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Author Topic: Where can I buy GLASS 6AG7 tubes?  (Read 25794 times)
W1BR
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Posts: 4179




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« Reply #15 on: October 17, 2014, 01:38:26 PM »

That is a good idea Tim, I did not realize 6V6's were glass too. They might not be "as good" as oscillator tubes, but this is a 80/40 transmitter only. Since I already have ONE 6AG7/6L6, maybe I will convert the other one to a 6V6/6L6 and see how it works. And I already have metal 6V6 tubes, so can see how bad the two glass tubes interact.

Now I have something to do this weekend.

p

I mentioned that earlier... if the suffix is G or GT, the tube is glass.

Pete
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K3STX
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« Reply #16 on: October 17, 2014, 01:56:21 PM »

I mentioned that earlier... if the suffix is G or GT, the tube is glass.

Pete

Yes, however when you said it I ignored you  Grin

I did not know how attractive the 6V6G tubes were, it is worth a try. I searched on Google under 6AG7G and saw a tube listed in "images", perhaps that was an error in the tube designation as that was the ONLY image I saw of a "6AG7G" tube.

paul
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G3RZP
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« Reply #17 on: October 18, 2014, 02:11:51 AM »

K1ZJH,

Pete, look at this data sheet..

http://frank.pocnet.net/sheets/095/6/6P9P.pdf

Nothing like the eBay pic......

73

Peter G3RZP
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W1BR
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« Reply #18 on: October 18, 2014, 07:09:29 AM »

Peter

Interesting! It is a glass tube in a metal shell. The only other example I can think of is the 0Z4 used in auto radios--they were also glass tubes with a metal shell over them.

The tubes in my link appear to be Chinese knockoffs?

Pete
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G3RZP
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« Reply #19 on: October 18, 2014, 07:45:10 AM »

Pete,

The 6P9P is Chinese, and as you say, glass tube in a metal shell. Philips/Mullard did a series of tubes like that - the EF50, which was originally meant for TV sets and was manufactured in millions in WW2, even in the US by Sylvania (the red Sylvanias were supposed to be the best), the EF54 which was a VHF amplifier pentode, predecessor to the 6AK5, the RL34 (very rare) a GG receiving triode and the EF55 which was pretty much a 6AG7. All on a 9 pin base, with no pinch, like miniature tubes.

There is a story, probably apocryphal, that the EF50 samples, designs and drawings were driven out of Eindhoven and taken to England as the German army entered the other side of the town. But the EF50 was the mainstay of a lot of the early radars, and post WW2, the TV sets after TV started again. A lot were used in 'Colossus', the computer used at Bletchley Park to break the German Enigma code. They were quite good in circuits such as the Phantastron and in Miller-Transitron timebases too.

So I wonder how close that eBay 6AG7 really is to a 6AG7..........Having had tubes from Eastern Europe, marked as 6BA6 but with cathode and suppressor grids strapped internally, I have suspicions...
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AC5UP
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« Reply #20 on: October 18, 2014, 08:04:08 AM »

Interesting! It is a glass tube in a metal shell. The only other example I can think of is the 0Z4 used in auto radios--they were also glass tubes with a metal shell over them.

The next time you have a metal 6SK7, 6SQ7, 6SJ7, etc headed for the circular file, pry the base off to see what's inside.

It's a glass tube in a metal shell.



This is what a shielded 6K7, 6Q7, 6J7, etc looked like before RCA decided to simplify the concept with the 'S' version of the same tubes. No separate grid cap, shield factory installed.



'Taint nothin' new.   Cheesy
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G3RZP
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« Reply #21 on: October 18, 2014, 08:09:45 AM »

I have opened a number of metal tubes and found them like the illustration in Terman - no glass tube inside.
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W1BR
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« Reply #22 on: October 18, 2014, 08:34:13 AM »

I have opened a number of metal tubes and found them like the illustration in Terman - no glass tube inside.

That is what I've always found as well. With very few exceptions, the metal 0Z4 being one of them.

AC5UP seems to be confusing metal tubes vs. glass tubes using an exterior metal goat shields, or similar.

The original octal based tubes were all metal and introduced by RCA. They were self shielding, using pin one for the shield connection. When competitors starting offering glass octal tubes in G and GT versions, the need for the external shield became necessary. BTW, those goat shield examples (labelled form fitting) were used on tubes with ST envelopes, which were not octal based and predated the RCA metal tubes.  Just because a tube has a shield does not make it a metal tube.

http://www.r-type.org/articles/art-018.htm

Pete
« Last Edit: October 18, 2014, 08:47:41 AM by K1ZJH » Logged
AC5UP
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« Reply #23 on: October 18, 2014, 11:24:40 AM »

Thanks for the link to the 6J5 teardown....

It was years ago that I removed the base of an RCA metal tube to find a glass bottom plate and assumed it was a complete glass tube inside a metal shell.  If the glass could be sized consistently it wouldn't take much to build the shields ever so slightly smaller, heat them enough to expand a bit, then slip the glass inside as the shield contracted around it.  Permanent jam fit.  Next time I have a metal tube headed for the trash I'll take it for a spin with a tubing cutter to see what pops up.  Learn something new every day.

BTW:  Back in the day magnetic CRT shields were made from " Mu Metal " which was probably a bit pricey for common tubes.  http://en.wikipedia.org/wiki/Mu-metal  I've seen rusted 6J7's and similar so the shield material was probably nothing exotic.
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W1BR
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« Reply #24 on: October 18, 2014, 11:46:03 AM »

There were some new manufacturer "metal" tubes that had miniature glass tubes hidden inside!  I assume they were specially made for military contracts. I've seen a few photos of them posted over on the antiqueradios.com website.

If you want to see a really odd metal tube, Google birdcage 5Z4.  The first RCA versions had two exposed metal plate anodes, surrounded with a circular metal grille to prevent electrocuting service folks while allowing some ventilation.  They were only made for a year or so, but there was reliability issues and they went over to the more conventional metal shell with internal anodes.

Perhaps Peter can comment, but I remember reading that the Brit's had produced metal tubes before RCA came up with the octal design.

Pete
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G3RZP
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« Reply #25 on: October 19, 2014, 07:06:43 AM »

Back in the 1920s, the M-O (Marconi-Osram) valve company (MOV) produced some big water cooled transmitting valves, with numbers like CAT1, CAT2 etc., the 'CAT' standing for 'Cooled Anode Transmitting'. By the end of the 1920s with the broadcast receiver market taking off, they were looking at methods of reducing costs and produced some receiving valves with a metal envelope which was also the plate. For screening and safety, these were mainly in perforated thin metal cases - but not so much the AF valves. At the time, the pretty much de facto standard for heaters (as opposed to filaments) was 4 volts: 6.3 came a bit later from the US being good for car radios. These valves, with a relationship to the big CAT series were whimsically called 'catkins', and were all on the standard British 5 and 7 pin bases.

RCA went a different route with a mild steel envelope and eyelets of a special glass called FerNiCo (Iron, Nickel, Cobalt) which matched the thermal expansion of the steel. The metal tubes could be exhausted with envelope heated to red heat, which helped to get rid of absorbed gas, had very good screening and were more robust. They tended to be smaller - compare a metal 6J5 with a 6J5G - but as both MOV and RCA found, they were more expensive. Glass is pretty cheap when compared with even mild steel....although special types of glass are not necessarily so. Thinking here of the high temperature 'Nonex' glass used for things like Eimac 35Ts and the like, which tubes with their tantalum plates were run white hot as the tantalum absorbed the gas.

AFAIK, only Tung-Sol produced GT versions of the 6SG7 and 6SH7 (which are basically the octal predecessors of the 6BA6 and 6AU6 respectively). Possibly this is because they wanted cheaper tubes for use in FM receivers...All the other manufacturers only did them in metal. Yet nobody ever did a 6SN7 or 6SL7 in other than GT versions...
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W1BR
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Posts: 4179




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« Reply #26 on: October 19, 2014, 09:34:04 PM »

ZGP:  Yes, I think those are the ones I heard about, and someone mentioned they were called "catkin" tubes for some odd reason.

Pete
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G3RZP
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« Reply #27 on: October 20, 2014, 12:07:49 AM »

Pete,

Now you know why they were called 'catkins' - little cats!

Meouw....
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W3RSW
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Posts: 606




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« Reply #28 on: October 23, 2014, 11:15:40 AM »

Back to the original question of wanting to "cool-up" a rig using a glass version of a 6AG7.

The metal 6aG7 is THE epitome of electron coupled oscillators, period proper on any bread board rig or otherwise.  As mentioned shielding is necessary even if you used an apparently non-existent glass version. Just using the 6AG7 denotes knowledge and coolness factor in your rig.
Not using it denotes a potential unawareness of the electron coupled osc. circuit.  Grin

Your QST reference mentioned:
http://faculty.frostburg.edu/phys/latta/ee/6cl6xmtr/6cl6why6cl6.html
as:
Quote
The 6AG7 is highly recommended in an article in the March, 1950 issue of QST
 In 1950, an important article in QST magazine, "Crystal-Controlled Oscillators, A Review of Modern Crystals, Circuits and Tubes" (QST, March 1950, C. Vernon Chambers, W1JEQ) addressed all of the issues above, as well as the choice of which tube to use. In that article, various electron-coupled circuits were tried along with a variety of tubes: the 6AG7, 6F6, 6V6GT, and 6L6. Among the many conclusions in the article, one came through loud and clear, which I quote here: "Of the four tubes tested the 6AG7 is by far the best from every standpoint." As a result of that article, virtually all crystal oscillator circuits in the ARRL handbook for the next 15 years featured or recommended the use of the 6AG7.

Very good info in the 6CL6/6ag7 literature cited, such reference mentioned many times in several blogs, sites and threads over the years.

In this day and age of weak or somewhat small form factor crystals it's more important that ever to go easy on a crystal; the electron coupled circuit which uses a single tube as 'gentle' oscillator and amplifier is a nice way to go.
« Last Edit: October 23, 2014, 11:23:22 AM by W3RSW » Logged

Rick, W3RSW
W3RSW
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« Reply #29 on: October 23, 2014, 12:11:30 PM »

My historic interpretation is somewhat different from the following:
Quote
The approach to circuit design in the 40's/50's ARRL handbooks heavily emphasized a minimal number of high gain devices.

This can actually be a little tricky when the high gain devices tend to self-oscillate in some wacky modes (including UHF parasitics and/or squegging) if not everything is bypassed with extreme care.

A more modern approach is to use multiple stages of moderate gain, and especially to use some negative feedback at each stage to enhance stability.

A lot of the problems that were attributed to "not having a hot enough oscillator tube" or "not enough shielding" back then, were really because the simple designs ran the oscillator and PA and the antenna all at the same frequency. There is a substantial advantage to frequency doubling somewhere between the oscillator grid and antenna. Frequency multiplication provides incredibly more decoupling, than all the metal tubes and aluminum sheet metal ever could.

If you read the 50's handbooks they knew that transmitters that had electron coupled oscillators aka "tri-tet" which doubled or tripled at the oscillator tube plate, had substantial immunity to many problems that plagued the straight-through designs. They usually attributed this to the concept of "oscillator loading" but did not seem to realize that the real advantage was frequency multiplication.

Didn't realize?
Take just about any 1950's vintage handbook, typically the 1957 version and you'll find frequency multiplier stages in the construction section of every transmitter except the single stage 6ag7 beginner's and the following single stage 6146, 75 watt novice circuit.
    "75 watts on 4 Four Bands"
     "A 7-Band 90 watt Transmitter"
     "75 to 300 watts with VFO control"
     "A 500-Watt Multiband VFO Transmitter" (THE classic 813 rig of the 50's)
     "A Remotely Tuned VFO" (using a single 6ag7 or 5763 with numerous dial scales for
                                           following doubled, etc. frequencies, etc.)

Pages 164 and 165 describe freq. multiplication, push-pull multiplication, etc. in detail in same '57 handbook in its earlier circuit 'theory' section.

Long before 1956 numerous circuits described 160 meter vfo's for 80 meters, and on and on.  Take the 1953 Handbook for example, pages 148 and 149 described multipliers, virtually same language as 1957.   The first single tube 6ag7 breadboard rig in the '53 book along with the "Novice 807 Amplifier," also breadboard ASSUMES operation of higher frequencies by second or more overtone. All the following transmitter circuits in the '53 have "amp/dblr" or similar description in intermediate circuits.  

Guess what pages in the 1945 book has Frequency Multiplication?  (hint, 112, etc.)
Sadly, actual transmitter circuits, except AC carrier current rigs, were left out due to
war restrictions.

Overall:  The number of tubes or stages was never a problem after, say, the 20's.  Great marketing wars were fought over how many tubes you could put in a circuit, particularly domestic receivers.  Amateur transmitters never received that hype, but each tube had function, in multiple freq. rigs -- an oscillator, buffer, two to three frequency doublers, perhaps a driver and final.  Proper bypassing, short leads, shielding (Faraday shield, FG's sake.), parasitics, .. all "discovered" and alleviated far before the 40's and 50's.

You want beaucoup multiplier tubes in an transmitter chain?  The 4-65A rig is a great example listed for several years.

Blanket historical opinions as factual statements, not researched, are the reason that the internet is often unreliable.  

I might also mention the Collins 32v series transmitters used freq/ multiplication, modular shielding, modern pi-net output, modern parasitic suppression, all rolled into essentially a three knobbed transmitter - band, plate tune and load - along with some meter switching and set and forget speech amp. gain.  Amazingly  Grin, a lot of glass tubes, every one except the oscillator.  I see they were already up to the TVI shielded 32V3 by 1953 adverts. 

« Last Edit: October 23, 2014, 12:29:53 PM by W3RSW » Logged

Rick, W3RSW
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