[ARC5] RF amp tubes...

[Brian Clarke]

Hello Ken,

There may be a few confounding factors here:
1. The G1 Voltage of the 12SK7 should be -3 and that of the 12SH7 should 
be -1.
2. The gain of the 12SK7 is 2 mA/V and that for the 12SH7 is 4.9 mA/V, when 
run at the correct G1 Voltages.
3. None of the 12S*7 family of pentodes was designed for low noise.
4. The 12SH7 is a sharp cut-off pentode, while the 12SK7 is a remote cut-off 
pentode. So, the 12SK7 will respond better to AVC than the 12SH7, which is 
better run flat out. One mod often made to Command receivers was to 
disconnect the 1st 12SK7 from both the AVC and RF gain controls to give 
better noise performance.

So, if you put a 12SH7 in place of the 12SK7 without altering the cathode 
resistor, it will be somewhat starved of plate current, which will lower the 
Johnson noise and the gain. And the increase in gain by a factor 2.45, 
according to my Valve book (at rated G1 Voltages), or 3 according to your 
figures is possibly about right. However, what happens if you lower the 
cathode resistor to give the 12SH7 its full head of steam?

I suspect that most of what you are seeing is the increase in gain - with a 
slight drop in noise because you are starving the 12SH7 plate. It will be 
interesting to see the S+N / N figures. I imagine THD as you increase G1 
Voltage would be interesting, too.

73 de Brian, VK2GCE

On Thursday, June 13, 2013 4:54 PM, you said:


> 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