Amps
[Top] [All Lists]

[AMPS] Blown TL922A... What to do?

To: <amps@contesting.com>
Subject: [AMPS] Blown TL922A... What to do?
From: 2@vc.net (measures)
Date: Thu, 7 Sep 2000 09:32:13 -0700
>
>> >The tank is typically a virtual short for VHF and UHF energy,
>> >because it has a capacitor shunting the input.
>> 
>> That's not quite right, Don. The tune C has inherent inductance, so you
>
>The series inductance LOWERS the impedance. It forms a virtual 
>short in the cases I have measured.
>
>> get resonances. The resonance frequencies vary with shape, size and
>> capacitance setting, but in all that I have put on the network analyser,
>> the lowest frequency resonance is 'series', or low impedance and the next
>> resonance is 'parallel' or high impedance. I'm using the term resonance to
>> mean points where the impedance goes purely resistive.
>> 
>> The one I looked at yesterday was 100pF max, .06" spacing and about 2.3"
>> cube overall. The first, series, resonance came in at 90-150MHz, depending
>> on C setting, and the higher one was in the 200-300MHz region. The higher
>> frequency one was low Q.
>
>I measured a transmitting capacitor commonly used in PAs. It was 
>series resonant, and a virtual short, at the frequency where Rich 
>claims considerable VHF voltages occur across the terminals of 
>the capacitor. We were talking about a specific tank system, and a 
>virtual short of a few ohms.
>
>I didn't find a high impedance (above the operating frequency) within 
>the range of my equipment, which is 1.2 GHz maximum.   
>
>The tank circuit with the transmitting type capacitor typically used 
>that I tested was a virtual short for VHF and lower UHF energy. 
> 
>> The impedance at the capacitor terminal is low either side of the series
>> resonant point. How low depends on how far you move in frequency, but it
>> doesn't suddenly go high impedance at any frequency 'close' to the
>> resonant point.
>
>Exactly. The series resonance produces a wide smooth dip in 
>impedance. Rich's guess that it suddenly goes high is a bad 
>guess, even though it fits his "theory". 
> 
>> Adding inductance in series with the capacitor (to simulate the lead from
>> an anode) changes the resonant frequencies, but doesn't change the overall
>> characteristic.
>
>The lead from the anode affect the impedance at the anode, not 
>across the capacitor or into the tank. That assumes the tank is laid 
>out with some common sense, and the lead is routed from the tube 
>to the tuning cap, and then from the tuning cap to the switch and 
>the rest of the tank circuit.
>
>The best lead arrangement generally is in one stator terminal of the 
>capacitor and OUT the other lead. This lets the capacitor act like a  
>low pass feedthrough.
> 
>> Adding a L/R parallel circuit in series with the capacitor introduces loss
>> as the frequency increases, but does not introduce multiple resonances. As
>> I understood what he said, Rich suggested that the separate current paths
>> through the L and the R should produce multiple resonances. I said then I
>> thought his analysis was wrong. I still do.
>
>Rich's analysis is wrong. 

Rich did not say this.  The discussion of current division in L-supp and 
R-supp had Nothing to do with the discussion about series resonances in 
C-Tune.  

> I can't measure multiple resonances here 
>either. There are some small ripples in impedance, but nothing like 
>Rich claims.
>
I have made no measurements of series resonance in Tune Capacitors.  I 
lack an Impedance Analyzer.   I depend on others' measurements.  

>I think the problem is Rich only has a grid-dip meter. All of his 
>resonance theories, for grids...anodes...whatever, are based on a 
>dip meter. 
>
>Dip meters do not measure system impedance or the resonance 
>mode or Q, 

... duhh

>they measure signal suck-out through magnetic field 
>coupling to an oscillator to whatever is in the field of the GDO tank 
>coil.
>
>Why not measure the impedance at the path loss where the switch 
>contacts "arc" from VHF energy? 

say what?

>I did that in one PA, and it would 
>take a tube driving many thousands of amperes at the frequency of 
>instability to arc the switch.
>
bad math.?
>
cheers

-  Rich..., 805.386.3734, www.vcnet.com/measures.  
end


--
FAQ on WWW:               http://www.contesting.com/FAQ/amps
Submissions:              amps@contesting.com
Administrative requests:  amps-REQUEST@contesting.com
Problems:                 owner-amps@contesting.com


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