Standby switch. Series vs. PT center tap method

[philo43]

Some amps switch the HV on and off at the PT center tap (to ground) vs. in the B+ line. I'd be interested in anyone's opinion on this. It seemed to be more common on older amps, however it's still used today. Pro's? Cons?

[David Root]

I would only use the PT CT position on an amp using cathode bias on the power tubes.

It's a long story but doing that in a fixed bias AB1 amp is not a good idea.

[philo43]

Yeah, I can see that. Wouldn't be practical for most B+ derived bias supplies. I'm thinking more about inrush current or effect on tube versus solid rectifier circuits. Or anything else that might be affected.

[paulster]

Switch the B+ as it leaves less of the amp energised to high voltage and ready to hurt someone being careless with it.

[philo43]

Switch the B+ as it leaves less of the amp energised to high voltage and ready to hurt someone being careless with it.

Not sure I see what you mean. Seems there would be fewer points energized with no CT ground. Certainly no charging of the filter caps. The only potential would be between the secondary terminals.

[paulster]

If you remove the centre tap then you've broken the circuit and left it floating, with the potential to have a user create the bridge between this level and ground.

If you break the high potential circuit then everything will sit at a ground potential.

[KT66]

Ampeg used the CT switching method with the whole portaflex line, some of which are fixed bias and their engineering is pretty solid. The B18n ,for example, even steals the bias from the HT line. One reason I have seen come up in discussions on this topic in favor of this method is that AC is easier on switches than DC, but they also use the method sometimes of switching the negative side of a FWB from ground, so that shoots down the AC on the switch idea. Maybe it's an anti standby thump thing ?

[paulster]

AC is certainly easier on the switches, and switching the centre tap isn't a bad idea; it's just not as safe as switching the B+ if (and this is a big 'if') the user does something stupid.

I use double-pole switches and switch the AC sides of the transformer, which is the best of all worlds, so you don't get the switch on/off thump and you also keep the energised parts of the circuit to a minimum.

[EFK]

Isn;t the 'standard' B+ switch as used by Marshall and most others technically allowing for hot-switching? Don't know how much of an issue it is in the real world, but all the data sheets make it a point to warn against it as damaging to rectifier tubes. I would think it would be better to put the switch after the first filter cap and before the choke, thus avoiding the hot switch issue?