Correct Screen Voltages for Federal 864 Limiter

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lassoharp

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Can anyone post some of the 6SK7 screen voltages (w/respect to ground) they're getting on their builds or from original units?

The manual says what appears to be 118V but I'm getting quite a bit lower on mine - all other voltages looking normal.  Typo?


Thanks
 
bump


. . Surely someone has documented the voltages for their Federal builds?  No?


If not - anyone want to venture a guess at which sounds correct:

118V - the value printed in the online manual

11.3V - value that was measured  on Soundguy's ailing unit here (was never confirmed):

http://www.groupdiy.com/index.php?topic=7517.0  


schematic:

http://www.waltzingbear.com/Schematics/Misc/AM-864.htm

 
typical data sheet examples show around a 3.25 to 3.5:1 ratio for scrren current VS plate current. So if you're drawing 13mA plate, you should have about 4mA screen current. What's your current through R6 (15k) VS both plate resistors combined?

11V sounds rediculously low, more like what you would find on a fender 5c1, where the 6SJ7 has a 2MEG screen resistor.
 
gemini86 said:
typical data sheet examples show around a 3.25 to 3.5:1 ratio for scrren current VS plate current. So if you're drawing 13mA plate, you should have about 4mA screen current. What's your current through R6 (15k) VS both plate resistors combined?

11V sounds rediculously low, more like what you would find on a fender 5c1, where the 6SJ7 has a 2MEG screen resistor.


Current through R6 = 12.6ma,  combined plate current = 1.5ma 

I've got a 5K plate balance (supply feed type) so that would add another 2K or so to each plate slightly lowering the plate current value.  It looks like the stock plate current value would be ~ 2.0 ma combined? (45V drop across 47K plate R ).  I tried inserting a 10ohm resistor in the screen supply branch to get a current value but this was too inconsistent - shows ~ .06ma.  It appears that nearly all of 12-13ma is being taken through the current control circuit.  That doesn't seem right. 

I'm not used to seeing that low value either but my screens have been between 12 and 13V depending on tube, and all other voltages look normal.  I must be missing something simple.

 
Current through both tubes combined is 1.5ma? That doesnt seem right. Check voltage drop across r6. Voltages readings on each tube and b+ readings all laid out would help to visualize this. Or if you're feeling up to it, add your voltages to the schematic and post it.
 
Thanks gemini86 and abbey R,

That's well within what I've been measuring.  My B+ is slightly low and my R6 is lower than stock (200V) for the time being, so that measures to scale ok.

I guess they must have meant 11.8V.
 

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abbey road d enfer said:
Screen voltage is derived from B+ via 16:1 voltage divider (R6 15k, R8 1k).
I can evaluate B+ at ca. 230V, so the screen voltage should be about 14V.
Thanks, I missed r8 somehow.
 
lassoharp: I've built many Feds and your screens are spot-on.  Low volatge there keeps the SK7s close to shut-off allowing the remote cut-off property of the input grid(s) to come alive in almost triode mode.  A poor man's 6386.

A trick: throw in a balance pot to balance the tubes via the screens. Do a search here and you will find it.  I chose a different way, equally effective, minimizing stray hum and motor-boating.

And don't fret the lower B+, it should keep you just under zero-dbfs output so you won't need an output attenuator.

PM if you like.
 
I planned on trying the screen balance pot.  I've got a pot on the plates right now that works fairly well when using a decently matched pair of tubes but isn't great for eliminating hum or buzz rejection.



 
I tried the plate pot and noticed it got rejected.  Works for about a second.  This is how I learned that the screens are the real gatekeepers to current flow.  Heck, some old tube amp designs switch the screens to ground for standby mode: not 100% effective though, hence the term 'old'.  ;)
 
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