The AM Forum

I would like to provide a low power, high power
selection mode for a small tube amplifier. The HV
transformer I have has a pair of primary winding
available for 120 or 240 V. No secondary CT.
So the relays Ry1 and Ry2 could be arranged to place the winding
series aid (LV) or in parallel for HV operation. All operation
is 120 V in. Any issues here? Do I have to protect
against hot switching. Should I have spark suppressor
R-C on the relay contact points?

Thanks, Alan

Depends fully on the relay type and size. We used to swich RF generator power this way between various power setting using contactors, up to 50 kW. If you use a hefty relay capable of switching motor loads in this power range, I don't see a problem. You Mains switch does the same without snubber networks.

Shouldn't be a problem to hot switch the primaries.  No different than when you turn on the on/off switch.  But better to do this with a single DPDT relay.

Using two relays could cause a problem if one or the other fails to trip.  Possible timing of the two relays could also be a problem.  Work out the circuit with one DPDT relay

Fred

Thanks for the inputs guys. Fred, agree, one relay.

One concern on the hot switch and I have to go through design details, is the tuned plate load Z in low power will be quite incorrect in high power. So I may force some sort of protection just in case.

The snub components are minimal overhead, I may add them just for insurance. The relay I have in mind is a motor control unit. Switches 15 HP 220 V motors.

Are you sure a 15HP contactor is going to be big enough ;D  How big is the xfmr??

Ha Ha Fred. Typo. 1/5 HP. The transformer, its not that large. 20 ohm secondary, 1250 V/1 A.

I think the contact ratings are 10 A. 

Alan

A xfmr with a 1250v/1a secondary can draw over 10 amps on the primary when run on 120vac.  You may need a heavier relay than 10amps.

Hi Fred. Yes, although I will not be even close to that secondary load current.
It is more transformer than I require for the application.

Seems to me it would be a bit more "rugged" to use both primary windings in parallel & phase for 120V operation.

Just to keep the VA through it linear, ya know.

73DG

Correct Dennis. However, I wanted a low power tune mode.

I am not demanding high current from this xmfr. Although
it will handle easily 1A plate Ip, I am down at 300 ma
probably tops.

I use a resistor with a relay across it for tune-operate in my 100 Watt.

If it's a tetrode, you can drop the screen voltage.

With the arrangement shown, if you tune for 1/4 power out in low, when you double the voltage, you should be pretty close to being in tune.

Double the voltage, quadruple the power output.  Tis how we amplitude modulate!

--Shane
KD6VXI