[Hallicrafters] The SR-2000 and sellers of "MATCHED" 8122's

[Carl]

Interesting comments Jim. Ive only worked on a few SR-2000's back when they 
were relatively state of the art and didnt have any neutralization issues 
that I can recall. RCA tubes in the -28 to -35V range were the most common 
pairs and in a bulk pack of 50 there were very few oddballs and those always 
found a mate sooner or later. National sold me the tubes at $24.95 at the 
employee discount and never complained about my moonlighting repair sideline 
(-;

The NCL-2000 bias range is around -25 to -45V with the -25 tubes being the 
easiest to drive. Ive had a few Eimacs at around -47 and had to tweak the 
bias range.

Ive also modified my HF and 6M NCL-2000's with individual tube bias pots. 
About a 3V delta barely affects the IMD and up to 5V within a few dB. The 
screen supplies are regulated and the voltages reduced a bit; the 1966 tubes 
in the 6M amp were finally replaced a few year ago when the power was down 
to 700W.

Carl
KM1H


----- Original Message ----- 
From: "james.liles" <james.liles at comcast.net>
To: <hallicrafters at mailman.qth.net>
Sent: Saturday, October 23, 2010 11:58 PM
Subject: [Hallicrafters] The SR-2000 and sellers of "MATCHED" 8122's


> If you are an SR-2000 user buying matched 8122's for use in the SR-2000, 
> please be aware of the following caveats regarding the 8122.
>
> First, the Hallicrafters SR-2000 finals are run in the AB1 configuration 
> with high impedance drive.
>
> Second, the Hallicrafters engineers lamented that they had to buy a bushel 
> of 8122's to find a half dozen pairs that were matched AND would work in 
> the SR-2000.
>
> Why wouldn't any matched pair work in the SR-2000??  There are vast 
> variations in the parameters from tube to tube and between matched pairs. 
> The  Burle 8122's were color coded Red, Blue, Green, and Black to 
> distinguish the tube parameters which would help to predict the idle 
> current bias as well as the ability to NEUTRALIZE the tubes.  The Burle 
> tubes are the only 8122's that are color coded this way.  The Red and Blue 
> Burle tubes will work in the SR-2000 without modifying the transceiver or 
> power supply.  The Green Burle tubes can be made to work with 
> modifications to the transceiver bias configuration and neutralizing 
> bridge as well as a change to the P-2000 bias configuration.  The Black 
> version cannot be made to work well in the SR-2000.  Requires too much 
> bias to idle and cannot be neutralized even with mods.
>
> Why should this affect neutralization?  I  believe the reason to be the 
> precision that is required to accommodate the close spaced elements.  It 
> would appear that the control grid which should be in the shadow of the 
> screen to plate is a bit off causing the idle bias and feed back to vary 
> wildly.
>
> I would guess that the Black version will work in the NCL-2000 because 
> it's an AB2* amplifier with a low impedance input and wider bias control.
>
> The 8122 can be tested and profiled to predict matching, neutralizing, and 
> output parameters to determine the suitability of a pair for use in the 
> SR-2000.
>
> Kindest regards Jim K9AXN
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