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Using Relays to switch Power and HT (stand-by)
Using Relays to switch Power and HT (stand-by)
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Old 15th August 2019, 04:14 PM   #1
jacksonsinliss is offline jacksonsinliss  England
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Default Using Relays to switch Power and HT (stand-by)

Does anyone know if Relays introduce noise into the Power Supply if they are used to switch the Power on/off and the HT on/off (stand-by function)?

My plan is to replace the two 240V toggle switches in the common design to use 240V and 300VDC Relays instead, switched from a240V rotary 3P3T switch. The switch would be Off in p1, Stand-By in p2 and On in p3.
This is for a stereo integrated amp that I am building from scratch using ECC83's and EL34's.

Roger and out
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Old 15th August 2019, 05:12 PM   #2
Parafeed813 is offline Parafeed813  Netherlands
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There shouldn't be any need to switch HT in an EL34 amp, so I would just use a single power switch.
You can use a relay to short a dropping resistor to apply full negative bias to reduce or cut off the current through the EL34s if you want.
No sonic impact.
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Old 15th August 2019, 05:28 PM   #3
petertub is offline petertub  Sweden
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Using Relays to switch Power and HT (stand-by)
Quote:
Originally Posted by jacksonsinliss View Post
Does anyone know if Relays introduce noise into the Power Supply if they are used to switch the Power on/off and the HT on/off (stand-by function)?

My plan is to replace the two 240V toggle switches in the common design to use 240V and 300VDC Relays instead, switched from a240V rotary 3P3T switch. The switch would be Off in p1, Stand-By in p2 and On in p3.
This is for a stereo integrated amp that I am building from scratch using ECC83's and EL34's.

Roger and out
First : do not switch the B+. There is no need and it can make more damage to the tubes then you like.


Second i would recommend SSR ( solid state relay ) for the task, they make and break at zero crossings, thus no pops or bangs.

Se the wikipedia intro :
Solid-state relay - Wikipedia
then search your favorite vendor for samples.
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Old 15th August 2019, 06:20 PM   #4
kodabmx is offline kodabmx  Canada
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Using Relays to switch Power and HT (stand-by)
I switch B+ and power the relay from the bias supply. If it fails, the output tubes are disconnected from B+. I put the relay on the DC B+ line (under 400V) and use one of these.

Waterproof Car Relay DC 12V 40A 4Pin Automotive Fuse Relay Normally Open Relayvb | eBay
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Old 15th August 2019, 09:13 PM   #5
petertub is offline petertub  Sweden
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Using Relays to switch Power and HT (stand-by)
An introduction to SSR :
https://www.electronics-tutorials.ws...ate-relay.html


And some devises for sale :
https://www.electronics-tutorials.ws...ate-relay.html


The really nice thing is that those relays may be controlled by logic circuits, such as a microprocessor or even simpler a 9V battery (that would last forever)
A simple switch closure will connect the battery to the device, turing on. When turned on a voltage, basically anything, will "hold" the relay on, another switch that shorts the SSR control will turn off. The end effect is that two cheap switched will turn on and off a device using no mains voltage at the switches and doing this job silent and without disturbances.
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Old 16th August 2019, 03:25 AM   #6
PRR is online now PRR  United States
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Turning power OFF always makes "noise". Relay same a s a switch. (Non-issue for turn-ON because tubes don't work right away.)

AC coil relay will throw hum around the chassis.

The only switch I would use on B+ is the Carling vacuum-cleaner switch which has served well on a million Fenders. Relays to switch 400V into a capacitor are rare and never rated for such work. If I "HAD" to relay a B+, I would start with the relay for a whole-house air-conditioner. It may stand the strain; if it fails every year they are readily available at competitive prices (<$20).
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Old 16th August 2019, 03:52 AM   #7
Keit is offline Keit  Australia
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Bridge the B+ (HT) relay contacts with a capacitor and resistor in series.

This will do two important things:-
a) Considerably ease the turn-off stress on the contacts. Instead of the HT load current causing the relay contacts arcing over, it's shunted into the capacitor until it charges to the B+ voltage.
b) stops the generation of turn-off transients which get into other places and cause the noise.

Size the capacitor to substantially charge in about 6-7 millisec. The resistor protects the relay contacts from overcurrent surge when they close. A resistance of the same order as would draw the load current from B+ will be about right. Use a resistor of 1 Watt rating or higher - not to handle the dissipation, which can be regarded as negligible, but to withstand the voltage.

Make sure the relay contacts are rated to switch significantly more than the B+ voltage or it won't last long. Any HT filter electrolytics should be before the relay contacts to protect them from the charge-up surge. Or, if you have a vacuum tube rectifier, you can get away with a filter cap after the relay contacts.

A better solution is to have separate transformers for B+ and heaters. Then you can standby switch the AC input to the B+ transformer, which doesn't stress the contacts and a cheap relay can be used with great reliability.

Last edited by Keit; 16th August 2019 at 03:59 AM.
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Old 16th August 2019, 03:55 AM   #8
stocktrader200 is online now stocktrader200  Canada
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I get a great audio fade in as the heater warms up. would not change it now.
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Old 16th August 2019, 12:25 PM   #9
Mark Tillotson is offline Mark Tillotson
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Most relays and switches are not rated for high voltage DC - its hard to break a DC arc if enough power is available to sustain it. Typically the current limiting of a vacuum tube circuit as load prevents the arc growing too big. For a high power vacuum amp I'd be very cautious about switching anywhere other than the AC supply.


As a datapoint a 1200V 0.5A supply of mine (basically big transformer / bridge rectifier / series stacked electrolytics) can hold an arc upto about 2cm in diameter. You'd need a Frankenstein-style knife blade switch to handle that!
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Old 16th August 2019, 12:41 PM   #10
TheGimp is offline TheGimp  United States
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Quote:
Originally Posted by petertub View Post
First : do not switch the B+. There is no need and it can make more damage to the tubes then you like.


Second i would recommend SSR ( solid state relay ) for the task, they make and break at zero crossings, thus no pops or bangs.

Se the wikipedia intro :
Solid-state relay - Wikipedia
then search your favorite vendor for samples.
Be careful of the spec sheet. Not all SSRs switch at zero cross. The ones that do will specify such. The ones that don't may say nothing.
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