[Amps] Ten-Tec Centurion

[Will Matney]

Rich and Tom,

I'm pretty sure there's no RF switching circuit in that amplifier. About 
the only use of this I remember in amateur amps was a mobile solid-state 
amp. I doubt it would have a relay jack if it had RF switching either. 
My guess is the same as Rich's about possible parasitics causing this. 
Tune C has a voltage rating high enough to withstand normal operation at 
the operating frequencies, but may not at VHF frequencies. The amp 
shouldn't operate at all with the switching jack un-connected. If the 
amp seems to be operating without it, that sounds like self oscillation 
to me.

Best & 73's

Will Matney

R.Measures wrote:

>
> On Nov 10, 2004, at 3:16 PM, Georgens, Tom wrote:
>
>>
>> I have a Centurion at my contest station in Barbados that works 
>> flawlessly
>> on 10-80 but will arc on 160. It is tuned into a resonant antenna with a
>> reasonable SWR.
>>
>> On 160, the amp will occasionally arc immediately on key down. If it 
>> does
>> not arc immediately, it will not arc if the key is held down. Also, 
>> when it
>> does not arc, it tunes smoothly and at the predicted settings. All of 
>> the
>> arcing is in the Tune cap and is not always in the same place.
>>
>> My theory is that there may be power overshoot from the TS850 on the 
>> leading
>> edge of the carrier that is causing the arcing. It is hard to see 
>> this on
>> the wattmeter and my 3 other amps all work fine. Does this seem like a
>> reasonable theory and is there any way to compensate for this?
>
>
>
> Tom -- Since the 3-other amplifiers do not exhibit this problem with 
> the TS-850, overshoot is not a very logical explanation. The first 
> thing I would check is the resistance of the vhf suppressor resistors. 
> If the resistance is >50% higher than the marked value, the 
> possibility exists that vhf energy damaged the vhf suppressor 
> resistors -- especially if the outside of the resistors shows no sign 
> of overheating. However, if the amplifier has been used for 10m RTTY, 
> there may be signs of over-heating and higher resistance. A common 
> problem with 2, 3-500Zs is that their 0.3pF of anode-cathode 
> (feedback) C amounts to –j4500-ohms at 110MHz. The workaround is to 
> reduce the vhf gain (and the amplitude of the vhf ringing as well) of 
> the amplifier by using low vhf Q parasitic suppressors -- the 
> trade-off of which is c 2% less power output at 29MHz.
>
>>
>> On my most recent trip, I performed a flawed experiment. For 
>> convenience, I
>> used a different TS850 to drive the amp and the problem occurred only 
>> once
>> out of dozens of tries. I then moved it to the TS850 where I had 
>> previously
>> noted the problem. In this case, I was never able to reproduce the 
>> problem.
>> I tried mistuning to some degree but no hint of the problem. However, 
>> when
>> taking the whole thing apart, I noticed that I had never connected 
>> the amp
>> relay cable. The amp was using RF sensing to key the relay.
>
>
>
> RF sensed switching invariably results in the amplifier hot-switching 
> because RF is already present when the relays in the amplifier begin 
> to switch. Hot-switching creates current transients and those in turn 
> ring the anode circuit's vhf resonant circuit formed by the Tune-C, 
> the Anode-C and the anode lead's L. In my opinion, such ringing is the 
> seed signal that gets fed back by anode-cathode C and initiates a 
> regenerative condition.
>
>> ...
>
>
> Richard L. Measures, AG6K, 805.386.3734. www.somis.org
>
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