[Top] [All Lists]

[AMPS] Re: Drive Level in amplifiers

To: <>
Subject: [AMPS] Re: Drive Level in amplifiers
From: (Jon Ogden)
Date: Mon, 23 Mar 98 22:51:47 -0600
>>One other point on drive level:
>>If the point were true that input impedance varies with drive, then in 
>>order to have an amplifier system that is operating properly, one must 
>>always operate it at maximum drive.  
>>If not operated then at maximum 
>>drive, the matching network is not properly attenuating harmonics, not 
>>providing the proper "flywheel" affect, etc.
>- The flywheel makes things mo betta, and it's always there.  

Again though, my point on matching network impedances still stands.  If 
at a low drive level my tube impedance is say 150 Ohms and at high drive 
levels it drops to say 75 Ohms then a matching network design to match a 
150 Ohm impedance to a 50 Ohm impedance will NOT work properly with a 
load of 75 Ohms.

In fact, the input impedance of a tuned circuit designed to match 50 Ohms 
on its input to some output impedance may actually look MUCH worse with a 
different impedance on it's output.  And you won't get proper power 
transfer either.  Optimal power transfer to a a load (such as a tube) 
occurs when the impedance that load sees looking back toward the source 
is the complex conjugate of it's impedance.  For example if the tube has 
an impedance of 125-j200 Ohms, then for proper power transfer from 
matching network to tube, the tube wants the matching network to have an 
output impedance of 125+j200 Ohms.  If the matching network therefore is 
designed for an output impedance of 125+j200 Ohms, but the tube changes 
to something like 80-j130 Ohms, optimal power transfer will not exist.  
And if the power isn't transferred to the tube, where does it go?  It 
goes BACK to the exciter and hence appears as an SWR.

So just having inductors and capacitors on your circuit doesn't 
necessarily make things always better.
>>It is not always beneficial to run an amplifier at maximum drive.  
>During ssb modulation, the drive varies widely during each spoken word.  
>However, max. peak drive and max. anode-I is essential to correct tank 

What does correct tank adjustment have to do with the input impedance?

>Please consider that:  Below a certain drive level, zero grid current 
>flows.  At max. drive, grid current flows.

I would say that this depends on how the tube is biased, but for argument 
sake, let's assume you are right.

However, what does the flow of grid current have to do with tube 
impedance?  Maybe I am showing some ignorance here, but again, I haven't 
seen this.

I guess we will see eventually who is correct here.  If I am able to take 
a noise bridge, measure the tube impedance and match it up, then I will 
be correct.  If however, it doesn't work than I am wrong.  Also, then I 
will conclude that 5 watts into the tube would be considered a large 
signal condition.



Jon Ogden

"A life lived in fear is a life half lived."

FAQ on WWW:     
Administrative requests:

<Prev in Thread] Current Thread [Next in Thread>