[Hallicrafters] SX28 hot & blue 6V6s

Richard Knoppow 1oldlens1 at ix.netcom.com
Thu Aug 25 21:21:04 EDT 2011

     A dark blue glow on the inside of the envelope is due 
to high voltage electrons hitting the glass and is normal. 
If the glow is lighter violet and inside the tube, 
especially inside the tube elements, its gas. Its possible 
the high voltage is too high or the screen grid voltage is 
too high. Check both. If one or both are the tubes will run 
very hot.  I haven't looked at the SX-28 schematic to see 
how the tubes are wired. Also, check the grid bias. The amp 
probably has a common cathode resistor with a by-pass 
capacitor. If the cap is leaking the grid voltage will go 
down and the tubes will run hot. Check all the socket 

Richard Knoppow
Los Angeles
dickburk at ix.netcom.com

----- Original Message ----- 
From: "Guido" <laffitte at prtc.net>
To: <hallicrafters at mailman.qth.net>
Sent: Thursday, August 25, 2011 5:40 PM
Subject: [Hallicrafters] SX28 hot & blue 6V6s

> Gents,
>  It has been a while since my last post but I keep a daily 
> track of this list. My SX28 had its first audio 
> transformer replacement about two years ago with two new 
> audio tubes. These are running hot and with a clear all 
> around bluish glow. They run too hot and I don’t think 
> this is normal although we know that 6V6s tend to run with 
> some extra heat but not this much. I am sure that this has 
> been discussed before but either I missed it or my almost 
> 59 years neurons are being depleted. Opinions will be 
> appreciated.
> Best 73s
> Guido KP4FAR

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