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How do you put in a standby switch?

1/16/2006 7:00 AM
Doug Patterson
How do you put in a standby switch?
I have built numerous amps but seem to have problems with the standby switches on some builds. My last build, a Spitfire with EL34s is an example. When I turn off the standby and power the amp to play, I get a flash in the 5V4 rectifier. I've tried a couple of rectifier tubes and they do the same thing. I put a switch in line with the power transformer high voltage center tap going to ground. I'm afraid to use the high power leads because the voltages are generally higher than the 250V rating on the different switches I use.  
What are your methods of standby switching?  
Do you use the high voltage power leads with higher voltage rated switches?  
Do you do anything to soften the power inrush?  
Do you know of any resources on the web?  
By the way... The EL34 Spitfire amp is a real ear bleeder, loud crunch and grind. Thanks.  
1/16/2006 8:24 AM
Ray Ivers
I really don't believe in standby switches myself, but here's what I'd do in this amp, using a DPDT switch rated for maybe 100VAC or greater:  
a) Connect a 3.9K to 4.7K, 1W resistor (I'm guessing at these values, but any in that ballpark should work fine) across one center/left set of poles, then connect the poles to the EL84 cathode R ground point, and the other pole to the EL84 cathode R/C pair.  
b) Connect the other center/right set of poles across the MV pot.  
This will allow a trickle of idle current through the output tubes during standby so they don't self-destruct via cathode-poisoning, but since they might still be able to pass a small amount of audio, the other pole connection will mute the PA signal input.  
High-voltage and rectifier-inrush-current issues are sidestepped completely.  
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1/16/2006 6:56 PM

8 million ways to do it. Most makers ignore the voltage ratings of the switches - check them out. Marshall likes to open the AC from the PT before the rectifiers. The flash from the recto is the inrush curreent as the empty filter caps charge up.  
Peavey likes to open the B+ after the plates and before the screens. As soon as the power turns on, the caps charge, so the downstream caps are all that charge when Standby is flipped. Quieter.  
And then you can kill the signal - short the output tube grids together - that way there is no change in the DC currents.  
You can open the cathodes, a number of makers do that, and it works, and I don't recall it being noisy at all. Again, the filtars are fully charged at power up.
1/19/2006 1:59 AM
Carl@Zwengel amps

Hey Enzo, you forgot lifting the center tap on the HT winding of the power tranny.  
But if the original poster is using tube rectifiers, I'm pretty much with Ray in that there really is no reason to use a standby switch at all unless you just want to follow convention. Rectifier tubes have a built in slow warm up time so the power tubes wont get hit with full B+ until the the cathodes are adequately warmed up anyway. Solid state rectification is another story and somewhat up for debate.
1/19/2006 5:21 PM

Yep, that will work too. There are tons of ways to do it.
1/18/2006 9:29 AM

Hi Doug:  
Where would I be able to find the EL34 Spitfire amp.  
Either in kit form or already built.  
Thanks bobtone
1/18/2006 10:00 PM
hey doug, what transformers did you use in the el34 spitfire? is it really just a spitfire with el34's or is there more mods to the circuit?
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