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eHam Forums => HomeBrew => Topic started by: VE3LYX on July 23, 2015, 04:31:46 AM



Title: 6l6 experiment
Post by: VE3LYX on July 23, 2015, 04:31:46 AM
Recently I found a copy of the 1943 ARRL book. In it was the simple breadboard transmitter which I promptly built and put on the air. I noticed the circuit was familiar to me and very similar to what is known as a TNT in that the crystal and choke were in place of the grid coil and GL assembly found in TNT designs. In fact I have a TNT with a 45 tube I use in the Bruce Kelly Memorial weekend.  I wondered how a 6l6 would operate in that mode as I had always used a old directly heated triode. So I built one on my experiment chassis which is wired for any tube using the 7AC pinout. The chassis also carries a simple on board power supply.
After a few minor snags I got it working. After a couple of days testing and modifying it has turned into a fairly decent low power transmitter. It is now running well enough to use on air so I hooked it up and called CQ on 40M. No takers but that is just sometimes how it is summer months. I have a couple more mods I would like to do to it but it is really a pretty decent little transmitter. Good enough I am not sorry I took the few hours to try it. Too bad there isn't a 6l6 group somewhere. My CW BTW is even improving. Very enjoyable experiment.
don  


Title: RE: 6l6 experiment
Post by: KD0REQ on July 31, 2015, 01:50:02 PM
you may have heard the 6L6 begat the 807, plate lead was brought to a top cap for better HF control.  outside from a base change to a Proper Transmitting Tube Base (tm) and glass shape, it's the same tube.


Title: RE: 6l6 experiment
Post by: VE3LYX on July 31, 2015, 03:17:59 PM
Yes I have heard that but from real life experience now believe it is an urban myth. I have both tubes in several varieties. Plus I also have several 1625s which are similar but 12 volt. This easy to say things but build a set and try to get a 6l6 to perform like an 807 and you will have to be quite creative. Anyway I completed the project tested it on air and moved on to other things. They are both beam power tubes.
don


Title: RE: 6l6 experiment
Post by: N2EY on July 31, 2015, 04:09:41 PM
you may have heard the 6L6 begat the 807, plate lead was brought to a top cap for better HF control.  outside from a base change to a Proper Transmitting Tube Base (tm) and glass shape, it's the same tube.

Not really.

The 6L6 and 807 are quite similar, but not identical. In particular the 807 has lower internal capacitances and higher plate dissipation, maximum plate voltage, and other differences.

But that's kind of beside the point.

The 6L6 was popular with hams in the 1930s and 1940s because it was relatively cheap and available, not because it was a good transmitting tube. The 807 and its 12 volt cousin the 1625 replaced them in most transmitter designs, particularly after WW2.

Today, the 6L6 is sought after by audio folks, which drives up the price. The 807/1625 is much more common and generally less expensive, and an overall better choice for transmitter design.

One more note: When looking at old transmitter designs, always remember that before WW2 the standard amateur transmitting crystal was the Bliley BC-3 or simlar - a big round holder with a huge piece of quartz inside. Those pre-war crystals, as well as some WW2 surplus types like the FT-171, could take enormous amounts of current and not have problems.

However, after WW2 the FT-243-type crystal became the amateur standard, and it can't take as much crystal current. Circuits that worked fine with pre-war crystals would destroy FT-243s. That's why, from the late 1940s onward, you see the 6AG7, 6CL6 and 12BY7 so often used as the oscillator in transmitter designs - and why the Tri-Tet circuit fell out of favor.

On top of that is the fact that in the old days there wasn't as much concern about chirps, clicks, yoops, and other oddities, That's all changed now - for over half a century we've known how to build small transmitters that have T9X signals.

There's a very good article in QST for March, 1950 called "Crystal Controlled Oscillators" which tries various tubes and circuits, and documents the results in terms of crystal current, relative output, ease of tuning and other factors. Recommended reading for anyone interested in hollow-state transmitters involving crystals.

73 de Jim, N2EY


Title: RE: 6l6 experiment
Post by: VE3LYX on July 31, 2015, 10:47:51 PM
It didn't seem beside the point to me. I run my 1625s at 770 volts.
Actually 6l6s are not that expensive. I have bought a few over the last few years. Just bought a pair of 807 replacements for my No 19 wireless sets. 5933s which have been around as long as I have, plus. They aren't  direct sub being shorter but were more expensive then my last 6l6s by quite a bit.
I actually have the round Blileys , 3561 and 3736 to be exact. I built the breadboard station as it was directly from the 1943 ARRL handbook. Works pretty good. Got a reply first call which is always a surprise. Its circuit bypasses some of that current with a parallel choke to the crystal so I am able to use the newer crystals as well (so far) without difficulty. Hardest part of the whole project was finding a breadboard. Everything else but brass hardware came from the junk box. I can also run a 6v6 but the rig is slightly better with the current 6l6G in the hole. Also tried 6l6EH. It can stand more plate voltage but this station has only around 300v key down so they were of no advantage. I was surprised though that the 6l6g outperformed the others as I thought at the lower voltage it probably wouldn't matter. I have a lot of vintage rigs. Those that are wonky I save for the Bruce Kelly event. This rig is quite usable which is a pleasant surprise. It is my go to rig and station for CW on 80M.(also has a breadboard regen (6sl7) for hearing.  Need to find a few more crystals (or build a VFO /buffer) I also built a 6l6 TNT just to see how it would work compared to my 45 TNT. I will stick with the 45. The beam power tube works in that mode but is not well suited to it. It was a fun experiment though. One can have a lot of fun with a few sticks of wood, some hardware and varnish,  a rainy day and a well stocked junk box. 
don


Title: RE: 6l6 experiment
Post by: W8JI on August 08, 2015, 09:39:57 PM
Yes I have heard that but from real life experience now believe it is an urban myth. I have both tubes in several varieties. Plus I also have several 1625s which are similar but 12 volt. This easy to say things but build a set and try to get a 6l6 to perform like an 807 and you will have to be quite creative. Anyway I completed the project tested it on air and moved on to other things. They are both beam power tubes.
don

Hi Don,

It is not a myth. Look at the tube characteristic curves. The early 6L6 and 807 are almost identical at the same voltages. Even the cathodes and filaments are common.

The stability change is a different issue. The base lead lengths and the anode lead routing and length are what mostly determine stability. Because the 807 and 1625 have a much shorter anode lead that is moved away from the grids, the 807 and 1625 are much better in radio frequency applications (despite sharing very similar or common internal parts).

Most tubes evolved from starting types through subtle changes to improve certain traits. The 811, for example, shared the 812 filament, envelope, base, and anode. Only the grid was rewound. Later when the 812 got welded cooling channel fins on the anode to greatly increase thermal lag and slightly improve dissipation, the 811 shared the anode and became an 811A.

If you dissect tubes or study the characteristic curves, you can see how the 6L6 evolved into RF tubes, and how the branching produced the 6146B and 6L6GC as elements common to multiple tubes were modified and shared.

This was done to save engineering design costs, as well as manufacturing and tooling costs.

Your observation of ease of use is correct, but the conclusion the 807 is not an evolution of the 6L6 is incorrect. It is.

73 Tom


Title: RE: 6l6 experiment
Post by: VE3LYX on August 09, 2015, 04:26:27 AM
Well feel free then to sub all your 807s and 1625s with 6L6s. I am not about to do that. I did not BTW say the one did not begat the other. I objected to "the same tube". To me the 6V6 is the 6l6s closest relative and some of the audio KT series the natural extension of the 6L6 series design. Anyway that 6l6 rig now has a triode in its spot. The 6l6 had some characteristics I didn't like so that part is for now over.
don VE3LYX


Title: RE: 6l6 experiment
Post by: W8JI on August 09, 2015, 05:41:36 AM
Well feel free then to sub all your 807s and 1625s with 6L6s. I am not about to do that. I did not BTW say the one did not begat the other. I objected to "the same tube". To me the 6V6 is the 6l6s closest relative and some of the audio KT series the natural extension of the 6L6 series design. Anyway that 6l6 rig now has a triode in its spot. The 6l6 had some characteristics I didn't like so that part is for now over.
don VE3LYX

No one said they sub. All I said was the insides are the same with very minor changes, with the major change he leads are rerouted. It's pretty simple and an interesting history of tube evolution.

The 807 is an extension of the early 6L6 design with only very slight modifications internally but different lead routing through the envelope.


Title: RE: 6l6 experiment
Post by: G3RZP on August 09, 2015, 08:42:32 AM
There was also an 8018, which was supposedly an 807 chosen to be able at full ratings at 125MHz in some rare WW2 RAF ground station transmitter. The 807W was a physically smaller but electrically identical device, while the British 5B/254M was an 807 on a loktal base, with the same diameter as a 7C5, but longer: there was a version with wire leads, and other variants available in 12.6 and 19 volt heater versions, with a choice of single ended or plate top cap. The plate top cap versions were better performers then the 807 above about 25 MHz because of the much shorter lead lengths. But the granddaddy of them all was the RCA 6L6.

I would suspect that there is a small difference between the metal 6L6 construction and the glass one because of the pinch supporting the electrode structure in the glass one.

I wonder how much I could get for a pair of NIB metal 6L6s made for the US Navy in 1944?


Title: RE: 6l6 experiment
Post by: VE3LYX on August 09, 2015, 11:48:47 AM
I have heard of metal 6l6s run upside down in cooling oil at 1000 volts. Could be baloney I don't know. Yes I agree it would be interesting to see what one could do with them. I had wondered about an RF quad four. I suspect some of the new ones can handle quite a bit more then one would think. I have spent a lot of time with many of this tube family (7AC pinout group) including 25l6 36l6 50l6 50c6 6v6 6l6g 6l6eh tall boys etc etc. Have learned a few things about them. Some good some not good. I did build a three tube 6l6 amp once long ago. I know if you don't have the higher voltage for the 6l6 you are better off with a 6v6 in most cases. I have a pair of large bottle 6v6s from 1943. seem like a big bottle for a small tube.
don VE3LYX   


Title: RE: 6l6 experiment
Post by: W8JI on August 13, 2015, 03:59:01 PM
I had to salvage parts from the local trash piles to build things. 6L6 tubes were very rare.

Common tubes in the junk piles were the 6V6, 6Y6G, 6K6GT, and 50L6. BW TV sets had 6AG7 video drivers as well as small sweep tubes.

When I built my first transmitter, I used a 6V6GT. I morphed that transmitter to the 6V6GT driving a 6L6 when I finally found a 6L6, and eventually saved enough money to get an 807 to replace the 6L6. To me, the 807 was like magic. 75 watts plate input power. A real transmitting tube.


Title: RE: 6l6 experiment
Post by: VE3LYX on August 16, 2015, 03:49:25 PM
I was not as you know happy with the 6l6 TNT. I wanted to see if it was the beam power tube that was the obstacle in reasonable stable performance so I rewired for a double triode. Since the on board PS had 12volts available and I had a 12sn7 it got the nod. It wasn't much improvement. I ran it as a parallel pair. So yesterday I rebuilt it. leaving the 12sn7 in but splitting the triodes. I wound a hartely coil (centre tapped )on a ceramic slug tuned core. Guessing at the turns.  I drove the second triode section from the plate with a 680pf ceramic cap and left the final tank circuit as it was. On initial fire up on a dummy (AKA Christmas tree bulb) it was tuned to 6990. A quick adjustment on the slug and re trimming the tank capacitor and I was on 7050 with a good stable signal and decent output. The osc section is only slug tuned or PTO and the coil has a 150pf cap in parallel to get it bang on. A bit of fooling around today  with a minor change here and there and I have a very nice working little transmitter putting out a few watts of CW and sounding good. I have been playing with it this afternoon and am very pleased with its behavior. No crystal but extremely stable. Not sensitive in anyway to hand capacity. Tunes easily and at a comfortable rate. All self contained including PS and it didn't cost a fortune to build. Maybe $10 at the most. I will be trying it on air this evening around 7030 to 7050khz. I really like the PTO deal. Output is reasonable and should be enough in decent conditions.
don Ve3LYX