[Hammarlund] Beware Russian 6SN7

[Richard Knoppow]

     The 6SN7 in my HQ-129-X measured weak on one side so I 
got a couple of Russian made ones from my tube supplier to 
replace it. They test fine in both my tube testers, an old 
Weston and a TV-7B/U but both have 60hz hum. Note 60hz not 
120hz so its coming from the heaters somehow. I got another 
6SN7GTB today, this one made by General Electric, works fine 
no hum. I don't know what the Russians are doing wrong but 
there must be some heater to cathode leakage, both tubes 
were identical.
     Another instance of a hummy tube was the 6K8 converter. 
Hum modulation on the higher frequencies. I got two 
replacements, I think both old RCA tubes, one a military 
one. One has some hum, the other is perfectly clean. All 
three test good on both testers.
     I checked the power supply before changing the tubes, 
its almost ripple free. I think the new 6K8 may have 
affected the frequency calibration but have not redone that 
yet.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com