Standby switch on CT - why is this bad?

[tubesornothing]

I am doing a little mod job on an amp that has its standby switch on the CT.  One Merlin's site he says this is frowned upon (http://www.freewebs.com/valvewizard1/standby.html).  I think I have also read on other sites that it is a bad practice.

why?  Really, I am not trying to stir the pot, I just don't know and would like to be enlightened.

thanks
ToN

[jjasilli]

My understanding is that ground lifts of any type do not conform to modern electrical standards.  Hence manufacturers will shy away from this practice as it may complicate legal liability issues.

Also, as Merlin points out, I believe that breaking the CT ground (like a standby SW which breaks the tubes' cathode to ground connection) will still leave full B+ voltage throughout the amp.  This is hazardous to the hapless tech who assumes the amp is safe when in standby mode.

[phsyconoodler]

I agree.
  Old Traynor amps do this and after getting zapped a couple of times,I remove this kind of standby.

[tubesornothing]

Very good - thanks guys.

[kagliostro]

Also if the CT is lift and you benefit in touching the two sides of the PT

assuming to have a 330v-0-330v PT

what you got isn't 330v but a healthy 660v

Kagliostro

[sluckey]

There is another reason too. In the case of a fixed bias amp that derives the bias voltage from the HT winding (either from a tap or directly from the end of the winding)... When you break the CT ground connection, you do kill the B+ current path, but you also kill the ground path for the bias voltage. However, now the bias voltage will find a sneak ground path thru the B+ supply circuit. The voltage on your bias caps will increase, probably to a high enough level to damage the bias caps. This is not a problem if the bias supply has it's own dedicated PT secondary winding. And it's a non issue in a cathode biased amp.

The other safety related reasons mentioned are enough to prevent me from switching the CT lead.

[Fresh_Start]

IIRC Merlin mentions lifting the power tube cathode(s) as an acceptable standby.  O'Connor talks about it too I think.  Not a complete separation from ground - just inserting a large enough cathode resistor for the tube(s) to be biased really cold (IOW almost inaudible).

While we're talking standby switch theory, can I hijack and ask what the pros & cons of the cathode standby switch are?

1 "Con" comes to mind right away:  there's still B+ everywhere.  OTOH there probably aren't too many old amp techs who just count on the Standby switch to cutoff B+ :Wink:

Cheers,

Chip

[sluckey]

My Magnatone M10A opens the cathode ground for STBY. One pro is much lower voltage rating needed for the switch. Another is no pop. From an operational viewpoint this is fine, but from a tinkerer's viewpoint, I don't like it because B+ is still everywhere in the amp. Sometimes during troubleshooting or tinkering, I like to just kill the B+ and discharge the caps, keeping the filaments lit, and quickly solder/desolder a component, then bring the B+ back on. Can't really do that with the Maggie.

[simonallaway]

IIRC Merlin mentions lifting the power tube cathode(s) as an acceptable standby.

I liked his idea of half-wave rectifying the AC filament supply as a kind of "soft" start. So many options! So much confusion for a newbie like me.