[HBR] HBR-11/2000. Comments?

[N2EY at aol.com]

The Class B 12AX7 requires some driver power and the plate current varies all 
over the place. Distorttion is likely to be pretty high for a receiver 
application.
 
The 12BH7 pushpull audio I described runs pure Class A. No plate current 
variation! Other triodes will work, too, including some odd TV tubes like the 
12B4. 
 
--
 
In the current implementation of the Southgate Type 7, and in past 
implementations of the Types 5 and 4[2], the basic receiver ended up with a triode (6C4 
or similar) driving an ARC-5 "B" output transformer[1]. This gave ~600 ohm 
audio at a level suitable for headphones. 
 
For speaker operation, a separate chassis carries the audio power stage. This 
chassis can be stowed in or near the speaker - it has no controls. Its power 
requirements are B+, heater, and ground.
 
LC audio filters of various design can be inserted in the 600 ohm line, where 
they can be used with both headphones and speaker. An article in QST for 
December, 1971 (IIRC - might be 1970?) has a very good article on designing LC 
filters without a ton of math. Followups in Technical Correspondence also gave 
good ideas on the use of 11/44 and 22/88 mH toroids in such filters. 
 
The above approach lets you use the same audio section (speaker/last audio/LC 
filters) for several receivers, and you don't have to hunt around for scarce 
audio iron each time you get a receiver idea. 
 
   
[1] - Look for ARC-5 output transformers with a terminal numbered "6". This 
is a tap on the secondary of about 600 ohms. The full secondary has output Z of 
about 4000 ohms.
 
[2] - "Silver" receiver pictured on HBR website
 
73 de Jim, N2EY 
 
-----Original Message-----
From: Hopperdhh at aol.com
To: hbr at mailman.qth.net
Sent: Wed, 26 Jul 2006 12:08 PM
Subject: Re: [HBR] HBR-11/2000. Comments?

 The 12AX7 has been used as a modulator in zero bias, so it would be a good  
candidate for AF output.  It reportedly gave about 7 watts  output.  
Personally, I like a single 6AQ5, 6V6 or 7C5 with some  negative feedback like the 
Drake 2B.  I don't like the class B pumping on  the B+ supply!Dan K9WEK