6GX8 as Audio Level Indicator? - diyAudio
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Old 21st October 2011, 03:49 AM   #1
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Default 6GX8 as Audio Level Indicator?

Hi all.

I'm building this amplifier:

http://hollowstate.netfirms.com/777schematic.pdf

my question is this: I have all the parts, tubes, iron (well maybe, see ahead) and I'm ready to go on this except for one thing. I have no experience with level indicators and eye tubes, so the 6GX8 is making me worry. I have 8-ohm speakers and right now i have an OPT that is 5k Primary PP with an 8ohm secondary. So- does this mean I can't use the level indicator? Or can I put the indicator on the 8ohm secondary?

I'm confused as hell on how that even works. It would seem that the 16ohm tap could be only used exclusively for the indicator, or am I wrong and you can also attach your speaker?

Or do I just put the indicator circuit at the 8ohm tap and run my speaker in parallel?

No clue here, any help appreciated.

-Steve
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Old 21st October 2011, 05:31 PM   #2
TheGimp is online now TheGimp  United States
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The 16 ohm tap would put out more voltage (1.4141 times as much) but the tuning indicator only represents a 10K load.

Put it on the 8 ohm tap along with your speaker. It will be less sensitive on the 8 ohm tap but based on the telefunken spec below I don't think it will be an issue.

That said, I don't see how it works.

It is configured as a 'gridleak bias' stage, but the input resistor is too small (470K) for gridleak operation.

Telefunken :

http://www.r-type.org/pdfs/eam86.pdf

Be sure the 0.18/200 is a low leakage cap. Any leakage will kill the gridleak bias operation.

Shows -4V bias to shut off the eye (close) but shows a 3M grid resistor in series driven by the signal source. I'd change the 470K to a 3Meg resistor.

The data sheet is not completely clear. It might be that the tube is meant to be biased off (hence the 470K) and negative going peaks cause the movement towards closure.

Last edited by TheGimp; 21st October 2011 at 05:50 PM.
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Old 21st October 2011, 06:46 PM   #3
Yvesm is offline Yvesm  France
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Quote:
Originally Posted by TheGimp View Post
. . .
It is configured as a 'gridleak bias' stage, but the input resistor is too small (470K) for gridleak operation . . .
Think to the grid / cathode diode, it's just a peak rectifier.
The link cap charges itself thru that diode on positive alternances making the grid more negative and reducing plate current.
It discharges thru the grid leak resistor.
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Old 22nd October 2011, 02:24 AM   #4
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Thanks for the help guys. I'll try it and see what happens!
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