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

Sun Oct 24 12:15:04 EDT 2010

Good morning Carl:
The design of the SR-2000 will function well with the 8122 's that idle bias 
from -26 to -31 volts.  That would be the Red and Blue Burle version or any 
other vendors equivalent.  The SR-2000 bias system will accommodate 
about -35 volts but neutralization becomes an issue for a tube that exceeds 
about -31 volts, simply too much feed back for the neutralizing bridge in an 
unmodified SR-2000.  The bridge and bias systems can be modified to extend 
the range to 8122's that idle at up to -34 volts --- that would be the green 
Burle.  I won't try for the black version as it's normally in saturation 
before it reaches 0 volts.  The AB1 configuration generates ALC when grid 
current occurs which is too late for the high bias Black Burle tubes.  I 
posted a schematic on the HHI site that has the bias and neutralizing bridge 
changes as well as numerous other enhancements and a notch filter for the 
SR-2000.  I have yet to finish the instructions for it's use and strongly 
recommend waiting for the supporting documentation as some of the changes 
are co-dependent and it would make sense to know why the changes were 
necessary and what they accomplished.

BTW Carl, I believe the NCL-2000 AB2* design will allow a swing to almost 
+12 or +15 volts. I believe that's unique to National --- how is that 
accomplished and what triggers ALC??  Would you be interested in selling any 
of those $24.95 8122's???????   Don't post an unedited answer Carl and 
remember thoughts are things.  A great day to you and yours and thanks for 
the response.                    Kindest regards Jim K9AXN

----- Original Message ----- 
From: "Carl" <km1h at jeremy.mv.com>
To: "james.liles" <james.liles at comcast.net>; <hallicrafters at mailman.qth.net>
Sent: Sunday, October 24, 2010 7:40 AM
Subject: [SPAM]Re: [Hallicrafters] The SR-2000 and sellers of "MATCHED" 
8122's

> 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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