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Standby switch

8/6/1999 6:53 PM
Standby switch
I've seen standby switches either right behind the rectifier diodes or behind the choke and the first caps.  
Which way better for the reliability and longevity of the amp?
8/6/1999 9:40 PM
Which way better for the reliability and longevity of the amp?  
The best way is to put the standby before the first cap.  
8/8/1999 3:26 PM
Benjamin Fargen

You can run a standby switch in between the PT secondary center tap wire and ground. This takes the stress off the first set of filter caps (note: a Fender style standby circuit) and the standby switch due to the fact that you don't have the B+ voltage running through the switch.  
8/9/1999 6:36 PM

Next problem: the standby switch is located  
right behind the rectifier diodes, but in front of  
the first cap. But the switch still makes a noise when I switch on the B+ at my Plexi clone.  
By the way, the switch is new, rated 250V/2 amps and a good brand.  
How can get Rid of this "plop" or "crack"?  
8/9/1999 9:09 PM
Benjamin Fargen

I have run this same standby configuration and I have never had a noise problem like this before.  
How much B+ are we talking about here? 350-450vdc? I have always been dealing with a lower B+ with this config.  
It might be caused by the high B+ hitting the PS caps full on. Notice how Fender ran all of their fixed bias amps with the standby switch after two stacked 80uf 450v caps.(the stacked caps are used to double the voltage rating because there is no load on the B+ during standby and this would exceed the voltage rating of a standard 350-450vdc cap. 450vdc + 450 vdc = 900vdc rating and 80uf used because the capacitance is halved when configured this way...80uf + 80uf = 40uf)  
This must have filtered out some of the AC ripple before the B+ moved on to the rest of the PS caps and the rest of the amp....and maybe this stopped any clicks or pops from the standby switch from becoming a problem  
Can anyone else add some insight?  
8/10/1999 3:03 AM
Carl Z

Unless I'm mis-reading the thread here's what's going on. If you lift the CT of the tranny on a fixed bias amp you're also removing the bias voltage from the power tubes. When you take the amp off standby you're getting a whopping huge current surge through the power tubes. That's what's giving you your popping. It's also not too good for your tubes either. If you notice the manufacturers that use that arrangement are also using cathode biased amps and current surges aren't nearly as big an issue.  
A really slick method I've used with good results is to put the standby switch between the rectifier and the parallel caps. then bridge the switch with a small cap in the range of 0.047uF. There's virtually no surge current and the amp just gently begins conducting.  
Hope that was of some help to you.  
Carl Z
8/10/1999 4:02 AM
Benjamin Fargen

Your right! I should have asked him if the amp was cathode biased or fixed bias before I suggested the center tap standby method (my apologies Carsten!)....very good point!  
But I don't think he is using the center tap stand-by method at the moment. From what I gathered from the post is that he is running a standard stand-by switch after the rectifier diodes and before the first PS cap and still getting a pop..........maybe Carsten should clarify before we go on.  
Thanks for the input Carl.  

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