Magnecraft HV relay sticking.

[Naysayer]

I have no experience with Vacuum switches.
The Magnecraft reed-relay (SPST) I had between HVPS and plate choke began to stick. Thankfully, a safety habit of 'discharge before touching' paid off with only a frightful bang! Autopsy to look inside relay: 2 flat metal strips in a glass tube (non-vacuum) passing through a coil -all covered in wax- in a plastic case. I'm surprised it lasted as long as it did given it's low 2-digit current rating. Another learning experience & now it's onward to a proper vacuum relay.

I ordered the cheapest but new SPST vacuum switch I could find: A Soviet relay rated for 10A, needs 22vdc to keep closed so it should substitute without a fuss. About 2" tall pic attached if I can upload. I like the G3SEK circuit that opens a relay disconnecting HV to RF deck when a fault is detected. I placed it between HVPS and plate choke. I do recognize DC is hard on relay contacts. The relay stays closed after warm-up, rather than opening & closing with each T-R cycle. If placed in an AC line it would be either: pre-FWB or on the primary side of Plate Transformer. Both those locations would conflict with the Soft-Start. I found this decision process confusing and I'm still not certain about it. Relay location was more of a default but the Magnecraft did work well in same location for awhile. Only remaining option I can see is not using the circuit at all.

gotta cut this short
neil, n2eye

 

[nomadradio]

Brings to mind the test that Eimac suggested to see if your amplifier circuit would properly protect the 4CX1000A tetrode. Its extremely high-gain control grid is made from exceedingly fine wire that has a grid-power rating of zero Watts. Their suggested test was to short your HV supply into a piece of #40 gauge wire. If it shut down before the wire suffered damage, you were probably okay. And if it was too slow to protect that piece of wire you ran the risk that it would not protect the tube from a surge. Pretty sure I never built a HV power supply that would pass that test.

73

 

[Naysayer]

Could be an expen$ive proposition.

 

[glassinsulator]

Try a diode across the relay coil.

 

[linearone]

What amp is this? What tube is it?
Why are you breaking the b+ and not the primary of the plate trans? Does the amp have sufficient cutoff bias? Why are you worried about it?

 

[nomadradio]

Breaking a hot circuit draws an arc across the gap between the contact points as they draw apart. Usual strategy is bigger contact points. Some vacuum relays are characterized for breaking a hot circuit, and others are not. Finding original specs for russky relays might or might not be possible. And interpreting them to determine what sort of service they're meant for is not my specialty.

I do remember finding that vacuum relays designed only to break a low-current/voltage circuit was cheaper than one characterized for hot switching. A vacuum relay won't make a bright oxy/nitrogen arc when it breaks a circuit, but if it's not meant for hot switching, that service won't be kind to it.

Oh, and linearone, this relay isn't being used for transmit/receive switching like they do in a D&A amplifier. This is a fault-protection circuit. Should not get activated all that often, but serves to protect components from fault surge currents.

73

 

[Naysayer]

A real struggle to find a voltage source for vacuum relay (24v but it works with 14). I tried to increase the 12vdc HV trigger voltage (replaced the controlling 7812 with a 7815 and even the 12v zener on the 7812 Base with a 15v zener but nothing raised the voltage above 12. Eventually placed a voltage tripler on filament transformer. Yes it another relay slows it down but it better than adding another transformer. The Tet Boards do not like circuit add-ons. Even when chassis grounding is not relied upon.

Now, I'm back to finding a Load Cap solution.

 

[Naysayer]

Not so fast.

Trying to use old parts when I can. but the 24v relay is troublesome.
Voltage Tripler is out, I'll spare the details, cap swaps, relay too big, etc.
Ordered 24v transformer to power the V relay and a tiny low-current SP NO relay to control it. Putting the T. Boards back the way they were. When I copy circuits, I should do so exactly and ignore the urge to get creative.