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Engineering question about standby switches


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9/27/2000 9:54 PM
nuke
Engineering question about standby switches
On the subject of standby switches in tube amps with SS rectifiers:  
 
Many, maybe most, amps that have SS rectifiers have one set of filter caps right at the rectifier, followed by the switch to the plate supply of the output section.  
 
rectifier->caps->switch->B+  
 
I prefer a different arrangment for various reasons:  
 
rectifier->switch->caps->B+  
 
The main reason is this avoids the "bump" when cutting the current on. It also helps keep the voltages on the first filters from getting out of hand, since sometimes the combination of high mains voltage and the naturally increased voltage present on an unloaded transformer.  
 
However, my question is whether there's any reason to not use my preferred arrangement?  
 
I can't think of any, except the switch will take the brunt of the initial current surge into the uncharged filters. That might compromise the long-term reliability of the switch. Given the fairly robust nature of the usual carling switches, I don't think it is a big issue, particularly with roughly 40uf or so of bulk capacity in the filters.  
 
What do you guys think?
 
9/27/2000 10:16 PM
Ray Ivers

For what it's worth, I think the SVT way's the best:  
 
AC mains>DPST switch>HV transformer>rectifier>etc.  
 
although admittedly it has limited application in most amps.  
 
In second place, the Marshall circuit:  
 
HV PT secondary>DPST switch>rectifier>etc.  
 
I believe that both these methods tend to minimize switch arcing, since the PT winding inductance will limit the initial current surge at the instant of switch contact.  
 
Ray Ivers
 
9/28/2000 2:44 AM
tommy

"rectifier->switch->caps->B+"  
 
The switch feeding into the capacitor will fail quicker than you think. For an instant, it may as well be feeding into a short circuit when the cap is empty.  
 
I used to work for a treadmill manufacturer, and we had a relay feeding a rectifier that fed a capacitor. It was line 120VAC, and 220uF of cap. I measured the surge current into the cap, and depending on the point of the sine wave where the switch closed, the surge could go as high as 120A!! Scale back for tube circuit values, and it is still ugly.  
 
I also tried this on a Dynaco ST-70 stereo amp, and the switch got blasted pretty quick. I changed the setup.  
 
Chris
 
9/28/2000 12:37 PM
tommy

I realized something after I wrote my first message. On my ST-70, I had actually set it up like this:  
 
tranny>rectifier>cap>switch>cap  
 
That's why mine failed so miserably, the first cap was dumping into the second one when the switch was closed, and the switch was taking the proverbial whip.  
 
As someone stated in another message, the inductance and resistance of the power tranny will probably help you out as far as surges go.  
 
So upon further consideration, your idea does not sound too bad. I still like Fender's plan, where the switch feeds only into a resistor or inductor and OT CT. Still pops a little though.  
 
Chris
 
9/28/2000 2:59 AM
Bruce

I think you're doing it right and welcome to the club.  
I'll trade off the $3.00 switch against running uneeded high DC filtered voltage and the subsequent big nasty pop.  
You can always have the standby switch jumper across a large value cap that will send a little DC voltage to the cap but take forever to charge the first filter cap, if you are worried about hitting the uncharged cap with a bunch of voltage/current.  
It happens anyhow with a SS rectifier so why worry?  
The cheap B+ standby switch is a cheap idea anyhow and no one says it will last forever.  
 
Bruce
 
9/28/2000 1:49 PM
R.G.
Switch the power tube cathodes
Switch the cathodes to or away from ground. Switch current is always limited by the power tubes themselves. One terminal of the switch remains at ground. If you like, parallel the switch with a 10K 5W resistor. This just biases the output tubes 'way off when you want standby, then biases them back on when you want it to operate.  
 
Even better, read MOSFET Follies at GEO for yet another way: Use a 1KV N channel power MOSFET between the power tube cathodes and ground, and switch the gate of the MOSFET between ground (standby) and +12V (operate; produced by a zener and a resistor to B+) with any switch you like. The switch lasts forever because there's almost no voltage or current on it at all. The MOSFET saturates to under an ohm in most cases, so it has no effect on the operating sound.
 
9/28/2000 4:10 PM
Matthew Springer
Switching the power tubes off ground
RG,  
How does this method compare to the other arrangmwnt nobody's mentioned in this thread (switching the PT secondary center tap off of ground)?  
 
-Matthew
 

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