# Stand-By Switching



## Lincoln (Jun 2, 2008)

What's the "best" method of stand-by Switching? 

I've been installing the stand-by switch between the high voltage center tap and ground. Seems to work great and everything I've encountered so far has a center tap. 

However, to confuse things......the deluxe reverb I'm building interrupts the B+ to obtain stand-by. Is this better? If so, why is it better? 

Is there a down side to switching the HV center tap?


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## dcole (Oct 8, 2008)

Tube rectifier?


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## Lincoln (Jun 2, 2008)

dcole said:


> Tube rectifier?


yes, tube rectifier on this one. But what if it was SS rectified?


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## dcole (Oct 8, 2008)

From what I understand, there is no need for standby on a tube rectified amp as the rectifiers typically have a controlled warm up. They typically don't pass voltage for the first 30 seconds after power is turned on to the heater. For SS rectifiers, you can use standby switching to make sure there is no voltage applied to the tubes until they have warmed up, but I am not sure what is the more technically better way. Maybe Wild Bill knows?


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## Church-Audio (Sep 27, 2014)

dcole said:


> From what I understand, there is no need for standby on a tube rectified amp as the rectifiers typically have a controlled warm up. They typically don't pass voltage for the first 30 seconds after power is turned on to the heater. For SS rectifiers, you can use standby switching to make sure there is no voltage applied to the tubes until they have warmed up, but I am not sure what is the more technically better way. Maybe Wild Bill knows?


The whole thing about cathode stripping really did not relate to receiving tubes it related to transmitting tubes. You need really high voltage and current to start stripping cathodes inside a tube. One of the other issues is when the tube is not drawing any current your filters might be drawing very high voltage maybe way above the rated voltage. This depends on the design of the amp.... But as long as your not much over 5-10% for a few seconds you should be fine. Again a good reason not to use shit caps... And for tube rectified circuits it's really not an issues. Although I did have a tweed low power twin with double rectifiers that did have a huge issue with Inrush currents. Turned out to be a oxidized fuse holder. Cleaned it put silicone grease over the contacts the problem whent away. I think another real issue is one of finding a switch with the proper doc voltage rating. I never use the standby switch on any of my amps ever. The method of using the hv centre tap is a good one. If your going to install one. Lots of amps out there don't have them. It's nice to be able to leave an amps hv off while on a set break but honestly I don't think it's necessary. And especially not necessary with a well designed power supply.


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## Wild Bill (May 3, 2006)

dcole said:


> From what I understand, there is no need for standby on a tube rectified amp as the rectifiers typically have a controlled warm up. They typically don't pass voltage for the first 30 seconds after power is turned on to the heater. For SS rectifiers, you can use standby switching to make sure there is no voltage applied to the tubes until they have warmed up, but I am not sure what is the more technically better way. Maybe Wild Bill knows?


Tube rectifiers don't suddenly snap on after a warmup time. There is a rise as they heat up till they reach the maximum voltage. Also, usually SS rectifiers don't HAVE a centre tap! They are wired as a full wave bridge. So there is no other place to put the standby switch than in series with the B+.

There are pros and cons to using an AC rated switch for DC. The overall rating to switch DC is approximately 2X the AC rating, which is a plus. However, there is a bit more likelihood of arcing, which over time can allow some carbon buildup on the contacts, causing noise.

Why did Leo Fenders and others start using a standby switch for tubes that don't really need it? All I can pass along is a story I once heard. Every player knows that his amp sounds better after a good warmup. With a standby switch, a performing musician can leave his amp hot after a set while he takes a break. When it's time for the next set he can throw the switch and instantly start playing, with no need to readjust his tone controls.

After all these years I have no reference to cite for this but it makes sense, at least to me.

Wild Bill


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## dtsaudio (Apr 15, 2009)

> Also, usually SS rectifiers don't HAVE a centre tap! They are wired as a full wave bridge. So there is no other place to put the standby switch than in series with the B+.


With all do respect Bill, not necessarily true. A lot of the Traynor amps, particularly bass amps have center tapped transformers and use full wave rectifiers. There are also Marshall and Garnet amps like this. Marshall by the way puts their standby on the AC side of the rectifier when they use a bridge rectifier.



> This depends on the design of the amp.... But as long as your not much over 5-10% for a few seconds you should be fine. Again a good reason not to use shit caps... And for tube rectified circuits it's really not an issues.


Usually you're correct Chris. But there are a couple of caveats. 
Power tubes take longer to warm up than a rectifier does (up to 2-3 times longer), the HV can exceed the cap ratings while the output tubes are still warming for a period I'm not comfortable with. The problem comes in when an amp still has its old caps in it (we know they're out there) and they may not be up to spec. Also a lot of the older amps had power transformers designed for 115 - 117 volt primaries. I've seen AC lines as high as 127V in some areas - Toronto, and Montreal to name two. That extra 10 volts can be significant on the secondary side especially during warm up.
One other warning for modern caps. It's been my experience that JJ caps don't like going over their 500V rating even for very short periods. Their lifespan is significantly shortened. If I know an amp is going to do that, I use something else. I mention this because I've seen pictures recently of JJ caps being used in guitar amps. Below their rating they are fine.
And don't get me started on the shit caps used in some modern Fenders and Crate amps.


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## Lincoln (Jun 2, 2008)

Ok, thanks for the input everybody. Where there is an HV center tap, I will use it for stand-by switching.

Switching the B+ always seems so "harsh" (to me anyway).


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## WCGill (Mar 27, 2009)

Not all tube rectifiers are slow warm-up. Directly heated tubes such as 5U4's and 5Y3's have almost instant full B+ and they're used in many amps. I do prefer standby switches for some of the reasons mentioned above and mostly because I'm "old-school". I've had a Princeton Reverb with a 5U4 for many years however, no standby switch, and tube life has never been an issue. 

To try and answer Lincoln's question, the CT-switched method is fine and seeing as the switch doesn't see DC, it doesn't have to be rated as high as one that interrupts B+. The only downside, if any, is that since no B+ is generated, there will be inrush current when it's flipped on, more of an issue for the main filter caps. When interrupting B+, I prefer to have the main caps on the hot side of the switch as I use mostly GZ34 rectifiers which don't put as much stress on the caps. SS, then it matters not. 
Other ways to put an amp in standby include output tube plate switching on the OPT CT, output tube cathode switching and screen switching, all excellent methods.


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## Church-Audio (Sep 27, 2014)

dtsaudio said:


> With all do respect Bill, not necessarily true. A lot of the Traynor amps, particularly bass amps have center tapped transformers and use full wave rectifiers. There are also Marshall and Garnet amps like this. Marshall by the way puts their standby on the AC side of the rectifier when they use a bridge rectifier.
> 
> 
> Usually you're correct Chris. But there are a couple of caveats.
> ...


Yeah when I say good caps I am not including jj  I think Ars make great caps so does Sprague. Not a huge fan of the grey caps we both know what ones we are talking about they are shit. Dont last long and they have had many bad batches over the years. I find them all the time in HRD amps. Funny you mention over voltage. I had a Soldano from 1985 in the shop that had a transformer made for Japan. The customer brought it to a shop that shall remain nameless in Kitchener ontario, they fixed it 2 times replaced caps replaced diodes ect ect. NOBODY noticed the high heater voltage almost 9 volts... I called soldano gave them the serial number they said it had the wrong transformer in it 100 volt primary instead of a 120 volt primary. The end result was the plate voltage was 565 and the heater was 9 the switching supply was supposed to be +- 15 volts * unregulated * it was almost 22. Brought the voltage down on my variac to 100 all of a sudden the hum is gone and amp is working properly. This amp had a stand by switch if it did not I doubt it would have lasted as long as it did.


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