EL84 6BQ5 unused pin question

[rp]

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Hello,
I need to put a screen resistor on an EL84 based guitar amp. I'd like to
use one the unused pins - 1, 6, or 8 - as a support terminal to
attach one side of the resistor to: e.g. resistor across 1 & 9 with the
B+ connected at 1. On a 6L6 I would use pin 6 as a support since it
is labeled NC - not connected. Looking up the basing for the EL84 shows
the 1,6 and 8 pins as IC - internally connected. I believe this
is what NC and IC stand for but I could be wrong. So I'm not sure what
would happen if I put the B+ on the 'blank' pins on the EL84.
Is IC the same as NC? Can I do it?

TIA -rp

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[Fred Nachbaur]

Robert Casey wrote:
> rp wrote:
>
>> ***NOTE*** Please remove -nospam- from address when replying ***NOTE***
>>
>> Hello,
>> I need to put a screen resistor on an EL84 based guitar amp. I'd like to
>> use one the unused pins - 1, 6, or 8 - as a support terminal to
>> attach one side of the resistor to: e.g. resistor across 1 & 9 with the
>> B+ connected at 1. On a 6L6 I would use pin 6 as a support since it
>> is labeled NC - not connected. Looking up the basing for the EL84 shows
>> the 1,6 and 8 pins as IC - internally connected. I believe this
>> is what NC and IC stand for but I could be wrong. So I'm not sure what
>> would happen if I put the B+ on the 'blank' pins on the EL84.
>> Is IC the same as NC? Can I do it?
>>
>>
>>
> No. "IC" means "internal connection", which means that the tube
> manufacturer made
> an undocumented connection to one of the electrodes inside the tube.
> Usually for
> mechanical support. The "IC" pin might be connected to the tube's
> plate, for example.
>
> You could visually inspect the base of the tubes you have, to see what
> if anything
> the IC pins are connected to. But when it comes time to replace the
> tubes sparks
> may fly and smoke released......

Good advice. While it's probably rare that an "Internal Connection" pin
would connect directly to one of the active elements, you could still
have "issues" if you use an IC pin as a tie point.

I remember looking at a tube some time ago - (I *think* it was a 6C4,
but I'm not certain, and don't have one here to check) - in which the
"IC" pin was connected directly to the getter! The thought occurred to
me, "Wonder if this would work as a diode?" but I never pursued it.

Also, different manufacturers might decide to do different things with
such uncommitted pins, so even if it worked out with one brand of tube,
it might result in unpleasant surprises if you ever switched to a
different brand.

Cheers,
Fred
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