SAG Switch

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ViperDoc
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SAG Switch

Post by ViperDoc »

I'm omitting the standby switch on my next 18 watt build, and I'll be going with a solid state rectifier with a SAG switch. Which is the best way to wire it?

1) SPDT switch after the diodes to switch between the sag resistor and the bare lead? or

2) SPST after the diodes to switch "around" the sag resistor? I thought if the switch fails, you just have sag. Thanks.
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Bieworm
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Re: SAG Switch

Post by Bieworm »

I would replace the standby switch with a 3 way switch, so there is still a standby switch. I think the standby switch is obsolete with a tube rectifier, but with SS there is the inrush current... bad idea imho.
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Re: SAG Switch

Post by ViperDoc »

Good point. Need to make sure I don't get my "wires crossed". Bwaahahahah!
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Re: SAG Switch

Post by Bieworm »

Or even better.. put the 3way switch in OFF-SBY-ON and use the other one for sag switch.
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katopan
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Re: SAG Switch

Post by katopan »

Second option. The switch should just short out the sag resistor when the contacts are closed. Then you can safety switch it online and it's kinder on the switch.
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Re: SAG Switch

Post by ViperDoc »

katopan wrote:
Mon 12/07/20 4:48 pm
Second option. The switch should just short out the sag resistor when the contacts are closed. Then you can safety switch it online and it's kinder on the switch.
Like this, I presume:

SAG SWITCH.jpg
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Re: SAG Switch

Post by katopan »

Exactly. I've implemented that on a SE amp build for someone else and it worked fine.
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Re: SAG Switch

Post by ViperDoc »

katopan wrote:
Wed 12/09/20 5:28 pm
Exactly. I've implemented that on a SE amp build for someone else and it worked fine.
Super. Thanks.

Regarding the standby switch, as talked-about as this is on the webs, I can't find the answer about it with solid state rectification. The veteran horde seems to advocate omitting a standby switch with tube rectifiers, but it seems to be a coin flip for diodes. Any opinions or good links? I found so many cathode stripping/poisoning rabbit holes, I lost my patience.
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Re: SAG Switch

Post by JMPGuitars »

ViperDoc wrote:
Thu 12/10/20 3:09 am
Regarding the standby switch, as talked-about as this is on the webs, I can't find the answer about it with solid state rectification. The veteran horde seems to advocate omitting a standby switch with tube rectifiers, but it seems to be a coin flip for diodes. Any opinions or good links? I found so many cathode stripping/poisoning rabbit holes, I lost my patience.
It doesn't really matter. If you leave your amp on standby for 2 days, it might mess something up. If you're leaving it on standby for a few minutes, it really doesn't matter.

You have two options if the theory of it bothers you:

1. Omit the standby switch because it isn't necessary anyway
or
2. Add a resistor across the switch per Merlin's instructions: http://www.valvewizard.co.uk/standby.html

Thanks,
Josh
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crgfrench
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Re: SAG Switch

Post by crgfrench »

Re: the standby switch, I'm omitting it on my current 6V6 build and using a thermistor instead.
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Re: SAG Switch

Post by TriodeLuvr »

crgfrench wrote:
Thu 12/10/20 9:43 am
Re: the standby switch, I'm omitting it on my current 6V6 build and using a thermistor instead.
There's another way to handle this in a SS amp. Essentially, you wire the coil of a 120VAC relay in parallel with the primary of the power transformer. Then, you add a power resistor in series with the incoming line supply, after the power switch, and wire the relay's NO contacts across the resistor. With this in place, turning on the amp with "empty" filter caps causes a significant voltage drop across the resistor. As the capacitors slowly charge, the voltage across the primary of the power transformer (and the relay coil) gradually increases. Eventually, after a time delay, the voltage is high enough to activate the relay, and it shorts the resistor, removing it from the circuit and bringing the caps up to full voltage. With the right dropping resistor, this can mimic the delay of a vacuum rectifier pretty well. I can look in some of my older texts and provide a starting point for resistor values if you're interested. Just let me know.

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Re: SAG Switch

Post by crgfrench »

Thanks! What is a "SS amp"?
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Re: SAG Switch

Post by TriodeLuvr »

crgfrench wrote:
Fri 12/18/20 11:52 pm
Thanks! What is a "SS amp"?
Sorry, meant to say SS (solid state) rectifier, not amp.
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Re: SAG Switch

Post by Bieworm »

crgfrench wrote:
Fri 12/18/20 11:52 pm
Thanks! What is a "SS amp"?
A roland jc120 😄😄😄
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Re: SAG Switch

Post by ViperDoc »

katopan wrote:
Wed 12/09/20 5:28 pm
Exactly. I've implemented that on a SE amp build for someone else and it worked fine.
I neglected to ask if a Carling SPST switch rated at 120/240VAC would pass class for such a sag bypass or would you recommend one rated for 400-500VDC?
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Re: SAG Switch

Post by katopan »

It'd be fine. The switch voltage rating is what it can break across its contacts, not the insulation rating to outside (which standards require to be much higher). In this application the voltage being switched is just the drop across the sag resistor, so much lower than the rating.
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Re: SAG Switch

Post by ViperDoc »

That’s very interesting, I appreciate the explanation. Carling SPST on the back then, perfect.
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