Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jeff on December 22, 2011, 07:47:21 pm
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I was looking at the data sheets for an EL84 and for pin 1,6, and 8 it says - internal connection.
What does this mean? I understand no connection but what are those pins internally connected to? Can those pins be used for tie points?
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> internal connection. Can those pins be used for tie points?
Exact answer? No.
If you need tie-ponts, add terminal strips.
The IC pins can be connected to ANYthing inside the tube, at the option of the maker.
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> internal connection. Can those pins be used for tie points?
Exact answer? No.
If you need tie-ponts, add terminal strips.
The IC pins can be connected to ANYthing inside the tube, at the option of the maker.
Sorry PRR , I must desagree with you .
EL 84 socket pins 1,6, 8 are not connected to the tube, so we can use it to tie points. Jeff is right.
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I agree with PRR
May be someone is interested to read, with atention, this
http://archive.ampage.org/threads/1/gadc/028895/Wiring_up_EL84s-1.html (http://archive.ampage.org/threads/1/gadc/028895/Wiring_up_EL84s-1.html)
Kagliostro
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those who say the pins are not internally connected should use them as tie points and see what happens :icon_biggrin:
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Sounds like an internal connection is "like a box of chock-o-lots"
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Some tubes connect to them for extra support for screen grid or whatever. You can look in the tube and see if they are connected but the next tube you get may be different.
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some manufacturers chose to use pin 1, and as a consequence those amps can only run some EL84's... :BangHead:
http://forum.metroamp.com/viewtopic.php?f=10&t=11245 (http://forum.metroamp.com/viewtopic.php?f=10&t=11245)
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Sorry PRR , I must desagree with you .
EL 84 socket pins 1,6, 8 are not connected to the tube, so we can use it to tie points. Jeff is right.
You might be able to use the pins as tie points, but it is bad practice. Murphy's Law ensures you will find out why it is bad practice as soon as you drop a $230 pair of NOS 6550's (http://www.kcanostubes.com/content/nos-jan-ge-6550a-matched-pairs) in your "swallows all octal tubes" amp that uses cathode bias.
On most octal output tubes of the 6L6/6V6/6550 class, pin 1 is not used. In the EL34, pin 1 is the suppressor, and must be physically connected to the cathode at pin 8. In general, many amp builders might tie pin 1 to pin 8 even in a 6L6 amp, just to be universal and allow the option of EL34's later.
Being a conscientious builder, you probably included some spring-type tube clamps that grip the bases of the tubes to keep them firmly in the socket. But the GE 6550 (http://www.mif.pg.gda.pl/homepages/frank/sheets/135/6/6550A.pdf) could have pin 1 not connected, or it could be connected to the metal shell around the base. Since your amp is cathode biased, and you strapped pin 1 to pin 8, the cathode of the tube is essentially connected to the shell if you have bad luck.
Your luck gets even worse, as that spring clamp is bolted to the chassis. That also means the cathode is connected to the chassis, which is probably grounded. As a result, your cathode resistor is shorted, and your expensive 6550's now have zero bias. They quickly redplate and melt.
Note that the data sheet said "S/NC" which means that pin 1 could either be connected the base shell, or connected to nothing, purely at the manufacturer's convenience.
You might be right to assume that "unused pins" could be used as tie points. You have a better chance of being right if the pins are simply not present on the tube's base (as is common in 5U4 rectifiers). But some manufacturers will add the extra (supposedly unused) pins to offer more stability in the socket. Are you sure you know they're not connected to anything in the tube? How about in the latest Chinese knockoff of a 50 year-obsolete tube, such as we commonly use?
The point is, while commonly seen, it's "best practice" to avoid any connection to those pins.
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the only pin you can use as tie point is the 6Th, for the control grid resistor ( usually )
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If you have official data: "NC" is safe, "IC" is hands-off.
And there were a few non-conforming batches, and lots of new-made tubes who may not always do what the original sheets say.
> included some spring-type tube clamps ... 6550
I am going to say there were no 6550-size bear-grips. The 6550s I've seen in mobile service were held down by top-clamps.
You did not mention 7027's extra G2 pin, which seems to be intended to blow-up sockets not wired for 7027.
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Good points. Well, go ahead and deduct bonus points for a bad example. :l2:
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Ok so IC can be anything.
That reminds me, I haven't played mine sweeper in a while. :l2:
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I have an old tube stereo that I was going to part out but on second thought I want to get it up and running. Pin 1 is connected to pin 2. I took the new tubes I'm going to be using, and with a multimeter, tested for connection between pin 1 and every other pin on and got no connection(on the tube itself).
Is it safe to assume I can use these tubes with this amp.
I really don't want to, but is it a good idea to rewire the original wiring so pin 1 is not connected to pin 2? I assume the only real reason to do that would be IF I replaced the tubes and IF those new tubes had pin 1 internally connected to something there'd be a problem.
This is how it was wired stock so I assume it's OK.
I'm just thinking if there's something kooky that I don't know about. Like, a certian mfg thought that the tube was better with a cap between plate and grid(or some other element) and it works better if that is inside the tube so the plate is connected through the cap to pin one, or something wacky like that. Another element in the tube, internally connected, but not directally connected.
Does internal connection always mean a pin that is directaly connected to one of the other pins or could it mean something else?
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I have an old tube stereo that I was going to part out but on second thought I want to get it up and running. Pin 1 is connected to pin 2. ...
I really don't want to, but is it a good idea to rewire the original wiring so pin 1 is not connected to pin 2? I assume the only real reason to do that would be IF I replaced the tubes and IF those new tubes had pin 1 internally connected to something there'd be a problem.
Widen your scope.
Are these octal sockets? So then pin 2 is one side of the heater wiring? If so, does the connection to pin 1 ultimately go to ground? I'm wondering if there is one wire going to pin 7 of the octal sockets (one side of the heater wiring) and the other side of the heater winding grounded (along with pins 1, 2 of the octal sockets).
If so, the issue may be whether you care to use the unbalanced-style heater wiring.
Does internal connection always mean a pin that is directaly connected to one of the other pins or could it mean something else?
Yes. :laugh:
Meaning, you don't know if the pin is unused or connected to some other element inside the tube. Safety dictates you simply not use this pin.
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See your point on the octals. But
They're EL84s. Just wondering why they are tied together if it's bad to use that pin. Just as easy to go only to pin 2 and leave pin 1 unused.
for a EL84:
1=IC
2=Grid
I measured NC between pin 1 and any other pin on the tube. OK to put these tubes in or should I rewire?
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They're EL84s. Just wondering why they are tied together if it's bad to use that pin. Just as easy to go only to pin 2 and leave pin 1 unused.
for a EL84:
1=IC
2=Grid
Maybe the amp is wired for 7189As also?
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Cool.
Thanks for all the help.
I think what I'm going to do is leave it wired as is because the tubes I'm using have no connection. I'll leave a note so I don't forget to check the new tubes if I need to replace them years form now.
I just need to get some cap cans before I try it out.
(and put the OT I "borrowed" for my other amp back in)
I'm glad now I decided to restore it instead of scavange it for parts. Can't wait to hear what it sounds like.
Thanks for the help.
Jeff
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"IC" can be a support rod. Intended to "float" with no applied voltage. It is possible (if unlikely) that applying a voltage will push the beam of electrons out of the normal path, change tube operation.
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OK. I'll disconnect them. That is the right thing to do. I guess maybe I was trying to talk myself out of changing the original wiring but this is the right thing to do. Having those pins connected doesn't help any, and can only lead to problems under certian, however unlikely, conditions.
"If it's worth doing, it's worth doing right"
Wonder why it's wired this way? Maybe They did do it for a different substi"tube"tion. I see what you mean with the 7189, that pin 1 is grid, but pin 2 is grid on the tube too so it would work anyway.
Thanks,
Jeff
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... Pin 1 is connected to pin 2. I took the new tubes I'm going to be using, and with a multimeter, tested for connection between pin 1 and every other pin on and got no connection(on the tube itself). ...
I have an old (late 50's) amp that connects the output of the driver tube to pin one of the el84. This works because the internal connection in the OLD el84 is pin 1 to pin 2. I have checked only GE & RCA. The amp won't work with some new tubes because pin 1 & 2 are not internally connected. To make it work with new tubes like the JJ's I have, I need to move the input to pin 2 of the EL84 or connect pins one & two externally.
You need the input to end up on pin 2 if you remove the external connection on yours.
RCA data sheet says "Pin 1- internal connection, do not use"
JJ just shows no connection in their diagram.