Re: [Amps] Ten-Tec Centurion

[Will Matney <craxd1@ezwv.com>]

Ian put forth a very good point. If the amp could be switched to 
transmit mode by RF sensing, it would be illegal under U.S. law and 
could not be produced. This holds true for any amps yet or until the 
rules are changed. To me, the amp should not be able to be keyed without 
the switching jack connected to the transceiver. I could not find a 
schematic for the Centurion on the internet to download to look at the 
circuit. If someone knows where one is, let me know.

The only way I have ever seen an amp run by itself is by 
self-oscillation caused by parasitics or no bias as Bill pointed out. 
One must remember, Tom said this problem only occurred when using the 
one transceiver to drive the amp, correct me if I'm wrong on this. If 
so, that would kill the idea about the bias being bad as it would do it 
with any transceiver wouldn't it? Next, if it's a parasitic oscillation, 
what is the difference between the one transceiver and the others that 
could trigger it? About the only hair brained idea I could think of is 
that the one transceiver had more drive going in the input jack, that 
something was sensing this, and maybe putting a small amount of RF on 
the cathodes of the tubes making them conduct, thus starting up an 
oscillation. This may, or may not, be from parasitics though. Without 
the relays pulled in, the RF should be bypassed from the cathodes and 
put across the load or antenna. I dont reckon that amp would have a 
faulty reed relay where the contacts are welded together allowing RF on 
the cathodes? The only other thing I could think of would be two 
unshielded conductors being side by side, one having the RF from the 
transceiver on it, and the other going to the cathodes. This would act 
like a strip line type wattmeter pickup, and put a small amount of RF on 
the cathodes. If it were over the tune C having a pitted plate allowing 
the B+ to arc, it would happen with any transceiver used. For an RF arc, 
the amp would have to be running for the RF to be across the tank circuit.

Anyhow, before I can comment further, I'd have to see a schematic on 
this amp to see how the RF is being switched around.

Best & 73's

Will Matney



Ian White, G3SEK wrote:

> R. Measures wrote:
>
> > > Assuming the spike is at the leading edge of the transmitted signal, 
> > > then an
> > > rf sensing circuit could delay the turn on of the rf amplifier until 
> > > after
> > > the passing of the spike.
> >
> >
> >
> > Colin -- The problem is that incoming RF does not go on a 
> > mini-vacation immediately after the spike, it merely backs down to 
> > the ALC level that is set.  Thus, there is RF on the closing NO 
> > contacts while they are bouncing.  This results in hot-switching and 
> > current-transients.
>
> Also there are two sets of relay contacts, at input and output. These 
> contacts will bounce, and even if they are on the same DPCO relay, 
> they will not bounce exactly together. This means the PA can have some 
> exciting moments when the input relay is closed but the output 
> contacts are not. The PA then has full drive but no load, which could 
> lead to very high voltages across the tank components.
>
> All of that can happen without any kind of oscillation, either at the 
> signal frequency or parasitically at VHF.
>
>
> > >  If that was the case, then the arcing from the
> > > leading edge spike might not occur.  According to the facts given us 
> > > by Tom,
> > > he did not experience any arcs while the amplifier was rf sensing. That
> > > would be one explanation.  It might be the most logical one, too.  
> > > It is a
> > > simpler, less convoluted approach than parasitics.
> >
> >
> >
> > Measuring the actual resistance of the parasitic suppressor resistors 
> > and eye-balling their appearance could eliminate parasitics as a 
> > possible scenario in under 2-minutes - if the soldering-iron is hot. 
> > Not measuring R-supp will not.
>
>
>
> Fair point...
>
> I certainly agree about the root cause of the problem, though: 
> RF-activated switching will *always* be hot-switching. (And isn't it 
> illegal under the anti-CB-amplifier rules?)

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