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Author Topic: AL80A T/R Relay and Bias Control  (Read 1486 times)
K0WA
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Posts: 114




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« on: March 26, 2008, 06:33:17 PM »


I have a great AL80A.  No issues.  Works great on all bands.  Used in hard in many contests.  The one thing I hate about the amp is the T/R relay.  It goes CLUNK!  I am in the process of putting in a Gigavac GH-3 Vacuum Relay (12VDC) on the output and I plan to use a small OMRON relay for the input plus the bias control.  The OMRON relay is a 4PDT relay (12 VDC).  I am going to parallel two of the poles for the Input RF and the other two for the Bias swtich.  I've done the math, and the contacts will take the RF current up to 2.5:1.  Now to the question....

The Bias switch relay just closes the B- to ground through a 7.5 volt 10 watt Zener diode going to the center of the filament winding.  Turns on the amp of course when the relay closes.

But, thinking about this a little more (and maybe way to much) is there plate potential going through the relay..after all it is grounding the B- line so some 2500 volts at 400 mills will be going through that relay or at least across the relay.  Right?  Or am I thinking wrong here?  So, that means that there is a potential of a breakdown inside this OMRON relay (very tiny relay - 16 PIN PCB type relay)  The relay contacts can handle 3 amp at 120 volts but can it handle the voltage that I think is going to appear there?  Is there a potential for breakdown of the little relay and throwing the HV all over the place?  What to the august sages of this forum think of this?  I am having second thoughts about this relay.

Lee
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KA5N
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Posts: 4380




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« Reply #1 on: March 27, 2008, 07:16:11 AM »

A pretty long wait for the August sages, still March.
There won't be 2500 volts across the relay.  Most of the 2500 volt drop is across the tube.  When the relay is keyed then the drop across it is the zener voltage, unkeyed the drop is greater (cutoff bias).  Now as far as the relay you have selected being suitable, well Huh?  The relay will have to withstand the cutoff bias voltage (maybe 100 volts) and carry the plate current of the tube.  Since the voltage is low when the current is high, the amount of power carried by the relay is small (bias voltage X plate current).
Allen
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