Valve voltmeters

[Ears]

I had a rush of blood last weekend to head and bought a Circa 1970 Marconi wide range oscillator. Tube technology. Re-capped it and now I want calibrate it.
Instructions are quite specific as to use of a valve voltemeter in calibration, which I have not.
Did they generally give RMS or average ac voltage readings?

[Phazor]

most VTVM's are average reading but high impedance. Analog Battery meters of that era would measure in ohm's pervolt and would load the circuit they were reading.

Modern DVM's Digital Volt meters generally have 1meg ohm input impedance and so should be fine


Michael

[skyboltone]

This is the one you want. For any sort of calibration of RF equipment you will not be able to get by with a DVM. Even a Simpson Mod 60 has better isolation than a DVM once you get over 50 volts. But the bigger problem is that DVMs aren't all that good at measuring high frequency AC.

http://cgi.ebay.com/HP-Hewlett-Packard- ... dZViewItem

If you put a one meg resistor to ground (DVM impedance) in any oscillator circuit it will stop oscillating. Your master oscillator is a really nice peace of gear. I've aligned more radio gear than I can count. You'll be better off with a scope than a DVM. Most times they are just looking for a peak.

By the way, the price on the 410B is currently way low. Automated bidders will hammer it up to near $100 sometimes more because of that probe. It's got a tube in it. You are measuring voltage using the grid of a triode.

[Ears]

Thanks for both replies.

This is the one you want. For any sort of calibration of RF equipment you will not be able to get by with a DVM. Even a Simpson Mod 60 has better isolation than a DVM once you get over 50 volts. But the bigger problem is that DVMs aren't all that good at measuring high frequency AC.

The proceedure requires a valve voltmeter cpable of 40V and 10M Hz AC. An Avo meter for DC measurements and a scope. I've got a 1970's Tektronix scope, and a cheapish chinese DVM, how high is high freq for a basic DVM?- damnit I knew I shouldn't have trashed the spec sheet.

If you put a one meg resistor to ground (DVM impedance) in any oscillator circuit it will stop oscillating.

The proceedure only inserts the valve-VM at the output terminals or load, but needs to measure up to 10MHz. Says to use AVO for internal DC checks (why not the VVM?).

Your master oscillator is a really nice peace of gear.

Are you talking about the Marconi TF1370A? its certainly got some nice parts to salvage if it fails to perform as an oscillator.
('')

[skyboltone]

Thanks for both replies.


The proceedure requires a valve voltmeter cpable of 40V and 10M Hz AC. An Avo meter for DC measurements and a scope. I've got a 1970's Tektronix scope, and a cheapish chinese DVM, how high is high freq for a basic DVM?- damnit I knew I shouldn't have trashed the spec sheet.

Use the scope for in place of the VTVM and your DVM in place of the Analog Volt Ohmeter (AVO). You are good to go.

If you put a one meg resistor to ground (DVM impedance) in any oscillator circuit it will stop oscillating.

The proceedure only inserts the valve-VM at the output terminals or load, but needs to measure up to 10MHz. Says to use AVO for internal DC checks (why not the VVM?). Absolutely no difference, one side of that output is grounded

Are you talking about the Marconi TF1370A? its certainly got some nice parts to salvage if it fails to perform as an oscillator.
('')

Yes, the military used those to establish the carrier frequency (frequency standard) on the big HF rigs for AM communications. They are rare and collectable. Don't gut it. Traditional AM ham operators search high and low for those. [/b]

[thousandshirts]

This is the one you want. For any sort of calibration of RF equipment you will not be able to get by with a DVM. Even a Simpson Mod 60 has better isolation than a DVM once you get over 50 volts. But the bigger problem is that DVMs aren't all that good at measuring high frequency AC.

http://cgi.ebay.com/HP-Hewlett-Packard-410B. . . .

+1

I grabbed a 410B and find myself using it all the time. I'm not sure if the 410B really is the Cadillac of VTVM's or not, because I don't have experience with many other VTVM's (the only other one I have is a HP 400D, which is a bit older than the 410B and without range for DC) but it is sure nice for some things. The digital multimeters I have aren't exactly gathering dust because of the 410B, but I definitely use them a lot less now. The only thing I fear is having to replace the tube in the probe, locating and paying for it. Everything else, calibration and repairs to the inside of the unit are user serviceable within my competency.