[TenTec] SB220-Tubes and QSK

[Gary Hoffman]

Why not, if the emissions and gain of the old, worn tube were down ?


----- Original Message ----- 
From: "Ken Brown" <ken.d.brown at hawaiiantel.net>
To: "Discussion of Ten-Tec Equipment" <tentec at contesting.com>
Sent: Thursday, November 01, 2007 2:21 PM
Subject: Re: [TenTec] SB220-Tubes and QSK


> The logic seems sort of sound, except:
> 
> If an amplifier is designed to work with 3-500Z tubes, that is it has 
> the bias set to the proper voltage for clean linear and efficient 
> operation with 3-500Z, and it has a output network designed to match the 
> plate impedance of 3-500Z operating at the plate voltage and bias 
> voltage the amplifier provides to the tubes, how can a tube that is also 
> a 3-500Z operate differently and be more efficient than a 3-500Z?
> 
> Seems like it is not exactly a 3-500Z, but rather a variant of a 3-500Z, 
> and if it is dissipating less power than a 3-500Z, I would be suspicious 
> that it actually needs a slightly different bias voltage to operate 
> properly. If it is dissipating less power, I would suspect that it is 
> operating more on the Class C side with current flowing in a smaller 
> portion of the RF cycle. I would wonder about the linearity.
> 
> DE N6KB
> >  I  
> > had seen the reviews saying that the SB-220 ran cooler after  
> > installing the new tubes. Reading this casually, I sort of  
> > disregarded it. But when I observed it for myself, I was impressed.  
> > It is beginnng to make sense. The input power was about the same  
> > before the new tubes. If the amp is 60% efficient now, it must be  
> > dissipating about 670 watts. If the additional power I am now getting  
> > out was previously being dissipated, the amp was dissipating 300 -400  
> > watts more before or about a KW. That explains why it is running so  
> > much cooler. If all this logic is sound, the efficiency with the old  
> > tubes was only about 40%. Now that is a good reason to buy new tubes  
> > even though only a dB or two power gain is realized.
> >
> >   
> 
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