[ARC5] RF amp tubes...

[Geoff]

----- Original Message ----- 
From: "Richard Knoppow" <1oldlens1 at ix.netcom.com>
To: "Ken Gordon" <kgordon2006 at frontier.com>; "ARC-5 List" 
<arc5 at mailman.qth.net>
Sent: Thursday, June 13, 2013 12:09 PM
Subject: Re: [ARC5] RF amp tubes...


>
> ----- 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


While that is still the way used for AM the current method for CW is MDS 
which is the minimum level a signal can be heard above the noise by 3dB.

Signal leakage of the generator, cables, and other accessories used in the 
test are hard to reduce to an acceptable level in the average basement shop 
with what is generally used. Even a 8640B requires regular attention to 
details.

Carl
KM1H