Sanity Check

[dartanion]

I am building a DC30 for a customer from one of Nik's kits and was wiring up the PT last night when I noticed an oddity in the layout. He has the standby switch lifting the HT CT from ground I have always thought that this situation was a recipe for disaster...fried PT! Someone please correct me if I'm wrong.

Anyway, I plan on implementing the standard Standby switch arrangement between the rectifier and first filter node/OT CT.

[bueller]

Yes the DC30 has the center tap lifted for the standby. I think there was a 2KV cap there as well around the switch. DR Z does the same thing on the Z-28. I'm not sure of the benefit of this versus just interupting the HT.

Note:If you ever use the transformer shunt method to check bias, when you turn on the power but the standby is in the off position, it makes a ugly loud noise that will go away as soon as the standby is turned on. First time my friend biased a Z28 he jumped a mile when he turned it on! Scared the hell out him.

[dartanion]

Well, Nik's layout doesn't have a cap in the standby switch at all, so it this still an acceptable practice? Any benefits or drawbacks to this besides the affect on the shunt bias method?

[VacuumVoodoo]

I am building a DC30 for a customer from one of Nik's kits and was wiring up the PT last night when I noticed an oddity in the layout. He has the standby switch lifting the HT CT from ground I have always thought that this situation was a recipe for disaster...fried PT! Someone please correct me if I'm wrong.

Anyway, I plan on implementing the standard Standby switch arrangement between the rectifier and first filter node/OT CT.

CT Standby switch doesn't have to break a HV potential so it basically doesn't arc. Also there's no HV on switch operated by user. Double the safety with a fuse in series with the switch, rate it 2x B+ current draw.

[mooreamps]

This is correct. By using a ground lift, there is no high voltage across the switch.
-g

[rhinson]

it's something you see used a bit in older cathode biased amps, i think maybe ampegs and some others. i've used it bit myself in my builds of amps like this---works fine. rh

[RJ Guitars]

This is interesting... do I understand this correctly? - you can achieve a HV standby action by lifting the Center tap ground and there is a not an actual voltage across this connection?? I haven't really though about it yet but intuitively it seems like something is wrong here.

From what I've seen of the usual on/standby switch, it appears that the switch rating (AC-250V) is overtaxed, even if we are talking about DC rather than AC...

Or have I got this all wrong and somebody out there can enlighten me as to the obviously safe and reasonable way to do a standby switch?

Thanks for any help you can offer...

If anyone feels tempted to draw and post a back of the envelope schematic, I would say kind things about you.

rj

[rhinson]

http://www.schematicheaven.com/ampegamp ... ini_iv.pdf

here's an example of an old ampeg that used this style. there are others as well that used this.

another way of course is the marshall fashion of simply making the hv sec leads themselves switchable.

http://www.schematicheaven.com/marshall ... w_1959.pdf

[sliberty]

Tweed Pro (5E5) did the standby by lifting the center tap too. Someone once told that this approach was a problem for the PT in some way, but I haven't heard that elsewhere.

[LOUDthud]

If there is a bias supply taken from the HV winding, either a tap or the big resistor on the red wire(s) (O'Connor usually uses a 160K), there can be problems. Because there is usually a small static load on the B+, like the resistors around two series connected filter caps, and eventually tubes as they warm up, the B+ is pulled towards ground. This makes the bias supply go way more negative than you would like but the adjustment divider network limits it. You can end up with the B+ being +200V and the bias supply being -150V when the amp is in standby mode . Not a problem if you recognize this and all the resistors and caps are rated properly. If the standby switch is in the B+ line, the bias supply operates normally from the start.

[rhinson]

If there is a bias supply taken from the HV winding, either a tap or the big resistor on the red wire(s) (O'Connor usually uses a 160K), there can be problems. Because there is usually a small static load on the B+, like the resistors around two series connected filter caps, and eventually tubes as they warm up, the B+ is pulled towards ground. This makes the bias supply go way more negative than you would like but the adjustment divider network limits it. You can end up with the B+ being +200V and the bias supply being -150V when the amp is in standby mode . Not a problem if you recognize this and all the resistors and caps can are rated properly. If the standby switch is in the B+ line, the bias supply operates normally from the start.

yep this is correct. that's one reason why, for the most part, you see it on cathode biased amps. i believe a couple of boutique amps use this stby method on some of their amps, top hat for one. rh

[VacuumVoodoo]

yep this is correct. that's one reason why, for the most part, you see it on cathode biased amps. i believe a couple of boutique amps use this stby method on some of their amps, top hat for one. rh

I thought we were all using PTs with separate winding for bias supply by now

[evan]

This is correct. By using a ground lift, there is no high voltage across the switch.
-g

I don't think that's correct.

Thought experiment #1: If there's no potential across the open swich... then closing it will make no difference.

Thought experiment #2: If I replace the switch with a resistor -- say 1 meg -- what do I see?

Thought experiment #3: If I have a voltage drop of B+ in a circuit (switch closed), and I interrupt the circuit at any point, what happens?

What happens -- I think -- is that you get a rectified AC voltage across that switch with peaks a bit higher than B+ voltage, but opposite in sign.

[evan]

From what I've seen of the usual on/standby switch, it appears that the switch rating (AC-250V) is overtaxed, even if we are talking about DC rather than AC...

Or have I got this all wrong and somebody out there can enlighten me as to the obviously safe and reasonable way to do a standby switch?

In switching, turning the switch off (interrupting) is generally much harder on the switch than turning it on. DC is much harder on switches than AC (same voltage, same current). As the contacts open, an arc forms. The advantage of AC is that the current drops to zero periodically and this extinguishes the arc. That doesn't happen in DC switching, so you end up with a lot more arcing and contact heating, erosions, etc.

In the case of a standby switch located after the rectifier and before 1st filter cap, two things help you a lot.

Thing 1: It isn't DC you are switching. It's rectified AC. the current drops to zero twice each mains cycle.

Thing 2: You have this big filter cap on the load side of the switch. This means it takes time for a high potential to develop across the opening contacts. This reduces arcing as well.

So the only unusual demand on the standby switch is that it has enough dielectric strength to operate at high DC voltage indefinitely. Most 250 VAC switches can withstand 1000V or so. The usual Carling, NKK & Mountain 250VAC switches hold up well.

[VacuumVoodoo]

This is correct. By using a ground lift, there is no high voltage across the switch.
-g

I don't think that's correct.

Thought experiment #1: If there's no potential across the open swich... then closing it will make no difference.

Thought experiment #2: If I replace the switch with a resistor -- say 1 meg -- what do I see?

Thought experiment #3: If I have a voltage drop of B+ in a circuit (switch closed), and I interrupt the circuit at any point, what happens?

What happens -- I think -- is that you get a rectified AC voltage across that switch with peaks a bit higher than B+ voltage, but opposite in sign.

Do the same for single winding half wave rectifier, disconnect grounded end of winding from circuit - what do you see?
Extrapolate to CT winding and dual phase full wave rectifier.
Bring it to the real world by assuming >100megs resistance in an open switch.
Same experiment: single winding, full bridge rectifier, lift bridge ground.
In each case have a realistic load on the rectifier.