[AMRadio] Viking II Power Mod

Thu Oct 20 15:39:34 EDT 2011

Hi Ron,

Hold on there before you melt three otherwise perfectly good 813s...!

Just because a trio of that FB tube might be good for 1.5 kilowatts of input, that does NOT mean they'll do the same in AM continuous carrier service. A good rule of thumb that I was told years ago by KA9QLF & W2UJR is this: the amount of watts of continuous carrier output you can SAFELY get from a tube in AM linear amplifier service (remember, you also have TWO sidebands in there!) is equal to HALF of that tube's plate dissipation rating. 

An 813 alone can safely dissipate 125-watts, correct? Half of that is 125 divided by 2, or, 62.5-watts. With three 813s, you can safely get 3 x 62.5, or, 187.5-watts of continuous carrier output in AM mode.

Again, that's LESS than twice what you're getting from your Viking II now, without doing anything.

A few years ago I used my paralleled grounded-grid 813s as an AM linear, boosting my 10-watts, or so, of carrier from a HB 2E26 final (modulated by a pair of 6AQ5As), to 125-watts...that's a 12.5-fold increase in power output, a VERY worthwhile effort. Boosting a V2's 100-watts of carrier to 187.5-watts is---again!---simply not worth the pain & the strain of doing so, IMHO...

~73~ de Eddy VE3CUI - VE3XZ

*********************************************************************************************************************

On 2011-10-20, at 2:28 PM, W6OM wrote:

> Thanks Eddy
> 
> Interesting point. 3 X 813's at 2800V and 500 ma delivers a lot more than 
> 180 watts if the 813 spec sheet is accurate.
> 
> It's just a hunch but I think I'll give it a try anyway, if not workable I 
> will just build a big 8 -10 db attenuator.
> 
> Thanks again for your thoughts, that is what I was looking for.
> 
> Ron W6OM
> 
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> ----- Original Message ----- 
> From: "Eddy Swynar" <deswynar at xplornet.ca>
> To: "Mel Farrer" <farrerfolks at yahoo.com>; "Discussion of AM Radio in the 
> Amateur Service" <amradio at mailman.qth.net>
> Sent: Thursday, October 20, 2011 10:30 AM
> Subject: Re: [AMRadio] Viking II Power Mod
> 
> 
>> Hi Ron,
>> 
>>> 
>>> Greetings everyone.  I have a simple question concerning my old Viking 
>>> II.   I am just finishing up a 3 X 813 linear and need to consider how to 
>>> drive it with the Viking II.
>>> 
>> 
>> I don't think it'd be worth your while to even bother...
>> 
>> Consider the math: three 813s would give you, what, some 180-watts of 
>> carrier output as an AM linear, right? Well, you're probably already 
>> getting 100-watts of carrier output from that V2 as it stands now. The 
>> difference would be a mere 80-watts with the 813s, not even twice what the 
>> V2 is already giving you...
>> 
>> And keep in mind, that for a 3 db. increase in signal strength, you'd need 
>> to DOUBLE the power...and for a just perceptible increase in power, you'd 
>> need at least a 6 db. increase.
>> 
>> It just ain't worth the effort, IMHO...
>> 
>> ~73~ de Eddy VE3CUI - VE3XZ
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