[Vintage-Audio] More info on Scott LK-72 biasing

[Phil]

> By the way--running the amp without the rectifier tube installed does
> NOTHING towards re-forming the power supply caps, since there is no B plus
> anywhere in the amp without this tube installed.

That would be true, EXCEPT, I put some silicon rectifiers and a current
limiting resistor on a old octal tube base, plugging it in the socket and
used that so that I could bring up the Voltage slowly over a very long
period of time (well over a week).  Bringing it up slowly with the regular
tube installed wouldn't do it as the tube doesn't conduct until the voltage
already gets rather high! This also allowed me to put a meter across the
resistor so I could monitor the "forming current" (I've been doing
'lectronics a long time).

Phil


> Chuck N1LNH
> H.H. Scott factory tech 1970
> -----Original Message-----
> From: Phil <k06bb@elite.net>
> To: vintage-audio@mailman.qth.net <vintage-audio@mailman.qth.net>
> Date: Saturday, July 13, 2002 3:56 PM
> Subject: [Vintage-Audio] More info on Scott LK-72 biasing
>
>
> >Hello all
> >As some of you know, I'm working on a Scott LK-72 amplifier here.  I'd
> >recapped the paper caps and then, 'hoping' the 'lytic cans were good
spent
> >over a week slowly re-forming them in with no tubes, just a silicon
> >rectifier.  The caps appear ok now as none blew up in my face at slightly
> >below rated Voltage.  I then put the rectifier and pre-amp tubes in it
and
> >set it to just slightly below rated line Voltage (to compensate for lack
of
> >output tube current) and let run again.  No smoke or flames <grin>.
> >
> >All this was taking place while I was waiting for the copy of the service
> >info to get here so I could 'properly' set up the bias on the output
tubes.
> >As some of you may recall, this amplifier has a slightly different output
> >circuit with two chassis mounted RCA jacks and a switch labeled Bias and
I
> >wasn't sure how to set it up.
> >
> >Well, after the manual arrived I discovered that there was no mention of
> >this 'mod' in the service instructions.  However, there were two,
slightly
> >different  schematics of the unit.  Come to find out that although the
> front
> >panel says LK-72, this Scott is actually a LK-72B with a schematic
drawing
> >dated 1963.  (the plain Jane LK-72 is dated 1960).  The schematic shows
the
> >jacks and switch.  Apparently, Scott added a 3.3 Ohm resistor in each of
> the
> >two output section cathode circuits and a bias pot for each channel in
> >addition to the DC balance control (4 pots total).  So this allows you to
> >adjust both bias and balance of each channel.
> >
> >Now, looking at the schematic, those jacks don't appear to actually be in
> >the cathode of the tubes  but elsewhere, though one side of the switch
does
> >switch between the cathodes of the two channels.  Either that or there is
> an
> >error in the drawing!  I'm going to dig a little further into the actual
> >wiring of the Amplifier,comparing it to the drawing.
> >
> >But, the way things are, I can just drop my  Voltmeter across the 3.3
> >cathode Ohm resistor in each channel and set the bias that way for rated
> >current and the DC balance for minimum hum as the 'manual' says.  Not pay
> >any attention to the switch and RCA jacks.  Those were probably installed
> so
> >that one could set the bias without removing the bottom cover.
> >
> >Of course in the paperwork that also came, Scott said the amplifier used
> >"fixed non Adjustable bias and the tubes were very conservatively run for
> >long life"  8^)
> >
> >This brings up another question.  In the RCA Tube manual RC-29 the
limited
> >specifications say that for Class AB1 Pushpull running at 450 Volts plate
> >Voltage the nominal plate current (for two tubes) should be 66 mA.  The
> >Voltage in this Amp is slightly higher, Scott says 463 (though I won't
know
> >for sure till tubes are installed).
> >
> >QUESTION:  Considering the high cost of 7591's, should I bias them
slightly
> >"cool" for long life?  I know that among guitarists, some like to bias
cool
> >for longer life while others run them somewhat hotter for that "special"
> >sound.  I don't have a scope and Oscillator to check crossover
distortion.
> >
> >73 de Phil  KO6BB