Hey guys! (and Mona)
I got some Scott 265a mono's here and the owner wants to use metal can 6L6's in them. The amps originally come with 1614 which are electrically equivalent from what I can tell except the data sheet shows that the 1614 is a 25 watt tube and the 6L6 is a 19 watt tube.
Some of you may have noticed as I have that all the 1614's I have come across have 6L6 engraved on the base. He was told that they are exactly the same tube despite what the datasheets say. This amp runs these tubes hot and I was always under the impression the metal can 6L6 is a 19 watt tube and therefore not a great match for this amp. Personally I would feel safer running 6L6GC's in them but he is dead set against modern production tubes.
So I am looking for anyone that has more information on the production of these two tubes and exactly what is inside them. What makes this difficult is the metal can doesn't allow us to see if the plates are turning red. I am sure George or someone here must have gutted one to compare the innards to a standard 6L6......or at least I was hoping someone has.
So are they the same or not? Are the datasheets to be taken as gospel or not? I personally always just go by the datasheet to be safe but I feel this is a good question for the tube gurus out there.
-bird
I got some Scott 265a mono's here and the owner wants to use metal can 6L6's in them. The amps originally come with 1614 which are electrically equivalent from what I can tell except the data sheet shows that the 1614 is a 25 watt tube and the 6L6 is a 19 watt tube.
Some of you may have noticed as I have that all the 1614's I have come across have 6L6 engraved on the base. He was told that they are exactly the same tube despite what the datasheets say. This amp runs these tubes hot and I was always under the impression the metal can 6L6 is a 19 watt tube and therefore not a great match for this amp. Personally I would feel safer running 6L6GC's in them but he is dead set against modern production tubes.
So I am looking for anyone that has more information on the production of these two tubes and exactly what is inside them. What makes this difficult is the metal can doesn't allow us to see if the plates are turning red. I am sure George or someone here must have gutted one to compare the innards to a standard 6L6......or at least I was hoping someone has.
So are they the same or not? Are the datasheets to be taken as gospel or not? I personally always just go by the datasheet to be safe but I feel this is a good question for the tube gurus out there.
-bird
Surely the answer would be to use higher rated NOS 6L6 versions, such as 5881s, 6L6GCs, or maybe even 807s (with a socket change and top cap). Wouldn't that keep it sufficiently vintage, as well as thermally safe?
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I'm no expert on this topic, but there are clear differences in the datasheets for these two tubes, with the 1614 apparently made of sterner stuff. Its continuous anode dissipation limit is 21W vs 19W for the 6L6, and it has greater max voltage ratings as well. See the class AB1 typical operation examples -- there's an option for the 1614 (only) with 530V on the anodes.
The 5881, 6L6GC and 807 are also made of sterner stuff. So as 1614s are very hard to come by, these would make excellent substitutes, and buying NOS should help keep the customer happy.
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NOS 6L6GC's are very expensive🙁 I wish he would use modern tubes but I understand. I think he got a good deal on metal can 6L6's and I think he was under the impression they are the same as 1614's.
He has some donor tubes to autopsy. A dremel could open em up me thinks. I would think the plate structure or thickness would be different or something
He has some donor tubes to autopsy. A dremel could open em up me thinks. I would think the plate structure or thickness would be different or something
Excessive dissipation, not mere voltage, kills tubes.
A look at the original RCA 6L6 data sheet suggests that the 19 W. number is design center that can be safely exceeded, by a small amount. Notice the 34 W., without entering 'B2 conditions, remark. 38 W. (for 2 tubes) comes in under 30 W. when multiplied by Π/4. Π/4 is the max. theoretical yield from a Class "B" PP pair.
Don't forget to chassis ground pin 1 of the tube sockets, should that not currently be the case. Pin 1, most definitely, can't be used as a tie point.
A look at the original RCA 6L6 data sheet suggests that the 19 W. number is design center that can be safely exceeded, by a small amount. Notice the 34 W., without entering 'B2 conditions, remark. 38 W. (for 2 tubes) comes in under 30 W. when multiplied by Π/4. Π/4 is the max. theoretical yield from a Class "B" PP pair.
Don't forget to chassis ground pin 1 of the tube sockets, should that not currently be the case. Pin 1, most definitely, can't be used as a tie point.
I have cracked open dozens of 6L6's and most of the variants including hundreds od 807 / 1625's, but I don't remember opening a 1614. I attended a technical high school in the late 60's that had a 3 year long electronics program and all our lab work involved tubes.
The local Air Force Base had recently donated about a ton (literally) of "scrap" which included a lifetime supply of metal 6L6's, many of which I melted. I won a bet with the teacher that proved that I could indeed make the outside metal envelope of a 6L6 glow red. Stunk up a whole wing of the school, but that was a minor inconvenience.
The metal 6L6 and many of the tubes of the day were rated according to "design center ratings." No two tubes are created equal, each will have minor alignment issues. If you were to take a dozen "identical" tubes and find the point at which plate glow starts there would be a spread in this value of several watts. Ideally the dissipation would be evenly spread across the target area of the plate, but due to cathode and grid alignment, there will be hot spots. The "design center ratings" account for this and assume that average of all tubes made will have acceptable lifetime when run at the "design center" maximum plate dissipation.
Sometime in the 40's most manufacturers switched to the "design maximum ratings" system where an ideal "bogey" tube would have acceptable lifetime when run at these maximum ratings. This resulted in about a 10% increase in plate dissipation and some other ratings on otherwise identical tubes.
The 1614 is a transmitting tube, and therefore subject to a totally different rating system.
The CCS value is the rating for Continuous Commercial Service. This is a radio transmitter where there is a high likelihood that the transmitter could be transmitting 100% of the time for the lifetime of the tube. A radio station of commercial radio base station (say for a police dispatcher) would use these ratings. Commercial service also implies regular transmitter maintenance and tube replacement (needed or not). The transmitter manufacturer specifies the maintenance interval, like the scheduled maintenance on your car. Also like your car, the transmitter manufacturers made money on service, so plenty of good tubes were pulled and, say donated to the local high school.
The ICAS rating is for the Intermittent Commercial and Amateur Service. Similar, but the radio is assumed to be receiving at least 50% of the time. We (Motorola) used 80% or 90% for mobile transmitting equipment. Therefore the tubes had plenty of down time, so could be pushed harder for the same lifetime.
TV sweep tubes also run at 100% duty cycle. this is why their ratings are so conservative.
I have no evidence to prove this, but I assume that the tubes are identical, but there could be several different versions of metal 6L6's and 1614's. All of the metal 6L6's that I cut open back in high school were from the same RCA batch made in the 50's and therefore identical. I have no memories of cutting open a 1614, but I could have, and just assumed it was another 6L6.
I got a huge lot (over 100,000) of tubes from a friend for free nearly 20 years ago. They were all scrap and headed for the landfill when someone discovered the mercury leaking from some broken rectifiers. There were many broken glass tubes including maybe 200 "807's." I was surprised at the wide variations of things called "807" mostly made for the US military during and after WWII. Most were 6L6GB's with a different base, a plate cap, and some RF shielding around the base leads. There were a lot of 6L6GC guts too (bigger plates) and a few with 7027 guts (like the super 6BG6).
I an not familiar with the Scott amp, but if it doesn't feed the plates over 400 volts, or the screens over 275, you are probably OK. I know that were squeezing over 100 watts from a quad of the old metal RCA's in HiFi and guitar amps back in high school without undue failures.
Note pin 1. It is connected to the metal can. We had stuffed the metal tubes into some old Stromberg Carlson PA amps intended for 6L6GB's. They worked fine except of a rather shocking moment when I learned that the foil cover on the second Steppenwolf album does indeed conduct electricity and Stromberg had used pin 1 as a tie point for the screen supply. Touching the record jacket to the tube resulted in a cardboard Frisbee which flew across the classroom!
We used the band saw in the metal shop down the hall. There is a glass to metal seal near the point where the can flares out to hold the phenolic base. Cut a ring around the tube above this and slide the top off. I don't remember how high up. Try about a half inch.
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Hi George, in retrospect I wish i had attended a technical high school although we most likely would not have had something as cools as endless 6L6's to melt. Probably just boring old opamps 😉
The clarification of how datasheets word maximum values and design center values clears things up for me a bit. Maybe the 1614 was a screened and re-labeled 6L6 that was able to cope with constant use better than others.
The Scott runs the plates @ 440v. The screens share a 1.5k resistor so they are probably ~300v. I will have time to look at them later and so I will measure to confirm.
The clarification of how datasheets word maximum values and design center values clears things up for me a bit. Maybe the 1614 was a screened and re-labeled 6L6 that was able to cope with constant use better than others.
The Scott runs the plates @ 440v. The screens share a 1.5k resistor so they are probably ~300v. I will have time to look at them later and so I will measure to confirm.
Yes it is a guessing game when pushing the limits.
I checked and the screens are running at 390v
300mA is what the output tubes draw. Yikes!
I checked and the screens are running at 390v
300mA is what the output tubes draw. Yikes!
No need to. Glowing anode radiates the heat to the envelope that in turn starts glowing.
Attempting to make a metal 6L6 glow red from excessive plate dissipation alone did not work. We were limited by the number of Eico 400 volt variable power supplies the we could connect together, so more power could have done the trick, but there is another way.....The same way I successfully created a plasma ball INSIDE a 6AS7G which kept growing in intensity until the glass envelope started to melt and the glass bulged OUTWARD.
Tube grids and cathodes are feeble things, and that little flat ribbon that connects the cathode to the base pins has a nasty habit of blowing in half like a fuse when things get interesting. The trick to really melting a tube is to not involve the weak links.
Upon cutting open several "identical" metal 6L6's I discovered that some had a non conductive coating on the inside of the envelope, but most did not. If you take a tube that does not have the coating on the metal and wire a BIG power supply (we used a Variac in series with a 100 W light bulb) and connect one end (negative if using DC) to the plate (pin 3) and the other end (positive) to the envelope (pin 1) there should be no current flow.
Now apply severe stress to the 6L6 (or other tube) in the usual manner until the plate glows red. What happens to a piece of metal when it glows red, it emits electrons and acts like a cathode.....and current will begin to flow from the plate to the metal envelope.....or in the 6AS7, from one plate to the other plate. Since these connections are made with heavy wire, they can sustain AMPS of current and make both objects very hot.
After a while enough ions will have outgassed from the glowing stuff to support the arc and the original stress on the 6L6 can be removed, as can the heater voltage. This plasma arc will eventually melt something. Usually the glass envelope, or the glass to metal seal in the base of the metal 6L6. Before that happened the paint peeled from the 6L6 and it's shell began to glow. a few moment later it died......sucked air.
Note, I did this stuff when I was 16 or 17 years old in the classroom of a public high school. Today I would be a little better prepared incase something went wrong. The 6L6 stunk up the whole vocational wing of the school, primarily from the burning paint and the phenolic base charring.
There should be no current into or out of pin 1 ( the case) under "normal" operating conditions. In reality some electrons will find their way to the case causing a build up of negative voltage. This is why they tell you to ground pin1. Can a resistor be put in this path and the voltage across the resistor be a measure of plate glow? I don't know, I have never tried it.
At idle? And with 440V on the anodes..... 🙄 Somehow, I don't think a metal can 6L6 is going to handle that very well.
Awesome. I salute you!
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Well I conclude they must be different valves. The 6L6 runs waaaaay hotter and I believe one didn't like it's screen over 400v and died.
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