv_zl
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Default Re: Taylor 45C Grid Volt Calibration

the focus of my question is the arrangement with the un-regulated grid voltage. I don't suggest that the 45C and the 45D are the same (they are not), but the grid voltage is controlled in a similar manner.

As the anode current is controlled by the grid voltage, having an unstable grid voltage, which changes with the anode current, seems quite wierd to me. On the other hand, I'm quite sure that the engineers who designed this tester had a decent understanding in tube testing. So I wonder what do I miss.

Cheers,
Aviv.
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Old 21st Oct 2020, 10:15 am   #28
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Default Re: Taylor 45C Grid Volt Calibration

Where are you measuring the grid voltage?

Lawrence.
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Old 21st Oct 2020, 10:26 am   #29
aviv_zl
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Default Re: Taylor 45C Grid Volt Calibration

Right at the grid banana jack in the top panel. The jack is conected directly to the grid pin in the tube socket (once the selsction wheels are set correctly), ralative to the cathode.

Aviv.
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Old 21st Oct 2020, 10:45 am   #30
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Default Re: Taylor 45C Grid Volt Calibration

When you press the button you short the resistors in the circuit which means that the voltage you measure from grid to cathode will change, you are removing the voltage drop that those resistors develop from the current flowing through them, that is part of the design of the circuit to make it possible to do the measurements correctly.
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Old 21st Oct 2020, 4:34 pm   #31
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Default Re: Taylor 45C Grid Volt Calibration

Good point Martin, you are obviously right. I missed that point. I will check later the correctness of the voltages when the switch is depressed. I will open a dedicated thread to discuss further aspects.

Thank you.
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Old 26th Mar 2021, 7:16 pm   #32
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Default Re: Taylor 45C Grid Volt Calibration

Trying to get my head around the grid bias voltages, i have replaced the worn out 2K pot albeit a bit of overkill with a 3W one but at least the taper is very linear!

Should the dial pointer ideally correspond to the Vpp reading and have a range of 0 - 15Vpp ?
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Old 27th Mar 2021, 2:41 pm   #33
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Default Re: Taylor 45C Grid Volt Calibration

Quote:
Originally Posted by Phil-G0SXY View Post
...........

Should the dial pointer ideally correspond to the Vpp reading and have a range of 0 - 15Vpp ?
No, I don't think so. The grid volt trace is of peculiar shape - random trace shown.

It is important to make sure Vpp is zero with the pointer at zero and that the pointer can travel back a little past that point. Obviously all other components should be within Taylor specs.

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Old 27th Mar 2021, 5:31 pm   #34
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Default Re: Taylor 45C Grid Volt Calibration

Thanks, the voltages I have measured are listed below, these are p-p and include the small +ve (mV) from the diode clipper, I have used a silicon diode as the original metal one was conducting both ways.

Dial Vpp
1 = 1.26
2 = 2.55
5 = 6.57
10 = 13.5
13 = 17.6
15 = 19.9

Do these look ok?
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Old 28th Mar 2021, 8:27 am   #35
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Default Re: Taylor 45C Grid Volt Calibration

The lack of original Taylor voltage specifications is annoying but you will see from previous posts that there are variations between 45Cs.

Maybe the use of a silicon diode has increased your voltages but I would have thought your results would render the instrument to be satisfactory in operation as long as it has been zeroed correctly. Many valves have to be set at grid 0V.

I find the Taylor 45C to be really handy but it's hardly a scientific instrument and users just have to get familiar with the results it provides. In my case I have purchased some popular new reference valves which generally correspond with data provided in the Taylor valve charts. I suspect some of the data in the Taylor valve charts to be erroneous and where possible I compare results to good known valves.

Happy Tayloring!
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Old 28th Mar 2021, 11:44 am   #36
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Default Re: Taylor 45C Grid Volt Calibration

Many thanks, my unit is actually the 45a from 1944 so its basically the same circuit.

I'm not expecting perfect results on this as long as its usable and gives some indication then i'll be happy, in fact i don't have a great valve collection but my eldest brother does so he may find it more useful than I.

It's a project and gives me something to do!
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