[ARC5] RF amp tubes...

[Richard Knoppow]

----- Original Message ----- 
From: "Kenneth G. Gordon" <kgordon2006 at frontier.com>
To: "ARC-5 List" <arc5 at mailman.qth.net>
Sent: Wednesday, June 12, 2013 11:54 PM
Subject: [ARC5] RF amp tubes...


> Tonight I swapped the 12SK7s in the RF amp and the 1st IF 
> of the 1.5 - 3 Mhz receiver I
> have been restoring with two NOS 12SH7s I bought some time 
> ago.
>
> The reduction in interally generated noise was amazing. I 
> would not have thought doing this
> would have made this much difference on the 160 meter 
> band, but it did. SSB signals I could
> just barely hear with the 12SK7s were completely readable 
> with the 12SH7s and sounded
> twice as strong. I did have to peak up the IF stage, but 
> the RF stage seemed to not need
> tweaking.
>
> Tomorrow I'll measure S+N/N ratio with both sets of tubes.
>
> The 12SK7s, by the way, are also NOS GEs. The 12SH7s are 
> RCA.
>
> Ken W7EKB

     The 12SH7 has about twice the transconductance of the 
12SK7 so it will have perhaps half the noise. However, its a 
sharp cut-off tube so the AVC will not work right. If you 
can find a 12SG7 it would be a better choice. This has about 
the same transconductance as the 12SH7 but is a semi-remote 
cut-off tube.
     The standard method for measuring signal to noise is to 
measure the difference between an unmodulated carrier with 
one modulated 30% with a 400 hz tone. Most manufactureres 
use 10db but some specs are at other ratios. This will give 
some comparison of performance when changes in tubes are 
made or to check performance where the manufacturer has 
given a value.


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