Standby on PT center tap

[panhead]

I'm using a 5AR4 with a typical full wave power supply. Can I put the standby switch between the PT center tap and ground, rather than before the first node of the power supply?

[rzenc]

I believe it would be rather dangerous, since "I" think a stand by switch is supposed to kill all H.T. inside the amp...I would install it between H.T. windings and recto tube, add fuses before the stand by switch.
For more thoughts:
http://www.freewebs.com/valvewizard1/standby.html
http://www.freewebs.com/valvewizard/fuses.html

Hope this helps
Best Regards

Rzenc

[sluckey]

Think of the PT HT winding and rectifier as a big ole battery. The cathode junction of the rectifier diodes is the battery positive and the CT is the negative. You can break either side to kill the current path to the circuit.

However, if the amp is fixed bias and the bias voltage is derived from the HT winding (either by big resistor or bias tap on the HT winding), don't do it because the bias voltage can rise to a dangerous level for the bias caps when the STBY switch is open. If it's cathode biased then switching the negative side has the same effect as switching the positive side.

I don't like this method because it still leaves B+ floating around inside the chassis, prehaps just waiting for your body to provide a path back to the CT. I really don't see any advantage in putting the switch in the CT side. Do you? The same current will flow thru the switch and the same voltage will appear across the open contacts.

[stingray_65]

However, if the amp is fixed bias and the bias voltage is derived from the HT winding (either by big resistor or bias tap on the HT winding), don't do it because the bias voltage can rise to a dangerous level for the bias caps when the STBY switch is open. If it's cathode biased then switching the negative side has the same effect as switching the positive side.

I do not mean to hijack this thread, But I am wiring up my version of the TOS right now and I am about to do just exactly what Sluckey is stating above.

The mains switch I have is one of those expensive Carling progressive switches and my face plate is made and labeled with a standby, so leaving it out is an unattractive option.

So I am wondering... my bias supply has a ground wire, could I land that wire on the standby switch on the same lug as my CT and avoid the possibility of causing over voltage in the circuit?

Ray

[Fresh_Start]

Ray - with one of those Carling "progressive" switches, you've essentially got two SP/ST switches controlled by a single toggle, right?  Why not just have the standby section of the switch cut the rectified HT right before the reservoir cap? IOW just where it's shown in tubenit's TOS schematic.

Here's one way to think about those switches:

S1 | S2
off | off
on | off
on | on

If the PT center tap is not grounded and you are taking bias voltage off the high voltage PT secondary, there won't be any voltage on the PT secondary when the center tap is disconnected.  The circuit for the PT secondary is open.  Doesn't matter where or whether the bias circuit is grounded.  You still won't have bias voltage and that's BAD for your power tubes (and other stuff too).

HTH

Chip

[stingray_65]

OMG Chip!

I have poured over this schemo for the last 3 weeks and never realized Tubenit and Geezer drew it that way.

I just started looking at the first filter cap and that was it.

I can be a real idiot sometimes :)

thanks
Ray