Working on a Seeburg JL with metal cased RCA 6L6's. (don't recall seeing those before) While trying to find an intermittent very loud snap, crackle, and pop, I get a bit of a shock off the metal case of the one 6L6. Turns out the plate is shorted to the metal enclosure! The base must be a little small because the tube clips weren't grounding out the B+, although it may have been intermittently and that may have been the noise it was making. Put my meter on it and it was about 395vdc to ground, .1 ohms from pin 3 to the metal case.
Tony
On 10/15/23 17:13, Tony Miklos via Jukebox-list wrote:
Working on a Seeburg JL with metal cased RCA 6L6's. (don't recall seeing those before) While trying to find an intermittent very loud snap, crackle, and pop, I get a bit of a shock off the metal case of the one 6L6. Turns out the plate is shorted to the metal enclosure! The base must be a little small because the tube clips weren't grounding out the B+, although it may have been intermittently and that may have been the noise it was making. Put my meter on it and it was about 395vdc to ground, .1 ohms from pin 3 to the metal case.
Interesting! On metal 6L6s and most metal tubes the case is tied to pin 1 and this is usually grounded to chassis in the amplifier.
Double-check the base of the tube and make sure that the center locating pin hasn't broken off or been forced. You might be measuring pin 1. The construction of the tubes has an interior glass envelope that insulates the tube elements from the case.
It's also possible that pin 1 was used as a hot tie-point, but in the era of the JL, metal 6L6s were still somewhat common and I would think Seeburg wouldn't have done that.
Another possibility is that the phenolic base insulation broke down and developed a carbon track between pin 3 and the case.
I'll check that out tomorrow and report back. Whatever did go wrong, it did have B+ voltage from it to ground (but zero from the other 6L6 to ground). I always wondered how they got metal cases to seal good enough, I didn't know they were glass inside! Now you have me thinking, if the keyed pin in the center was off, and the tube was turned a few notches, that would make sense for it to have B+ on the case.
Tony
On 10/15/2023 8:38 PM, Jay Hennigan via Jukebox-list wrote:
On 10/15/23 17:13, Tony Miklos via Jukebox-list wrote:
Working on a Seeburg JL with metal cased RCA 6L6's. (don't recall seeing those before) While trying to find an intermittent very loud snap, crackle, and pop, I get a bit of a shock off the metal case of the one 6L6. Turns out the plate is shorted to the metal enclosure! The base must be a little small because the tube clips weren't grounding out the B+, although it may have been intermittently and that may have been the noise it was making. Put my meter on it and it was about 395vdc to ground, .1 ohms from pin 3 to the metal case.
Interesting! On metal 6L6s and most metal tubes the case is tied to pin 1 and this is usually grounded to chassis in the amplifier.
Double-check the base of the tube and make sure that the center locating pin hasn't broken off or been forced. You might be measuring pin 1. The construction of the tubes has an interior glass envelope that insulates the tube elements from the case.
It's also possible that pin 1 was used as a hot tie-point, but in the era of the JL, metal 6L6s were still somewhat common and I would think Seeburg wouldn't have done that.
Another possibility is that the phenolic base insulation broke down and developed a carbon track between pin 3 and the case.
My experience with metal 6L6's is that if you touch one they give you a hell of a burn.
On Sun, Oct 15, 2023, 8:49 PM Tony Miklos via Jukebox-list < jukebox-list@lists.netlojix.com> wrote:
I'll check that out tomorrow and report back. Whatever did go wrong, it did have B+ voltage from it to ground (but zero from the other 6L6 to ground). I always wondered how they got metal cases to seal good enough, I didn't know they were glass inside! Now you have me thinking, if the keyed pin in the center was off, and the tube was turned a few notches, that would make sense for it to have B+ on the case.
Tony
On 10/15/2023 8:38 PM, Jay Hennigan via Jukebox-list wrote:
On 10/15/23 17:13, Tony Miklos via Jukebox-list wrote:
Working on a Seeburg JL with metal cased RCA 6L6's. (don't recall seeing those before) While trying to find an intermittent very loud snap, crackle, and pop, I get a bit of a shock off the metal case of the one 6L6. Turns out the plate is shorted to the metal enclosure! The base must be a little small because the tube clips weren't grounding out the B+, although it may have been intermittently and that may have been the noise it was making. Put my meter on it and it was about 395vdc to ground, .1 ohms from pin 3 to the metal case.
Interesting! On metal 6L6s and most metal tubes the case is tied to pin 1 and this is usually grounded to chassis in the amplifier.
Double-check the base of the tube and make sure that the center locating pin hasn't broken off or been forced. You might be measuring pin 1. The construction of the tubes has an interior glass envelope that insulates the tube elements from the case.
It's also possible that pin 1 was used as a hot tie-point, but in the era of the JL, metal 6L6s were still somewhat common and I would think Seeburg wouldn't have done that.
Another possibility is that the phenolic base insulation broke down and developed a carbon track between pin 3 and the case.
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Yes, they were running hot as hell! It's because the metal absorbs the heat that would have passed through the glass through radiation. Sort of burnt my fingers on one earlier but I was quick. I think it just about melted a micro fiber cloth when I used that to pull them.
Tony
On 10/15/2023 9:39 PM, Paul Howlett via Jukebox-list wrote:
My experience with metal 6L6's is that if you touch one they give you a hell of a burn. _______________________________________________ Jukebox-list mailing list -- jukebox-list@lists.netlojix.com To unsubscribe send an email to jukebox-list-leave@lists.netlojix.com %(web_page_url)slistinfo%(cgiext)s/%(_internal_name)s Searchable Archives: http://jukebox.markmail.org/
On 10/16/23 01:51, Tony Miklos via Jukebox-list wrote:
Yes, they were running hot as hell! It's because the metal absorbs the heat that would have passed through the glass through radiation. Sort of burnt my fingers on one earlier but I was quick. I think it just about melted a micro fiber cloth when I used that to pull them.
The original 6L6 and 6L6G were rated identically and lower than the 6L6GC. There was also a 6L6GA which like the 6L6G had the ST-14 "Coke bottle" envelope and the 6L6GB which like the 6L6GC had the ST-12 straight glass envelope.
Absolute maximum plate voltage went from 360V to 500V and plate dissipation from 19 to 30 watts between the 6L6 and the 6L6GC.
6L6/6L6G datasheet: https://frank.pocnet.net/sheets/049/6/6L6.pdf
6L6GC datasheet: https://frank.pocnet.net/sheets/049/6/6L6GC.pdf
If the amplifier nameplate specifies 6L6GB or 6L6GC, you risk running a 6L6 metal or 6L6GA tube above its ratings depending on the design of the amplifier.
I'd examine the base of the metal 6L6 that shocked you under a magnifier. See if there's a carbon track in the base between pin 3 and the case. If so, that's probably the cause of both the noise and the shock.
On 10/15/23 17:49, Tony Miklos via Jukebox-list wrote:
I'll check that out tomorrow and report back. Whatever did go wrong, it did have B+ voltage from it to ground (but zero from the other 6L6 to ground). I always wondered how they got metal cases to seal good enough, I didn't know they were glass inside! Now you have me thinking, if the keyed pin in the center was off, and the tube was turned a few notches, that would make sense for it to have B+ on the case.
Definitely inspect the socket. Pin 1 should be either left floating or preferably tied to ground. If it's used as a tie point there's no problem with glass 6L6s but it will be connected to the case if a metal tube is used.
On 10/15/2023 10:27 PM, Jay Hennigan via Jukebox-list wrote:
On 10/15/23 17:49, Tony Miklos via Jukebox-list wrote:
I'll check that out tomorrow and report back. Whatever did go wrong, it did have B+ voltage from it to ground (but zero from the other 6L6 to ground). I always wondered how they got metal cases to seal good enough, I didn't know they were glass inside! Now you have me thinking, if the keyed pin in the center was off, and the tube was turned a few notches, that would make sense for it to have B+ on the case.
Definitely inspect the socket. Pin 1 should be either left floating or preferably tied to ground. If it's used as a tie point there's no problem with glass 6L6s but it will be connected to the case if a metal tube is used.
OK, I did have the pin numbers mixed up when checking for a short with a meter. But the tube was in the socket correctly. This morning it made the noise (turned the wattage down to 1 so it didn't blow a speaker). I tried only the suspect 6L6 without the other one in and it acted up. So I swapped the tube to the other 6L6 socket and had my hand on it so I could yank it out if it acted up. It did act up big time, and with each crackle I got one heck of a shock until I got my hand off of it. I had tested the tube before, but tested it again and if checking the emission for 20 seconds to really warm it up, it did show shorts, but with this tester I don't know where the short is. Also when hot it starts to show grid leakage and it keeps going higher as I keep running the test. The socket #1 pins are not grounded, maybe luckily or it may have fried a transformer. I won't bother grounding them since I'm installing 2 matched glass ones. By the way, none of this happened while it was on the bench.
Thanks for your insight on metal tubes! Now to test it more. It normally acted up as it was warming up so it will get a lot of on/off cycles to test.
So, just out of curiosity, since this is something I've been wonderingabout for decades : -) What is the purpose of metal envelope tubes? Durability? Shielding? Conducting heat?
David Breneman david_breneman@yahoo.com
On 10/17/23 07:48, David Breneman via Jukebox-list wrote:
So, just out of curiosity, since this is something I've been wonderingabout for decades : -) What is the purpose of metal envelope tubes? Durability? Shielding? Conducting heat?
Ruggedness and shielding are the main advantages. Possibly a cost issue, not sure of the manufacturing costs vs. glass at the time.
Between semesters I was working for a co. that handled A.V. repairs for the NYC public schools. I perused their OEM files and service bulletins. One from Rheem Califone mentioned a model of their portable phono/PA units using 6L6 metal tubes causing shocks due to "capacitive conduction" internal to the tube. It sounds to me like it was really the sort of thing you experienced. The company offered free 6L6G replacements.
RobNYC
On Sunday, October 15, 2023 at 07:14:30 PM GMT-5, Tony Miklos via Jukebox-list jukebox-list@lists.netlojix.com wrote:
Working on a Seeburg JL with metal cased RCA 6L6's. (don't recall seeing those before) While trying to find an intermittent very loud snap, crackle, and pop, I get a bit of a shock off the metal case of the one 6L6. Turns out the plate is shorted to the metal enclosure! The base must be a little small because the tube clips weren't grounding out the B+, although it may have been intermittently and that may have been the noise it was making. Put my meter on it and it was about 395vdc to ground, .1 ohms from pin 3 to the metal case.
Tony
_______________________________________________ Jukebox-list mailing list -- jukebox-list@lists.netlojix.com To unsubscribe send an email to jukebox-list-leave@lists.netlojix.com %(web_page_url)slistinfo%(cgiext)s/%(_internal_name)s Searchable Archives: http://jukebox.markmail.org/
On 10/15/23 17:13, Tony Miklos via Jukebox-list wrote:
Working on a Seeburg JL with metal cased RCA 6L6's.
[snip]
The base must be a little small because the tube clips weren't grounding out the B+, although it may have been intermittently and that may have been the noise it was making.
The metal base tubes have a different profile than the glass based ones. Metal bases have a smaller indented main profile with little notches where the case is crimped on. Glass bases are larger and straight. The clips are to hold the tube in place. They aren't for grounding. That's what pin 1 is for on metal tubes and grounding obviously isn't needed with glass tubes.
The fact that pin 1 was not grounded on the amplifier tells me that it was probably designed for glass tubes. Is there a printed designator next to the socket, and if so does it just say 6L6 or something like 6L6G or 6L6GB ?
On 10/16/2023 3:17 PM, Jay Hennigan via Jukebox-list wrote:
The metal base tubes have a different profile than the glass based ones. Metal bases have a smaller indented main profile with little notches where the case is crimped on. Glass bases are larger and straight. The clips are to hold the tube in place. They aren't for grounding. That's what pin 1 is for on metal tubes and grounding obviously isn't needed with glass tubes.
The fact that pin 1 was not grounded on the amplifier tells me that it was probably designed for glass tubes. Is there a printed designator next to the socket, and if so does it just say 6L6 or something like 6L6G or 6L6GB ?
Thanks Jay, yes I know what the clips are for, I was just saying that if they _were_ making good contact with the metal base, who knows what damage may have occurred to the amp, like the transformer in particular. I don't know of any jukebox that came with metal 6L6's, but they come to me in all kinds of conditions. Not saying that no jukebox ever came with metal 6L6's, just none that I remember seeing. Even though this particular amp comes with a 2 amp slow blow fuse, I normally test them with a 1 amp fast blow. During that shocking excitement today, the 1 amp fuse did blow.
I wasn't really asking any questions, just stating the fact that I got shocked by a metal 6L6, and again it happened today while I had a good grip on it, I didn't just brush my hand across it like the first time. I've been shocked more times than I can remember and it was a short for certain. The crackling from the speakers was in sync with the shocks (it happened fast). It's a first for me and I thought I'd share it with you guys. Yes, I did get the pins wrong when looking for a short, thanks for catching that and it's good to know about grounding metal tubes.
Tony
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