[Drake] 6jb6 or 6jb6a

[Garey Barrell]

Richard -

MOST 6JB6(A) tubes work fine.  The "A" on the end merely indicates that 
the tube is designed for an 11 second warmup time to equalize series 
filament string currents.   There are SOME 6JB6s that are difficult to 
neutralize, but all that I have seen that exhibit this are "weird" 
brands such as Standard, International, etc.    There are also some 
"life" differences among even name brand tubes, but these aren't really 
significant.  I have some Zenith branded 6JB6s that appear to have been 
made by Sylvania, and they work just fine.

It IS best to use tubes of the same brand for all three if possible.  
It's also preferable to have three tubes that have close to the same 
idling current for a given grid bias, but few have the option to be that 
selective any more!.   If you have access to more than three tubes of 
the same brand, you can "match" them by plugging ONE into the final of 
the TR-4.  Turn on the power and set the idling plate current to 30 mA 
(1/3 of the nominal 100 mA for all three tubes).  A more accurate 
measurement can be made by measuring the voltage drop across the cathode 
resistor of that tube if you are so equipped.  Turn off power, verify 
that the plate voltage has discharged to zero, replace the single tube 
with another, and repeat the measurement.  DO NOT readjust the bias 
supply, leave it as set for the first tube.   Select the three tubes 
that draw closest to 30 mA (or 0.45V DC across 15 ohm cathode resistor 
for voltage method) each for your triplet.

73, Garey - K4OAH
Atlanta

Drake C-Line Service Manual
<http://hr99.home.mindspring.com/R-4C_Servicez/> 



Richard Fuoco wrote:

>hi, Is it ok to substitute 6JB6/A tubes for the 6JB6 finals in my  TR-4 Also is it a problem to use Sylvania "made" not branded tubes as opposed to Sylvania branded tubes/ Thanks in advance, Richard KM5WX
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