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Old 2nd October 2008, 03:07 PM   #1
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Default Where do you put the standby switch

Hi guys
This question is borne from another recent post where someone had problems with the standby switch kind of exploding when he switched in b+.
I have used a setup where the stanby switch is on the centre tap of the Power tranny to ground cause I figured that running 450V B+ on a switch rated at 250V 10A might be a tad risky.
No doubt my reasoning is flawed cause its too straight forward a method.. or is it?
What does everyone else do in this scenario
Thanks
Nick
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Old 2nd October 2008, 03:55 PM   #2
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In the rare case where I do use one I have generally placed it in the center tap lead as you have done. This works great of course unless your PT has a bias tap on the secondary winding which you are using, in which case strange and bad things can happen...

My more usual approach is to switch the primary of the plate transformer as I tend to use separate transformers for filaments and bias in my more complex designs.. This of course is a total non starter with guitar amplifiers..

In all cases I use an rc snubber across the switch contacts to reduce arcing.
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Old 2nd October 2008, 04:07 PM   #3
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Default I wont be using the bias winding

Hi Kevin
This will be for a guitar amp. The original puts it in the b+ line but i could no doubt put it on the ct as I dont intend on using the bias tapping.
Pardon my ignorance but in case i change my mind could you please elucidate on a snubber circuit with may be a schematic... please
Nick
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Old 2nd October 2008, 06:21 PM   #4
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I've put a standby switch on the center tap of the secondary. I paralleled a 1 meg ohm resistor across the switch to prevent the secondary from floating while the switch is open. I've heard people theorize that nasty voltage potentials could develop. With the resistor in parallel you won't truly cut off the B+, but it'll be a couple of tens of volts at most.

I've been careful never to open the standby switch while the amp is running, for fear of arcing. I really only use it to warm up the output tube's cathodes before applying B+. I open the switch after the amp is powered down and the power supply caps have had sufficient time to bleed down.
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Old 2nd October 2008, 08:05 PM   #5
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Switches don't like DC. As I understand it... if you open a switch with DC current flowing, if (when) an arc forms, it will tend to sustain because the current never crosses zero. If there is AC current flowing, the arc will self-extinguish as the current passes through zero.

Putting a switch in the CT might work; even though the current is only flowing in one direction at that node, it does at least theoretically go to zero twice per cycle. Not sure if this holds true in practice.

So (assuming you cannot switch the primary) it would be safest to use a 2-pole switch and open the secondary of the transformer. Your 250VAC-rated switch will probably be OK for a bit more voltage than the rating - assuming it is a UL/CSA/VDE-type safety rating there is quite a lot of margin. Of course, to be safe you could go out and find a switch that is rated for 440VAC or 600VAC. Probably hard to find, though.

Some info here:

http://www.allaboutcircuits.com/vol_4/chpt_4/2.html

Pete
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Old 3rd October 2008, 06:00 AM   #6
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Default Values for snubber circuit?

Thanks to all for replying. Hi pete, I hope you are well.
I might stick with the switch on the CT of the transformer but place a snubber accross it.. Anyone suggest the values of the cap and resistor in the snubber?
Thanks again.
Nick
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Old 3rd October 2008, 05:00 PM   #7
kevinkr is offline kevinkr  United States
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I'd start with 1K and something like 0.01uF - 0.1uF, make sure the cap is rated higher than the pk ac on the winding.. (1KV film or ceramic ought to be fine) Some iteration in values may be necessary to reduce the arc..
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Old 3rd October 2008, 05:13 PM   #8
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I hold my breath when folks talk of standby switches and HV equipment -- not done in HV ham gear -- use a relay (if you have to) -- put one of these across the contacts:

Click the image to open in full size.


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Old 3rd October 2008, 06:27 PM   #9
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Quote:
Originally posted by jackinnj
-- use a relay (if you have to) --
I agree 100%, especially over 300V. This is what I use:

http://catalog.tycoelectronics.com/T...PN=KUP-11A15-6

Though not clear on the datasheet, the relays I have are labeled with a 600VAC contact rating for both 2 and 3 pole verisions. The 6V AC coil version is nice to use with standard filament power, or you can use the 120VAC coil version.

Pete
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Old 3rd October 2008, 09:55 PM   #10
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Default I have found the perfect Standby switch

I think I have the solution... Dont use a standby switch!!!!
I just wouldnt be happy knowing that my newphew's fingers are a heartbeat away from HV disaster with this arrangement, so the more i think about it the more I just will rely on the slow turn on characterisitics of the gz34.
Of course I could issue each nephew with rubber soled boots and HV gloves although strumming/paying guitar may well be difficult.
I may well use the relay for my applications but ditching this setup for others amps also reduces the complication factor and possibility of burnt relays etc etc. I note these relays in Australia are around $125 dollars each yikes!!!!
Thanks for all the help
Nick
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