[AMPS] Technical Question #1

[Rich Measures]

>>As I recall, he mentioned that, however, we were discussing bandswitch 
>>arcing -- like that shown in photographs A and B on page 33 of the Oct. 
>>1990 *QST* article, "Parasitics Revisited", which is why he called me.  
>>According to Mr. Paul Pagel, Ameritron customers whose bandswitches 
>>seemingly had failed quite similarly were telephoning Mr. Rauch and 
>>asking questions -- which I presumed were somewhat "unfriendly". 
>
>However, Rich, I have problems with the concept of VHF oscillations 
>causing bandswitch damage.  

See page 33 of the Oct., 1990 *QST*.  The most severe damage you see in 
photographs A and B is to the 10m bandswitch contacts and the 160m Tune-C 
padder contacts, which are the contacts nearest to the VHF-resonant anode 
circuit.  

>The tank circuit rejects VHF signals.  

Would the HF tank reject VHF signals that are near a resonance in the 
C-Tune?

>So I fail to see how the VHF voltage could get high enough.
>
The typical L in a 10m tank coil is approx. 1uH.  Assuming that the open 
bandswitch contacts have 2pF of C (f=112MHz) and the frequency of 
oscillation is 110Mhz, could there perhaps be a problem if the Tune-C is 
resonant in the vicinity? .  

>Rather I would guess that the damaged bandswitches are either under-rated 
>for high power use on HF 

The bandswitches used in the TL-922, AL-82, SB-220, AL-80, and SB-1000,  
have a withstanding ability of approx. 6000vDC.

> (and more likely) that the amplifier in 
>question is unstable at HF and is potentially oscillating at HF.
>
>I haven't proved this, and I put if forth on the reflector for comment:
>
>Has anyone measured the impedance of the plate RFC at VHF?  The reason I 
>ask is that by the time we get to 100 MHz, the RFC has already gone 
>through several points of series resonance.  With each resonance each 
>succeeding peak impedance is less and less.  So, my unproven hypothesis 
>is that at 100 MHz the choke could actually look like a low impedance.  

In the SB-220, there is no HV-RFC resonance near the anode-resonance.  To 
know where the resonances are, one needs to employ a dipmeter.    

>Certainly much lower than the impedance of the Tune C.  I would expect 
>any VHF energy then to be shunted to ground via the choke.  So blown 
>chokes I would expect.  

In Henry 2K-4s and 3K-As, toasted, but not blown chokes, are somewhat 
common.  Typically, a few turns near the hot end have burned insulation.  
The anode resonance is near 95MHz.  

> Blown bandswitches from VHF oscillation is something I have a real problem 
with.
>
I have heard from a number of people who had real problems from blown 
bandswitches. TL-922s are probably the worst.  .   Have you perused the 
(TL-922) bandswitch photos in the 10/90 *QST* and those on my Website?


Rich...

R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures  


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