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Cathode Disconnect Standby

7/10/2004 11:22 PM
Carl Gigun
Cathode Disconnect Standby
Maybe I sniffed too many solder fumes last night, but I tried doing a standby switch this way and my meter reads over 200 volts DC at the plate cap in standby mode. 90 volts DC at the now ungrounded CT. I don't read any voltage across the current sense resistors at the power tube cathodes though, so I guess it's working which makes me think it's just my meter being smarter than me again. Then again, there's a 100K bleeder resistor from the screen node to ground, but this voltage develops despite the approx. 3ma of current it must be drawing.  
Can anyone enlighten me? I got curious and lifted the center tap of the heaters. With no tubes in I read 6.8 Vac across the winding, but almost 100v DC from the CT to ground. ? I'm definitely going back to switching the B+ if this is normal.  
-Carl Gigun
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7/11/2004 2:07 AM
Don Symes

That is normal --  
Think about the B+ standby switch. You go to SB and the plate voltage drops to ground and ground stays where it is and so does B+ BEHIND the SB switch.  
Well if you switch the other end, the B+ stays on the plates, but you let go of your power tubes' connection to ground so everything else rises toward B+.  
The nice thing about the cathode-lift SB is you aren't trying to switch a 300-600V node - no arcing or breakdown.
7/11/2004 3:08 AM
Carl Gigun
Re: oops, HV Center Tap Disconnect Standby
Oh man I really did stay in the basement too long last night (3am), I meant to say center tap disconnect standby... geez I'm out of it today. Your comments about cathode standby make perfect sense, but what I did was lift the ground connection to the CT.  
-Carl Gigun
7/11/2004 5:53 AM
Carl Gigun

Forget it, I already changed it back. This particular standby switch is really old and small, but it's lived this long so if it ain't broke...  

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