Valve Itch phono

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Should cause less than 1% additional error with 600 ohm generator source impedance.. LOL Were I concerned I would select the closest resistor for the 50 ohm case, and select one that is about 0.5% or so low for the 600 ohm case.

If mu follower just measure at output capacitor of the stage before the RIAA EQ.. I remember the valve itch has a mu-follower first stage.

The key is not to change anything else but that resistor between the first and second measurement!
 
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There is actually a coupling capacitor only after the passive RIAA in this but he could set his scope to AC input so its creating a coupling and measure at the top section's cathode follower output. To get more HF level easier there if careful with some HV at that node.

So after proactively clarifying all possible mysteries for Merlin, he will now be ready for some lab magic :)
 
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Should cause less than 1% additional error with 600 ohm generator source impedance.. LOL Were I concerned I would select the closest resistor for the 50 ohm case, and select one that is about 0.5% or so low for the 600 ohm case.

If mu follower just measure at output capacitor of the stage before the RIAA EQ.. I remember the valve itch has a mu-follower first stage.

The key is not to change anything else but that resistor between the first and second measurement!

I don't have 50 ohm resistor, I have 57 ohms.

I made a voltage divider with 14K in series signal & 14 ohms between signal & ground, could I use to convert my 600 ohm signal generator to 50 ohm?
 
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I used true rms dvm because my scope is old and can't give numbers.

100mVpp= 0.035Vrms output SG
measurement at output capacitor of the stage before the RIAA EQ 1.257Vrms = 3.555Vpp
measurement at output capacitor of the stage before the RIAA EQ (100K inserted) 1.261Vrms = 3.567Vpp

3.567Vpp / 3.555Vpp = 1.00337 log = 0.001463 x 20 = 0.0297

Used directly the 600R output of generator.
 

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diyAudio Chief Moderator
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Your generator is 600 Ohm. But that only adds 1% error to Kevin's look up table. You set it at 100kHz 100mV sinewave. You remove the 47k Itch's load resistor (no load). You measure ACV between V3 cathode and GND (V1.2 Itch schematic, prefer oscilloscope). Then you put 100k in series to input. You measure again. You translate how lower in dB. You look Kevin's table to find approximate pF Miller.
 
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100mVpp= 0.035Vrms output voltage divider
measurement at output capacitor of the stage before the RIAA EQ 1.240Vrms = 3.507Vpp
measurement at output capacitor of the stage before the RIAA EQ (100K inserted) 1.243Vrms = 3.516Vpp

3.516Vpp / 3.507Vpp = 1.00256 log = 3.173 x 20 = 0.0634

Used voltage divider with 14K in series signal & 14 ohms between signal & ground and put between the signal generator & the Itch
 
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Yeah the numbers are wrong. Inserting the 100K resistor will result in a much smaller voltage reading.

The meter reads in rms volts which doesn't matter - what matters is the ratio which should always result in a value of < = 1.0, the more capacitance there is the smaller the number becomes..

Just use the 600 ohms output directly into the phono stage (You must have removed the input 47K from the circuit first!) then without changing anything else connect the 100K in series (not parallel!) with the output of the generator.

EXAMPLE:
Let's say you measure 3Vrms at the cathode of V3 before putting the 100K in series with generator, and 1Vrms with the resistor in place.

1/3 = ratio of 0.3333 take the log of this value which is -0.4472 and multiply by 20 which gives you -9.544dB. This value lies between the given values of -8.7dB[40pF] and -10.4dB[50pF] so extrapolating this would be about 45pF.
 
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Hopefully it will turn out to be less than 100pF or so. In my final non cascoded design I used D3A triode connected which resulted in close to 300pF of input capacitance, older designs with multiple paralleled 12AX7A or 5751 were between 200 - 400pF, but I was mostly using HOMC so it did not matter that much. A Shure MM however was pretty unhappy with the 400pF input stage.. :p
 
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I had determined less than 100pF theoretical goal during its design (that would vary per build parasitics and/or alternative input tube types) but I wasn't too worried about it since it was meant for MC with SUT or HOMC direct. When using it straight for MM though its obviously good to know exactly what pF each build presents so to add and match external & internal wiring's pF best to the cartridge in hand.
 
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About the best I have done so far is in the range of 30pF with the cascode topology which is only achievable with great care in layout and construction. A few inches of thoughtlessly routed input wiring (me to a nutshell) and it's undone - still way under 100pF forunately; Ironically the cartridges I use shouldn't care too much, but I find do to an extent with the specific SUTs in use.
 
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Attached scope measurements, SG set to 100kHz 100mV, 1st pic scope set 0.1V/cm 2us shows the measurement 100kHz 100mV only attached SG, 2nd pic without the 100K input resistor scope set 5V/cm 5us, 3rd pic with 100K input resistor scope set 0.5V/cm 5us.

2nd measurement result 8Vpp

3rd measurement result 0.5Vpp

0.5/8 = 0.0625 ratio log = -1.20411 x 20 = -24.0823dB = aprox. 260pF
 

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Provided you have a function generator, and a scope this is not too hard to do.

Disconnect the 47K load resistor at the input to your phono stage. Connect a function generator with 50 ohm output directly to the input - set the output voltage to around 100mVpp @ 100kHz sine wave. Make sure the function generator output is DC coupled. (Most are) Measure the AC signal voltage at the plate of first stage with your scope and make this your reference voltage. Insert a 100K 1% or better resistor in series with the generator output. Measure the output voltage at the plate of the first stage and call this Vmeas. Find the ratio of the two voltages, convert to dB and look for the closest value in my table.

(Vmeas/Vref)log x 20 and match to closest value in table. If you are careful you might get within 10% or so of the actual value. (If the scope will give you numerical voltage measurements so much the better.)

Thank you very much for the table I saved in the PC for future measurements, very good tool.