[Boatanchors] SX-25 (Help Needed Again)

[Richard Knoppow]

----- Original Message ----- 
From: "Joe Connor" <joeconnor53 at yahoo.com>
To: "boatanchors" <boatanchors at mailman.qth.net>; 
<hallicrafters at mailman.qth.net>
Sent: Sunday, June 26, 2011 7:25 PM
Subject: [Boatanchors] SX-25 (Help Needed Again)


A supposed lack-of-sensitivity problem that has been 
bothering me from the outset turned out to be an open half 
of the audio output primary. (Only one 6F6 has plate 
voltage. A resistance measurment from the center tap to the 
plate of one 6F6 is 270 ohms while the resistance to the 
plate of the other 6F6 is infinity).

For a replacement, I'm looking at the HX125D. Here are the 
specs: http://www.hammondmfg.com/125a.htm Is this a good 
replacement? I understand that I will be limited to an 8 ohm 
or 4 ohm speaker but that's fine. For purposes of installing 
the replacement, what is the primary input impedance of the 
push-pull 6F6s?

By the way, this problem existed when I got the set. I 
replaced the 'lytics and audio caps (including that 1,600V 
one across the primary) before I powered the set up. The 
audio has been the same level and quality since I first 
tried the set. I discovered the problem tonight only when I 
measured resistors and checked voltages in the audio section 
for the first time.

With your help, guys, I think I'm into the home stretch with 
this receiver. The mechanics of the replacement seem to be 
pretty straight-forward. I will finish recapping and 
checking resistors while I wait for the new transformer to 
arrive.

Again, thank you for all your help!


Joe Connor

    The RCA tube handbook gives the value of 10,000 ohms 
plate to plate for the transformer for a Class-A push-pull 
amp. The same value applies to both fixed-bias and cathode 
bias.
    If you have an original Hallicrafters speaker it will 
have a 5K matching transformer in it. Just bypass it and go 
directly to the voice coil which is probably 4 or 8 ohms. I 
don't know for certain why H used 5K outputs on many of the 
older receivers but think its because originally the output 
transformer was in the speaker. This was common at one time. 
5K is about right for many single tube amps.
     If the transformer is not sealed in a can have a very 
close look at the terminals. Its possible the winding opened 
up right at the terminal in which case some solder will fix 
it.


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