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

Sun Jul 14 17:03:01 2002

Do not try and set the bias at some other "random" spec. Either sonic
performance or component life will be adversely affected.
It is true that 7591-A tubes have a lifetime that is far less than desired.
In the early '70s, I was working as an audio tech in a large retail
electronics store, and there were still many of these amps in general use. I
was shoveling in bucket loads of these tubes. The culprit is extreme heat.
These tubes were grouped rather tightly, and would sear the flesh  right off
your fingers if you weren't careful.
No one at that time was hip to the concept of using cooling fans on tubes.
At that time most tubes were still  available, and cheap, so nobody cared.
Things are drastically different today, with the 7591-A being basically not
available, and quite expensive if you can find them at all.
If you intend to use one of these old Scott amps, or any other amp /
receiver with 7591-A or 7868 tubes, set the bias as per the instructions,
and use a cooling fan ( or 2 small ones ), and your amp should sound fine,
with long output tube life. The little DC brushless types that are available
today work great, and you can build a speed control circuit to slow 'em down
a bit if they are too noisy for your liking.
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.
Chuck N1LNH
H.H. Scott factory tech 1970
-----Original Message-----
From: Phil <[email protected]>
To: [email protected] <[email protected]>
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
>
>
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