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Re: So.......


 
1/11/2005 6:16 PM
jeremy
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Re: So.......
yes.  
 
jeremy
 
1/11/2005 9:24 PM
Steve A.
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Re: Is this the correct place for a standby switch?
jeremy said:  
 
put the switch after the bridge, but b4 the first filter cap.  
 
    Well, there seem to be two schools of thought on this... the other opinion would be to connect the filter caps to the bridge as a load, but disconnect the B+ going to the plates and the rest of the circuitry. (That is how Fender wired it up in their BF/SF amps.)  
 
    Not to mention a completely different alternative: leave the B+ connected but disconnect the cathodes of the power tubes. :(  
 
Steve Ahola
 
 
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1/12/2005 12:08 PM
jeremy
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Not to mention a completely different alternative: leave the B+ connected but disconnect the cathodes of the power tubes.  
 
you can get current draw between the plates and the sctreens with this approach. happens more with oddball tubes, that have a lower screen rating, and lower voltage on them.  
 
jeremy
 
1/13/2005 9:32 PM
Jack Koochin
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"Not to mention a completely different alternative: leave the B+ connected but disconnect the cathodes of the power tubes."  
 
I read somewhere that you shouldn't just leave the power tube cathodes disconnected ... don't recall the exact jargon, but something about a slow stripping (or something like that) of the cathode taking place because of this...? The author suggested tying them to ground with several K of resistance instead. Anyone know about this ?  
 
Jack
 
1/14/2005 5:37 AM
Ray Ivers
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Jack,  
 
This has been written about in a number of different places. Here's two I could put my hands on quickly:  
 
RDH, p. 84: "Where it is necessary to maintain cathodes and filaments at normal operating temperatures during stand-by periods, better life in general will be obtained when the equipment is so designed that some, rather than no, cathode current flows during such periods." It goes on to say that for no-B+ stand-by periods of less than 15 minutes, the heaters should be maintained at full operating voltage, and for periods longer than 15 minutes they should be switched off.  
 
'Getting the Most out of Vacuum Tubes', Robert Tomer, p. 33: "Ordinary receiving tubes... were designed to operate in circuits where normal amounts of cathode current are drawn, more or less continuously, over long periods of time. [The} condition... known as cathode interface resistance, or 'sleeping sickness'... comes about after long periods of operation at low plate current."  
 
My own personal experience; I left my 'project amp' on standby (heaters full power, no B+) all weekend once by mistake, and while the preamp tubes (two 12AX7's, 6AV6, 6SL7, 6SN7) still functioned very weakly, the power tubes (NOS Sylvania 6L6GC's with less than 10 hours on them) would barely pass idle current. Apparently these tubes could have been rejuvenated, but I didn't know that at the time... :(  
 
So to answer your question... I think it would be the safest bet to keep the cathodes connected thru large value resistors when using a cathode-lift standby, to allow some current to flow. If the standby periods are kept short there shouldn't be any problems anyway, but you'll be covered in case you end up leaving the amp in standby for a long time.  
 
Ray
 
1/14/2005 1:22 AM
Enzo
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The cathodes will be fine. You can open them as a standby. High power radio transmitters need to worry about cathodes, not 30 watt guitar amps. On a number of amps that have half power switches, the cahtodes of half the tubes get opened. Yes there are many other ways to do it as well.  
 
The point of the standby is to turn off the amp but leave everything warmed up and ready to go. The standby switch is not an excuse to forget safety procedures. SO the fact that one scheme allows voltage to remain in parts of the amp has no bearing on the standby issue. STandby or no, ALWAYS assume the caps are charged.  
 
The standby switch does nothing more than offer a certain convenience, and possibly makes you feel more comfortable. The amp doesn't NEED one and tube life and all those other things are really not serious concerns here.  
 
BUt if you want one, we can sure help get it right for you. Where does it belong? ANywhere you want it to be. The easy place is to put a switch in the red wires from the power tranny to the board. You don't even have to pull the damned three sided board thing out for that. This amp has no center tap, but in amps that do, you can open the center tap as a stanby.  
 
Putting the switch before or after the bridge doesn't matter a whit as far as anything I can conceive goes. Putting it before or after the first filter does matter. Before the first filter means every time you flip it on, ALL the caps have to charge. That means more noise when you do. AFter the first filter - which is where you planned - leaves the first filters charged. The following caps charge through resistors so it is not as sharp an impulse.  
 
Having said that though, I still don't think the amount of pop you might get is so substantially different one way from the other that it matters a lot. There are commercial amps doing it both ways.  
 
Putting the break between the plates and screens of the power tubes is the way Peavey has done it on most of their amps. It works fine on the others, so there is no reason for it not to work just as well here. Yes it leaves voltage on the plates. They stay nice and toasty. Without B+ on the screens the power tubes won't conduct. That sounds like standby to me.  
 
Jeremy, I am sure it is possible with some odd tube or other that some small current might flow where we don't want it, but every part in the high voltage system has potential trouble. Switches can arc or weld shut, wires break off and touch things, caps short, caps leak conductive goo, big heavy power resistors like the 390 ohm there break off at the legs. And so on. I don't think that odd failure mode is likely enough to design around it. Bottom line is, even if some odd screen current finds its way around the standby switch, will it be enough to have sound coming out the speaker?  
 
If I were doing it, I would think about inserting a switch in the red wires. SImple and easy. But if you really want to do it on the DC side... There really is no convenient way to cut into the tracery around R58, so we wind up lifting one end of it and inserting the wires to the switch. This leaves an already vulnerable part unsupported. You could probably glue it on top of R66 right next to it. Or better yet, mount R58 on a terminal strip on the chassis somewhere and run wires back to the board where it was. That terminal strip would also be a handy place to run the wires to the switch from.  
 
There is plenty of open chassis space behind the boards, and R58 is not in the signal path, so you can put it anywhere you like. If you do the cathode bias mod in the manner I did, you will mount a couple terminal strips back there anyway. Might be a nice time to look into that mod as well.
 
1/14/2005 9:08 AM
jeremy
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Jeremy, I am sure it is possible with some odd tube or other that some small current might flow where we don't want it, but every part in the high voltage system has potential trouble. Switches can arc or weld shut, wires break off and touch things, caps short, caps leak conductive goo, big heavy power resistors like the 390 ohm there break off at the legs. And so on. I don't think that odd failure mode is likely enough to design around it. Bottom line is, even if some odd screen current finds its way around the standby switch, will it be enough to have sound coming out the speaker?  
 
 
agreed. it is the least of one's worries when thinking of a standby. thanx enzo!! ever get that email?  
 
jeremy
 
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