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Old 4th December 2012, 05:24 AM   #1
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Default Oscilloscope voltage limitations valve amps

Hi.
Its been a while since I have worked on valve gear. I was wondering if someone could clear out something for me. I have a Tektronix 453 Scope, as well as a Rigol DS1102E scope.
Can I probe those 250V-550V DC Valve- plate pins if I set my probes to x10 and the scope to AC coupling????
If not, how would I go about doing so?
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Old 4th December 2012, 05:34 AM   #2
freax is offline freax  Australia
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the user manual should be able to fill you in on this.

but yes please do wait for an experienced reply from people more experienced people. or go here: www.syscompdesign.com/AppNotes/probes.pdf

I too would like to know this answer in relation to tube rectifiers and oscilloscopes.

this might help: How do you measure ripple on the B+?
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Last edited by freax; 4th December 2012 at 05:47 AM.
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Old 4th December 2012, 06:32 AM   #3
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Check the voltage rating of your probe. 550 V is possibly too high. I recommend you get a 100x probe, e.g. Tektronix P5100, P6007 or P6009.

Samuel
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Old 4th December 2012, 01:34 PM   #4
oshifis is offline oshifis  Hungary
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It does not make sense and it is dangerous for the scope and for you. The usual spec is 250V for a 10:1 probe. Why do you want to measure the AC on the anode? Measure the B+ with a DMM, and measure the AC at the secondary of the OT. Or measure the anode AC and DC current by inserting a 10 ohm resistor between the cathode and GND.
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Old 4th December 2012, 03:51 PM   #5
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by diy didi View Post
Hi.
Its been a while since I have worked on valve gear. I was wondering if someone could clear out something for me. I have a Tektronix 453 Scope, as well as a Rigol DS1102E scope.
Can I probe those 250V-550V DC Valve- plate pins if I set my probes to x10 and the scope to AC coupling????
If not, how would I go about doing so?
Most x10 probes have typical voltage ratings in the 400 to 600V.
However, you have to be careful with the AC coupling: the input blocking cap of the scope will see the full DC voltage, irrespective of the presence or not of a x10 probe.
The rating of the scope input is generally lower, more like 250V.
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Old 5th December 2012, 02:16 PM   #6
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As far as I understand, the 100x probes I've listed attenuate down to DC. So with those AC coupling would be possible.

My 10x is just 300 V.

Samuel
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Old 5th December 2012, 03:10 PM   #7
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by Samuel Groner View Post
As far as I understand, the 100x probes I've listed attenuate down to DC. So with those AC coupling would be possible.

My 10x is just 300 V.

Samuel
The 40 Meg impedance ones include an additional resistor in the foot of the divider, and are safe with capacitive coupling.
The 100 Meg aren't:
Example:
Quote:
The P5122 probe should not be used on the TPS2000 Series oscilloscopes for AC-coupled measurements on signals with greater than 300 V DC offset.
High Voltage Probe Passive High-voltage Probes | Tektronix
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Old 5th December 2012, 03:16 PM   #8
SY is offline SY  United States
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Use an external series cap. I clip one lead on to the end of the probe, then use its other lead (insulated except for the end!) as the probe.
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Old 5th December 2012, 03:19 PM   #9
jrenkin is offline jrenkin  United States
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Quote:
Originally Posted by oshifis View Post
It does not make sense and it is dangerous for the scope and for you. The usual spec is 250V for a 10:1 probe. Why do you want to measure the AC on the anode? Measure the B+ with a DMM, and measure the AC at the secondary of the OT. Or measure the anode AC and DC current by inserting a 10 ohm resistor between the cathode and GND.
Seems some people are using high voltage probes and others are not, what is really needed in assessing a tube amp? I could use a little education on this too. Thanks!
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Old 5th December 2012, 09:06 PM   #10
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Most decent 'scopes have the max Vp-p printed by the Y [vert] amp BNC sockets. Take note and do not exceed this. X100 probes are available but not common. Don't forget the BNC outer and probe ground clip are connected to ground so don't try and "float" a scope to make differential measurements by disconnecting the mains earth connection.
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