leedsradio.com Blog

I will never forget the first time I saw metal tubes; it was a pair of 6L6′s in a Bogen amplifier from the early 1950′s. Metal tubes were introduced by RCA in a Technical Bulletin in 1935. The first metals were: 5Z4 rectifier, 6A8 pentagrid converter, 6C5 triode, 6F5 hi-mu triode, 6F6 power pentode, 6H6 dual diode, 6J7 pentode, 6K7 remote cut-off pentode (called a super control pentode in the bulletin). Metal tubes were also the first vacuum tubes to have an octal base.

Most people currently building tube gear don’t like using metal tubes. This is strange because they seem to always like older versus newer. It’s almost as if they think metal tubes as lesser just because you can’t see the filament glow. That’s too bad because the earliest metal tubes are superior in construction and quality compared to the older ST enveloped 4, 5, 6 and 7 pin tubes produced earlier. Sorry folks, the mystique of an ST envelope and nostalgia don’t change that. By 1935 tube manufacturers had learned quite a bit.

The 6F6 is an interesting tube. Unlike the more common 6V6 the 6F6 is not a beam power tube but an actual pentode. It precedes RCA’s 6L6, the first popular beam power tube, by a year.

On the left the history changing 6L6 (not part of the original metal tube offerings) along with 5Z4, 6C5, 6F5, 6H6 tubes.

I haven’t tried it, but you can operate 6L6′s upside-down in Ethylene glycol bath at a plate dissipation considerably above the 19 watt maximum called out in the specifications. Do not do attempt this if you have small children, pets, or a fussy spouse at home.

Someone ordered an Amphenol 86CP12 plug and backshell a week ago. Amphenol made a whole series of these to mate with tube & connector sockets. They were available as 4 pin, 5 pin, 6 pin, 7 pin, octal, 9 pin, 11 pin, and 12 . The great thing about them were that they all mounted in the same size hole, with the same ring, or on the same backshell. They are simple connectors designed to do their job without you having to think about them. They are also ridiculously easy to solder and assemble. The 9 pin was used as the AC line connector on the $3,000, Collins 51-S1 communications receiver and also on some Hammond Organs. The 11 pin is used as the power connector on the R390A, triple conversion receiver.

More than 50,000 390A’s were made for the US government between 1958 and 1984. The first ones cost $1500 and the last $38,000. The 86CP8, octal plug is used on the McIntosh C8 phono preamplifier’s umbilical cord. The 86 series males and their 78 female mates are used on more pieces of equipment than you can count and people still use them today when home building when a simple, relatively rugged connector is needed. Not a week goes by when I don’t sell a least one from the series.

Back to my customer. He ordered the connector by part number and received it yesterday. His email to me states:

“It is not anywhere near the quality that we need. I’d like to return it. “

I reminded him by email that he had ordered it by part number and he replied:

“…cosmetics and mechanics of the housing are very low quality and not compatible with our high end equipment. It has a very poor cosmetic appearance and we just can’t use it on our products…”

Now I really had to find out what unbelievable “high end” equipment he was making!

Answer: He’s rebuilding consumer grade Technics reel-to-reel tape decks from the 1970-80′s. So, the 86CP12 is not good enough for a late 1970′s, 240,000 yen (Approx $2,200 back then) but the series is good enough for some of the finest tube communication gear of all time? And what’s he charging for these pieces of “high end”? Nearly $5,000.

It’s late in the game for tape machines. You can buy a Studer 810A for less than half that. One of my customers got a Stevens, with interchangeable head stacks, that originally cost nearly $80,000 new for about $5,000. Technics machines are home machines. I’ve never seen one in a real recording studio. I might have seen something like that in an episode of the Partridge Family. Technics decks have unbalanced inputs using RCA jacks. Nothing is less high end than an RCA jack. You can gold and rhodium plate them all you like they are as connectors go garbage. The cheapest Switchcraft XLR is infinitely better than the most expensive RCA jack

It’s the end times. It’s no longer enough that a part is reliable, fairly priced and above all works. Function is out the window. I guess I could take the backshells, removed the finish, burnish the metal, polish it and then gold plate it, and charge 10 times the price. Then it would be “high end.”

Clockwise from the left: 4 pin, 5 pin, 6 pin, 7 pin, 8 pin, 9 pin, 11 pin, 12 pin. In the center two style of backshells—with and without strain relief. The 8 pin through 12 pin have the same pin circle and pin diameter. To avoid sounding like a noob never refer to anything other than the 8 pin as octal. The nine is NOT a 9 pin octal!

This tube is more than 9″ (23 cm) tall. Maximum plate dissipation is 1000 watts. Max plate voltage rating is 30,000 volts. The 4PR1000B is pulse rated version of its cousin the 4-1000A.
Filament takes about 21 amperes at 7.5 volts. Filament power alone is so high that you can damage the glass seals if you operate it without forced air cooling

I am surprised that most builders of vacuum tube equipment don’t know about production tube sockets. Production sockets are designed for a very large number of insertions and extractions. The contacts are extremely rugged and can be replaced individually. The terminals are threaded brass studs. Ideal uses for production sockets are in tube testers, pre-heaters and burn-in fixtures. At Leeds we have a pod fitted with banana jacks that accepts production sockets, of all types from acorn all the way to compactron, for use with Hickok 118A/B tube testers. It saves an enormous amount of time when testing large numbers of the same tube, because the insertion and extraction are smooth and medium to low force. You can test a quantity of tubes in an afternoon that would ruin the regular sockets on a tube tester.

The 127A is the same as the 3-100D2 transmitting triode. Filament is 5 volts @ 10.5 amperes. Max plate ratings: 2000 volts, 210 ma, Pd = 100 watt. If you look closely you can see that the plate is similar to that on the 100T. The top two pins are for the plate connection, the middle pins are for the grid. The VT-327A is similar but has only one plate connection at the top. Note the plate looks the same as used in the 304TL. Both tubes have uranium glass at the bottom.

This is a directly heated (10 volts @ 4.05 amperes) triode with 150 watt plate dissipation. Maximum plate volts is 2500 VDC and max Ib = 200 ma. You won’t see many of these around. Overall height from the pins to the top of the anode connection is 10.5 inches, a little under 27 cm. The cap coming of the side is the grid connection

I got into an interesting conversation via email with a customer about use of BNC connectors to carry high voltage. The HIPOT tester was already out so off I went. The Hi Pot tester used goes to 4000 volts RMS. At anything less that 2200 VRMS there is no sign of leakage. At somewhere above 2400-2500 volts breakdown occurs. This is with the connector un-mated. I then tried plugging in mate without cable attached. The breakdown voltage actually increased. Here’s a photo of the connector arcing over at 2500 VRMS

A couple of things… High voltage is dangerous. Don’t play with HV when you are tired or distracted Do not Hi Pot and let the DUT arc over for a long period of time.It produces ozone This kind of testing is potentially destructive to the DUT. With repeated testing the breakdown voltage decreased.

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