[Hammarlund] Hello list - HC-10 and 6AU6s.

[Richard Knoppow]

----- Original Message ----- 
From: "Kenneth G. Gordon" <kgordon2006 at frontier.com>
To: <Hammarlund at mailman.qth.net>
Sent: Monday, April 02, 2012 10:14 AM
Subject: Re: [Hammarlund] Hello list - HC-10 and 6AU6s.


> On 1 Apr 2012 at 22:29, David  Thompson wrote:
>
>> Al, W8UT says the list needs more traffic and I have not 
>> been on the
>> list since the original reflector in 1998.
>>
>> I just got a HC-10 SSB adapter and started checking it 
>> out.  I will
>> have more questions later but for now wonder why a 
>> previous owner put
>> 6AU6's in both IF's which call for 6BA6's.
>
> Probably because the 6AU6 has much greater 
> transconductance, and
> therefore lower noise and greater gain.
>
> However, since it is a sharp-cutoff tube, it will royally 
> screw up any AGC
> applied to it, and therefore makes the circuit much more 
> prone to overload.
>
> Also, in my experience, they are not particularly reliable 
> either.
>
> When a circuit calls for 6AU6s, I always use 6AH6s or the 
> military
> equivalents thereof. Those are far more reliable, cheaper, 
> more available,
> and are somewhat closer to being "semi-remote cutoff" 
> types.
>
> Even so, IMHO, I would never use either of those in the 
> HC-10's IF strip.
>
> As far as I know, all of the above tubes have the same 
> base-connections and
> are direct plug-in substitutions.
>
> If I had your HC-10, I would go back to 6BA6s before I did 
> any tune-up or
> restoration of that adapter.
>
> There was an excellent article in the latest issue of ER 
> Magazine on the HC-
> 10, BTW.
>
> Ken W7EKB

     The 6AU6 and 6BA6 have virtually the same 
transconductance with equal plate and screen voltages. The 
6AU6 has no advantage over the 6BA6 where the design calls 
for the second.  The 6AH6 has about double the 
tranconductance of either of the others but is a sharp 
cut-off tube like the 6AU6.  The increased gain may cause 
problems where its not designed for.
     As you point out using sharp cut-off tubes in an AVC 
circuit will screw it up.  These tubes have a little range 
of gain variation but not much and can cause distortion. 
The 6BA6 was so widely used because it works very well as a 
variable gain tube and is reasonably quiet even as an RF 
tube at communications frequencies.  In a SSB adaptor like 
the HC-10 the signal levels are high enough so that tube 
noise is of no consequence anyway plus its all at low 
frequencies.
     These are good converters and rare as hen's teeth.


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