Re: [Drakelist] VTVM Probe with DMM
Paul, The best probe I have found for the purpose are the old 100X scope probes. They have a very low capacitance on the order of 2.5pF in parallel with 10Meg so circuit loading is minimal. Most have extra voltage capability which is handy for working around transmitters. While designed for AC DC measurements via scopes, you can use them accurately with DMM's because the 100X divider swamps out the loading effect of the DMM. The numerical accuracy of the DMM compensates for the high voltage division ratio. Dennis AE6C On 11/17/12, Paul Christensen w...@arrl.net wrote: I finally got around to trying a good 'ole fashion VTVM probe with a Fluke DMM. Recall that most VTVM probes have a switch that allows for either DC Volts in one position, then AC/Ohms/mA in the other position. The DC position typically has a 1-meg isolation resistor, highly useful for VT grid measurements. Without value compensation, a modern DMM cannot use such a probe and is otherwise useless for serious tube receiver work. I took a Simpson VTVM probe with a BNC connector and connected it to a Pomona BNC-to-Banana adapter with standard 3/4 centers. With the adapter, the probe easily connects to a DMM. My first measurement was a precise +12V source. When using the VTVM probe in the DC position (series 1-meg resistor), the DMM displays +10.93V. So, +1.07V is being dropped across the 1-meg resistor. Assuming the resistor is close to 1-meg in value, the input Z of my Fluke 8060A calculates to 10.215 meg. Essentially, a 10:1 voltage divider is being created between the 1-meg iso-resistor, and the internal Z of the DMM. The drop is creating the value discrepancy. VTVMs are compensated in design and manufacture for this. I have several Fluke DDMs, including 8060A (my favorite DMM), and an advanced model 189. However, peering through the manuals, I see no setup routine to create a user-defined DC offset. What I want is the ability to measure a precise DC voltage, then enter a menu that allows me to assign a new display value to compensate for the voltage drop across the iso-resistor. I have a lab-grade Keithley bench-type DMM that does allow for such an offset, but hauling it around is a pain. So, does anyone know of a DMM that allows for DC voltage offset? This is different than the Relative button seen on many DMMs. Relative is used to zero the display for any input value. I want the same thing but instead of zero, assign a new value of my choice. Paul, W9AC ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
[Drakelist] R-4C Antenna Input Direct To Mixer (Chuck Pool)
Hi Chuck, I did! Just unplug the RF-tube, plug-in two small pieces of ISOLATED copper wire but stripped of of its coating so as to connect , fit and be squeezed into the anode and 1st grid slots of the socket of the removed tune Twist the 2 isolated parts of the wires. Gives you a loose coupling of say 2.. 5 pF between both LC-circuits in front and behind former first RF-amplifier - still yielding very good preselection. Works fine with me! Just getting a bit of less gain, t.b. compensated further down the signal path ( or audio), in any case behind narrow Xtal-filters. Result, as has been asked for? Even better IMD3 in CW if with 600 Hz Sherwood mods . Moreover: doing one of W8JI mods ( triode first mixer ) enhances performance even further of a stock version otherwise of poor performance ( as Sherwood benchmark shows). One other step still further is to replace that first mixer with a 7360 tube ( Re Squires Sanders stunning ham-receivers I think you know). But that takes more of an effort I'm afraid to acknowledge. 73 Pierre F3WT Le 19 nov. 2012 à 18:00, drakelist-requ...@zerobeat.net a écrit : Send Drakelist mailing list submissions to drakelist@zerobeat.net To subscribe or unsubscribe via the World Wide Web, visit http://mailman.zerobeat.net/mailman/listinfo/drakelist or, via email, send a message with subject or body 'help' to drakelist-requ...@zerobeat.net You can reach the person managing the list at drakelist-ow...@zerobeat.net When replying, please edit your Subject line so it is more specific than Re: Contents of Drakelist digest... Today's Topics: 1. R-4C Antenna Input Direct To Mixer (Chuck Pool) [Drakelist] R-4C Antenna Input Direct To Mixer.eml___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
Re: [Drakelist] VTVM Probe with DMM
Dennis, Many thanks. I'll give that a try once I find a set of 100x probes. Darn, I didn't know 100x probes even existed for scopes as I've only worked with 10x types for the past 30 years! Paul, W9AC - Original Message - From: Dennis Monticelli dennis.montice...@gmail.com To: Paul Christensen w...@arrl.net Cc: drakelist@zerobeat.net Sent: Wednesday, November 21, 2012 3:38 AM Subject: Re: [Drakelist] VTVM Probe with DMM Paul, The best probe I have found for the purpose are the old 100X scope probes. They have a very low capacitance on the order of 2.5pF in parallel with 10Meg so circuit loading is minimal. Most have extra voltage capability which is handy for working around transmitters. While designed for AC DC measurements via scopes, you can use them accurately with DMM's because the 100X divider swamps out the loading effect of the DMM. The numerical accuracy of the DMM compensates for the high voltage division ratio. Dennis AE6C On 11/17/12, Paul Christensen w...@arrl.net wrote: I finally got around to trying a good 'ole fashion VTVM probe with a Fluke DMM. Recall that most VTVM probes have a switch that allows for either DC Volts in one position, then AC/Ohms/mA in the other position. The DC position typically has a 1-meg isolation resistor, highly useful for VT grid measurements. Without value compensation, a modern DMM cannot use such a probe and is otherwise useless for serious tube receiver work. I took a Simpson VTVM probe with a BNC connector and connected it to a Pomona BNC-to-Banana adapter with standard 3/4 centers. With the adapter, the probe easily connects to a DMM. My first measurement was a precise +12V source. When using the VTVM probe in the DC position (series 1-meg resistor), the DMM displays +10.93V. So, +1.07V is being dropped across the 1-meg resistor. Assuming the resistor is close to 1-meg in value, the input Z of my Fluke 8060A calculates to 10.215 meg. Essentially, a 10:1 voltage divider is being created between the 1-meg iso-resistor, and the internal Z of the DMM. The drop is creating the value discrepancy. VTVMs are compensated in design and manufacture for this. I have several Fluke DDMs, including 8060A (my favorite DMM), and an advanced model 189. However, peering through the manuals, I see no setup routine to create a user-defined DC offset. What I want is the ability to measure a precise DC voltage, then enter a menu that allows me to assign a new display value to compensate for the voltage drop across the iso-resistor. I have a lab-grade Keithley bench-type DMM that does allow for such an offset, but hauling it around is a pain. So, does anyone know of a DMM that allows for DC voltage offset? This is different than the Relative button seen on many DMMs. Relative is used to zero the display for any input value. I want the same thing but instead of zero, assign a new value of my choice. Paul, W9AC ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
[Drakelist] T-4X RF Tune Vernier?
Were there two versions of the T-4X? Mine has a straight through RF Tune shaft, indeed the manual cover shows a T-4X with a bar knob like mine. However I have seen T-4X's with R-4A style red pointer vernier RF Tune controls. Is that a mod or perhaps a later production change? 73 - Luke - K6LO R-4A / T-4X ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
Re: [Drakelist] VTVM Probe with DMM
Good solution, Paul! Mentally dividing by ten is easy enough and you retain all of the advantages of using the VTVM probe. Dennis AE6C On 11/21/12, Paul Christensen w...@arrl.net wrote: Dennis' suggestion of using a 100x probe got me to thinking of an alternative as the 100x scope probes are sorta' pricey, even on the used market. As Dennis points out, the 100x scope probes are useful when sampling high voltage, typically greater than 1KV. Since my focus has been with the grids of tube receivers, these measurements (even into the hundreds of volts) are adequate with the right 10x probe. The 100x probe uses a 10 meg sampling resistor with a 100K shunt at the scope input. Most scopes have a ~ 1 meg input Z. The input Z of my Fluke 8060A computes to 10.5 meg and is a value that is probably close to most portable DMMs. Taking Dennis' suggestion, I used a 1 meg Simpson VTVM probe and shunted it with a value close to 100K to create the necessary 10:1 ratio. A 200K pot was used to find a value that produced an exact 10:1 ratio. For the Fluke 8060A and my Simpson VTVM probe, that value is 112.5K. I then created shunt consisting of two resistors in series whose total value is 112.5K and inserted this combination into a Pomona dual banana jack. Different shunt jacks can be created if one wants best accuracy over a choice of DMMs in the shack. In the links below, notice that the Simpson probe is connected to a BNC-to-dual banana adapter with standard 3/4 pin spacing. The adapter has holes to allow shunting from a dual banana connector; the latter of which has the 112.5K resistor combination. The black plug simply plugs into the red plug. http://tinyurl.com/bc6ggo8 http://tinyurl.com/b56tfm5 Testing this idea in the real world shows that Dennis' idea works in a 10x configuration. For example, when a +12V source is measured, the DMM reads 1.2V. It's then just a simple matter of multiplying displayed results by a factor of 10 which is a whole lot easier than using the DMM's RELative button and trying to find a 1.07V source as the calibrating reference. It appears good accuracy can be achieved when using a DMM with a traditional VTVM probe. The VTVM probe has the benefit of a 1 meg isolation resistor which is highly useful for grid readings. The trade-off is the requirement of multiplying displayed readings by a factor of 10 -- the same as mentally moving the display's decimal place to the right by one digit. Paul, W9AC ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
