leedsradio.com Blog

OK. It’s a little modern for Leeds but the 610C does the impossible: it measures resistance, voltage, charge (in coulombs) and current down to 100 pico-amperes full scale – that’s .0001 billionths of an ampere. A wonderful and versatile instrument. A dozen years ago I used a 610C to study the grid current of common vacuum tubes. I was motivated by the rather informal limits listed on common tube testers. It turns out that “modern” output tubes (6L6, 6550, etc) have grid leakages (gas current + grid emission) of around 1/3 of a micro-amp per watt of dissipation. Older ones (211, 845 etc) have up to 1 micro-amp per watt.
Eye-opening considering that the Hickok 118A/B and many other common tube testers reject at 3 micro-amps. Clean calibrated 610C’s go for $1000-$2000.

One of my favorite pieces of equipment. Unbelievably well made. It’s fairly big; approx. 23 cm wide, 18 cm tall and 15 cm deep.You can use it as a 0-1.1 pf or o to 11 pf variable depending on which of the panel mounted 874 series connects you used. The 874 series was developed in the late 1940′s. GR 874 connectors are hermaphrodites – they mate to themselves. Take a look at the cable sitting on top of the 1422. You can still buy a 1422-OD if you have about $11,000 burning a hole in your pocket.
(I just moved this from the Leeds site)

Mica capacitors were invented in 1909 by William Dubilier. He’s the Dubilier in Cornell-Dubilier. Here’s a late 30′s/early-40′s Aerovox .0011 mfd 5000 volt transmitting mica cap. Transmitting mica are different than regular mica caps because they can pass significant RF current. This one is rated for 5 amperes at 1000 KCS (That’s 1 mHz for all you young folks). As transmitting micas go this one is not particularly big – 75mm wide and 52mm tall. Look closely and you can see Aerovox’s beautiful art deco logo

A gorgeous part in search of a project! There are about a dozen of these, new and in original boxes sitting around the shop. The FJ-125 weighs 10 kg and is just under 18 cm long. It has a single 34 volt secondary rated, interestingly, at 11 amps DC. Built-in to the case is a “swinging choke.” A swinging choke’s inductance varies in proportion to the current running through it. This allows improvements in power supply regulation. The choke is rated .050 henries @ 1 ampere/.01 henries @ 11 amperes. The case is hermetically sealed and has output terminals that pass through glass seals. There is also an electrostatic shield between the primary and secondary. I thought about designing a single-ended transistor amplifier around this transformer.

Both have 100 watt maximum plate dissipation. The 7241 has a maximum DC plate current of 1.2 amperes and the 7242, 0.9 amperes. So you are thinking about OTL (output-transformer-less) circuits, huh? Do not let anyone fool you, both of these tubes are pigs.  Both take nearly 48 watts of filament power (6.3 volts @ 7.5 amps) and they both require very well designed drivers, as the maximum allowable grid resistance for fixed bias is only 50,000 for the 7241 and 20,000 for the ’42.. On the other hand, they are beautiful beyond belief, and have plate resistances of only 67 and 82 ohm respectively. The basing is JEDEC #A7-17 and they mate with  the increasingly hard to find  Johnson 122-237 – the same socket that is used for the 813B. The 7242′s transconductance is 110,000 micro-mhos. Oh, do I have your attention now?

This has three decks and can give you attenuation from 0 to 111 db in 0.1 db steps. Unlike the Hewlett Packard and General Radio equivalents each deck has 11 steps. The VH-795-G has 600 ohm input and output impedance. Very, very useful on the test bench.

Some mastering engineers use these (and the GR version) as precision level attenuators.

Today it’s overcast and much cooler in NYC.  Here’s the view of Manhattan today  from East River State Park in Williamsburg, Brooklyn.  In the center is the Empire State Building

If you connect the filament pins (2 & 7) together a 1B3GT, or similar vacuum diode makes an excellent HV capacitor capable or at least 15KV. Filament power is not applied. You can then use it as the upper cap in a high voltage capacitive AC voltage divider. The plate to heater capacitance is typically 3-4 pf. With a 100 pf capacitor in series you will have an approximately 34:1 divider. Note that the lower (larger) capacitor does NOT have to have an especially high voltage rating; a 100 pf 500 volt cap will work fine. If you use a 1000 pf lower cap for a 340:1 divider a 50 volt cap has sufficient ratings. The usual warnings apply; Don’t touch high voltage devices or associated circuitry when under test.

An interesting box rarely seen today. Internally, it’s just an attenuator and a tunable bridged T network. Operation is simple: Select 400 Hz or 1 KC. Apply signal to the input terminals at corresponding frequency. View residual on a ‘scope. Fine tune oscillator/source frequency for minimum deflection. Adjust the balance control for minimum deflection.  Repeat if needed. Percent distortion is peak-to-peak output voltage divided by input voltage times 100. Unlike HP’s active distortion analyzers this does not suffer from ground loops. The 320A can also be use as an attenuator. I am not sure when this was first manufactured but it is mentioned in the August 1951 issue of The Hewlett Packard Journal.

From the sublime (866) to the insane: Tung Sol 7791A hydrogen diode. Current rating: is 8 peak/2 ave. amperes and 25000 peak inverse voltage. Filament eats  15 amperes @ 5 volts. At 20 amperes forward current (Way outside its ratings) the 7791A drops only about 20 volts. It’s not as big as you would think: approx. 15 cm high, base flange 11.5 cm diameter.