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Switch for cathode to fixed bias, what's it called?

10/5/2005 8:11 PM
Switch for cathode to fixed bias, what's it called?
I know (I think)the correct switch to use is a DPDT or is it an spst, that has no off during the switching process. I forgot what it's called and I do not know were to get one. My ex-amp tech used one to have a switch so I can go from fixed to cathode bias. It's a neat little feature to have. It doesn't take a rocket scientist to make one but I don't know were to get one or what it's called. It was my idea to have him install it, that's one reason he's my ex-tech. TIA
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10/6/2005 1:34 AM

Well, I guess it makes a difference what circuit one is modifying and how. I am not aware of a "standard" bias select switch.  
I used a DPDT switch to do that to a PV Classic 30. A DPDT - double pole and double throw - switch is essentially two switches that select one circuit or the other in the same part. So one side of the switch selects where the grids go - either ground or to bias supply. And the other side of the switch selects whether the cathode is grounded or goes to the cathode resistor.  
In my case rather than select either ground or resistor, I just made it short across the resistor. That way there is never an open circuit. Doesn't really matter much, but I liked it that way.  
SPST means single pole and single throw.  
A switch is assumed to have two positions unless otherwise noted. A DPDT with OFF in the center would be a DPDT/center off. Sometimes it is called on-off-on. There are also switches where the center position selects BOTH ends rather than OFF. These switches are on-on-on types, you might see them as pickup selectors in guitars.  
To select a switch is easy. Neither the grid circuit nor the cathode circuit conduct much currernt, and the voltages are not very high either. SO select a toggle or other format switch that suits your space and construction methods. A toggle switch mounts in an easily drilled round hole. A rocker switch does not stick out like a toggle, but you have to cut a rectangular hole for it. You can use a push-push switch like many amps use for front panel channel switching. These mount often with a screw and spacer on either side of the pushbutton shaft.  
SO the choice is more aesthetic than anything as long as you get enough poles and throws for your circuit.  
DPDT would be enough for most conventional mods, but if you had separate bias feeds to the push and the pull, then you light want an extra pole - 3PDT. That is an example of something more complex than the usual.
10/6/2005 3:34 AM
Steve A.
It's called the Sal Trentino switch...
... after the amp tech who managed to put a patent on it. (Yes, he knows Randall Smith... ;) )  
    We were grumbling about this patent here a few years ago and who should pop in but Sal himself. He had no objection to experimenters using the circuit, but for commercial applications you do need to pay him a licensing fee. Supposedly not too expensive for the small builder... Randall Aiken was going to look into it (this was maybe 5 years ago).  
    The switch itself is usually DPDT with no center off position, but Dan Torres published a circuit with a SPDT switch in one of his Vintage Guitar articles. For the DPDT version you can see how I wired it up in my Pignose G40V amp:  
Steve Ahola
10/6/2005 7:07 AM
Make before break switch
That's what it's called. Does anybody know were I can get one?
10/6/2005 11:44 PM
Steve A.

    "Make before break" and "break before make" apply to rotary switches not toggle switches AFAIK, and are also referred to as "shorting" or "non-shorting", respectively.  
    A toggle switch will usually be "break before make" or "non-shorting" simply because of the way the switch is designed.  
    A good example of a shorting switch would be the original 3 position strat switch. If it wasn't "make before break" you would have never been able to get the notch positions by jimmying the switch. Eric Clapton would have never been able to play "Badge" with Cream and would have gotten back into laying bricks (rather than birds) for a living. ;)  
    Maybe someone can jump in and help me out here- I get a little confused when I try to think which switches should be shorting and which should be non-shorting. But I have used non-shorting switches for cathode/fixed bias with no problems (and no switching noise to speak of).  
    Speaker impedance selector switches- should they be shorting or non-shorting? On one hand I see how you always want a load on the OT secondary (shorting) but on the other hand, with a shorting switch you will need to deal with the combined impedance for a split second.  
Steve Ahola
10/12/2005 9:03 PM
Don Symes

Not that you'd flip the impedance selector 'hot', or leave it (deliberately) in a notch, but it seems that you'd prefer a shorting switch there - too low an impedance is much less fatal to a tube amp than too high. At least that's the lore I learned at the feet of the giants around Ampage.  
Then again, Mr. Aiken's speaker switcher uses non-shorting relays, so .. Whiskey Tango Foxtrot?
10/6/2005 11:32 PM
Joe Movich
Re: It's called the Sal Trentino switch...
Is this for real? I cant believe someone other than Randall Smith would patent this. I have in my collection of schematics a switch diagram for cathode to fixed bias. Does this mean I cant publish it? This is pretty much common knowledge. I certainly can't recall where I got it from. What kind of jerk would go through the patent process?

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