[ARC5] 10 meter BC-454 - screen voltage

[Geoff]

Several years ago I found it far more beneficial to do the evaluations in an 
already built receiver that had wide open access. It was a S-40B that was 
near pristine inside but a former owner had painted the full exterior with 
white latex paint and did a sloppy masking of the control lettering.

The price was right....free; and the only way to do RF evaluations is at the 
RF frequency desired and the S-40 covers from the BCB to 44 MHz.
I used that test bed to evaluate a whole range of octal tubes as well as a 
few others using adaptors.

Running tests at just DC is no better than using a tube tester that has or 
has been modified to allow a wide range of element voltage variations.

OTOH I do have a rather elderly TEK 570 curve tracer which has come in handy 
many times.

Carl




> Hello Ken,
>
> The tube characteristics written in tube manuals (whose?) are for 
> establishing other characteristics, like life, gain and emission. They are 
> not set in stone.
>
> If you want a quiet receiver, I would leave the Voltage regulator tubes 
> out - they rely on ionisation of the enclosed gas = very noisy. I know 
> 'coz I tried it once - with a BC-454.
>
> If you want to experiment with tube element Voltages, set up an octal 
> socket on a breadboard, and measure noise and distortion vs Voltage. 
> Trying to do that inside a built receiver makes it very difficult to 
> discover what's causing what.
>
> 73 de Brian, VK2GCE.
>
> On Friday, March 08, 2013 8:14 AM, you said:
>
>
>> Looking at the tube tables, I see that the screen voltage to the 12SK7s 
>> is 150 VDC when the
>> plate voltage is 250 VDC. When the plate voltage is 100 VDC, the screen 
>> voltage should also
>> be 100 VDC, according to those tables.
>>
>> Both ratings are for when the tube is operated in Class A.
>>
>> This receiver is still using the original screen-voltage voltage-divider 
>> (the two black wire-
>> wound resistors standing up in the back). Screen voltage is 75 VDC to all 
>> the tubes in the
>> receiver, except the 12A6. Plate voltage, with the power supply I am 
>> presently using, is 275
>> VDC.
>>
>> I wish to reduce the plate voltage to no more than 180 VDC. I plan to use 
>> a VR-75 and a VR-
>> 105 in series for this.
>>
>> What is the consensus concerning screen voltage to the RF and IF amps?
>>
>> My plan is to raise it to at least 100 VDC via a resistive voltage 
>> divider across the 180 VDC
>> regulated input.
>>
>> In your opinion, what might be the result, both pro and con?
>>
>> In my opinion, this will result in greater gain (due to the higher 
>> relative SG voltage) and lower
>> noise (due to the reduced plate voltage) at the RF and IF amps.
>>
>> Ken W7EKB
>
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