• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Relays for tube voltages

I'm looking for relays to switch voltages at different stages in my DIY tube poweramp. Switching mains is not a problem, but I can't find any relay that can switch voltages over 400 VDC. It needs to be of the PCB mountable type. Most PCB relays will only do 30VDC.
Or am I reading it wrong. They all say they will do e.g. 30 VDC @ 5A. Does that mean they will do 150VDC @ 1A??
So, what relays are you guys using for high DC voltage switching? Or does any type do, as long as I use high current types at low currents??
Thanks.
 

FoMoCo

Member
2012-12-04 10:04 pm
They all say they will do e.g. 30 VDC @ 5A. Does that mean they will do 150VDC @ 1A??
Not at all. The voltage and current ratings are maximums, not a total wattage. At 1A you still switch 30Vdc. And, beware: AC and DC ratings for relays are not the same. DC is very difficult to switch at high voltages. A relay that is rated for 125VAC may only be rated for 30Vdc.

High voltage DC relays are usually expensive vacuum relays. Personally, unless mechanical isolation is needed, I'd use a MOSFET or IGBT.
 

Rundmaus

Member
2005-08-21 10:46 pm
What are you going to power with 1000µF @ 530V? Aiming for obscenely low ripple or is it a really big power stage?

I'd use different transformers for the HV and the heaters. So you can switch them independently and still keep the switching stuff on the primary side.

Rundmaus
 
Moving it to the primary side might be an option. But with a 1000uF @ 530V capacitor bank plus 7 amps of heaters, that would make a big power resistor. Or am I wrong? Any tips on values and ratings?

Then put the limiting resistor on the secondary side, but before the rectifier so it switches AC (OK, pulsating DC really). An oversize mains relay should handle that.
 
Then put the limiting resistor on the secondary side, but before the rectifier so it switches AC (OK, pulsating DC really). An oversize mains relay should handle that.

Off course! Why didn't I think of that! 380 Vrms should be doable.
Moving it to the primary does have the advantage of limiting the inrush current for the other suplies as well though (DC heater for phase spiltter etc.)

I.I.R.C., 7 amps at 6.3 volts will present about 192mA at the primary side. And although the heaters are still cold at startup, so only a small amount of HT current will flow, I could acount for an idle load of 400mA at startup. I believe a 22k 5W limiting resistor would be sufficient. But I could be wrong...

@Rundmaus: it's for a 125 Watt poweramp.
 
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Are there any drawbacks of running the heaters (4x KT88) at lower voltage/current for a couple of seconds at start up??
No, you can put a high wattage resistor in series with the KT88 heaters and bypass it with a relay. This will surely limit the inrush current.

For such projects with high power I always use a SS relay as JCX suggest.

Never ever switch high voltage DC, to switch off a stage consider switching off the filament of the tube in use.
 
In your OP you stated that you are only intending on switching out the current limiting resistor. If that is all you are doing, then you are not actually switching the full HT voltage, only the voltage that is dropped across the resistor. Of course the relay will need to be insulated from other parts of the circuit but that is generally not an issue.