For maximum ratings, typical operation as A1 amplifier, and curves refer to refer to glass-octal type 6SL7GT

A quick check in Tube Test Conditions for Hickok Cardmatic Tube Testers, published by Hickok in 1962, shows that they are tested with exactly the same electrode voltages and have the same full-scale gm. The Triplett 3444A, however, does test these with different parameters. The 7F7 is tested in a region of the curves where the gm is lower than the conditions where 6SL7GT is tested. This is an anomaly and gives beginners the impression that the 7F7 has lower gm. Gm is not a fixed number for any and all conditions. If it were the characteristic curves would in fact be straight lines.

In RCA tube manuals you can find nomographs to estimate gm, rp, Ib, etc at voltages/currents other than those listed as the test conditions.

A Sylvania JAN 7F7 loctal with its close relative, an RCA JAN 6SL7GT

A few years ago a man called me and said he needed a replacement meter for his Mercury tube tester. I wasn’t sure I had one but I went in the back and dug for an hour and found not a substitute but the factory meter new and in the original packaging. When I told him the price was $50 he told me that was way too much.
The man had a non-functioning tube tester and fifty bucks was going to put it back in service. Wasn’t it worth that much to get it going? If the answer was no, then he really shouldn’t have wasted any of his time looking for the part. He should have simply thrown the thing in the garbage. He’s a member of the “I have no idea what it’s worth to me so it better be cheap” club.

Four quarters equals one dollar and one dollar equals four quarters. When you buy something your profit is supposed to be your utility and/or enjoyment from the item. The seller gets a profit that allows him to buy something that gives him utility and/or enjoyment. It’s win-win. Low-ballers don’t play that way. They only way they feel like they have won is if you lose.

More recently, a man requested a quote on a part. I wanted the sale and was willing to make considerable adjustment in price because of the quantity. I assumed he was fair so I told him to see what other dealers were offering and that I would be happy to match the lowest price he found. I had not looked myself. A few days later he came back with a surprisingly low number—around $4. I thought that maybe market volatility had pushed the price down. When I did my own search I found only 7 dealers who actually had the part in stock, and the lowest selling price was more than 3 times higher than what the man told me was the average price. And if he wanted to buy current production from the factory, the cost would be over $100.

Last week a man appeared at the door. He said he was having a problem with excessive plate current on his Dyna ST-70, a piece of equipment I am so familiar with that I can draw almost the entire schematic from memory. He kept insisting that the “bias” was too high. I told him he had insufficient bias. He replied “But the plate current is too high!” He has confused grid bias (often called bias for short) and the plate/cathode current that bias controls. He’s measuring the voltage across the 15.6 ohm cathode resistor that is brought out to the front-mounted octal test socket.

I received a follow-up email from him this morning saying “…it turns out bias level is not the problem…simply tested the bias range available without the power tubes installed…plenty of negative bias available.”

But he hasn’t actually tried it. He hasn’t tried it.

And if the bias voltage wasn’t a problem, the bias this morning is exactly what it was a week ago. If he puts the tubes in they will still have excessive plate current. This person simply does not understand that if everything is right with his ST-70 then the tubes he has are not good, and if the tubes are good there is something wrong with his ST-70. High plate current is a serious issue on any amp but particularly on amplifiers with smallish power supplies like ST-70′s. The transformers already run hot and the single GZ-34 rectifier is well stressed; the four EL-34′s alone take 200 ma of the GZ-34′s rated 250 ma.

And he absolutely refuses to supply any numbers as to what he feels is “plenty.” I’ve used more voltmeters more times than I’d like to remember and I’ve worked on thousands of pieces of gear. I’ve never seen a voltmeter with “plenty” on its scale. They have numbers. It’s numbers that make technical conversations possible and meaningful. (right : a typical analog meter that can be used to measure bias. Where’s the “plenty” indicator?)

Meaningful. That’s crux of the matter. People who call or stop by and play these games are not looking for meaning or a solution to a problem. They want to engage you. Ensnare you in their world. And it’s a topsy-turvy world. A world where everything is relative, there are no facts and no one knows more than anyone else. It’s also a world where they think I am supposed to have unlimited time to answer their psychological/spiritual/emotional problems disguised as technical queries. I am not supposed to get paid because according to them, my job is too much fun, and fun jobs don’t pay. Please don’t misunderstand. I love giving free technical advice as time permits but my requirement is that conversation is fruitful and enjoyable.

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.

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!

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.