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Re: [Amps] Ten-Tec Centurion

To: "amps@contesting.com" <amps@contesting.com>
Subject: Re: [Amps] Ten-Tec Centurion
From: Will Matney <craxd1@ezwv.com>
Date: Thu, 11 Nov 2004 14:22:40 -0500
List-post: <mailto:amps@contesting.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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