[R-390] 6BF5 S line

[Roy Morgan]

At 09:19 PM 11/17/02 -0800, Dan Merz wrote:
>Hi,  sorry to post this here but at least it's off the unmentionable
>subject,  which I think has been put to rest,  I hope.  I am wondering
>how the 6BF5 was wired in a 75 or 51 S line receiver,  e.g. cathode
>resistor, (by-pass cap?) ,  screen resistor/voltage,  plate voltage.

Dan,

Here is what I know and think. Some of this is fact, some is my opinion:

Facts (as near as I can remember them):
1) In the 51S-1, the 6BF5 has a 39 ohm (unbypassed I think) cathode 
resistor, fixed grid bias, plate and screen at full B+.
2) The nominal 51S-1 plate voltage is 150 volts, but I found it to be 
something like 180 volts at normal line voltage.
3) The 6BF5 is rated at 5.5 watts plate dissipation MAXIMUM
4) Total tube dissipation from cathode current in my 51S-1 was nearly NINE 
watts (this includes screen dissipation, I now realize)

Opinion:
1) The 6BF5 in Collins S-line radios is run way too hot
2) If you add a cathode resistor, or change the fixed bias resistors to 
reduce the plate current, it will be a good thing.
3) Maximum audio output for a given distortion will be reduced, but 
almost  nobody needs full audio output from these radios.. If you really 
do, consider using an external audio amplifier.

There is a web site with a 6AQ5 audio mod for the R-390A:
<http://www.geocities.com/husbeck/CONTENTS.HTM>
I recommend you go look.

Roy

- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
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Home: 301-330-8828 Work: Voice: 301-975-3254,  Fax: 301-948-6213
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