switching the earth of HT & LT DC& AC all on one switch ?

Wanting to switch 290VDC 12.6VDC & 6.3VAC on my vari mu limiter all with single pole switch by switching the earth.....is this a bit naughty.......it runs from separate  PSU that has a master switch on it

getting Hum from 6.3VAC mabe a could use two pole switch one side ac other DC....

Without a schematic, it's impossible not to be sure that you're not in great danger of reverse-polarizing and over-volting much of your circuit.

In short, I wouldn't do this.

Keith

Ok not sure why need schematic its just the HT 295VDC one side is neg and grounded 12.6VDC the neg is grounded so I was just gonna put a switch between the PSU neg and the other LT & HT negs......Ive given up with the AC gonna put on separate pole it works but hums gonna have that floating....I cant see how the HT and LT can interfear with each other, anyway Ive tried it with crock clips all seems ok...

many thanks

gary o said:

Ok not sure why need schematic....

Click to expand...

In my experience it's really difficult to give advice about particular setups- particularly high voltage power supplies- without seeing a specific schematic for your project/circuit. Even if you're building a "well known" Lab project, we can't assume we're using exactly the same schematic, as many of us adapt/arrange the schematic to fit with parts we have on hand- PSU's are probably the most flexible part of some projects, depending on what transformers we have available- some peopleuse one transf per rail, others use voltage multipliers etc to create the rails- so asking a question about switching a single rail is tricky to judge without seeing a schematic.

It's like when friends phone me up to ask help with "re-wiring a domestic light switch"- we all know how it works, and how it should look like in the switch or ceiling rose, but in reality, there are so many variations in actual practice, that we can never presume from afar (especially over t'internet) that things are "as we think they are"

...presumption is the mother of all f**kups...

...is an important phrase I keep in my head at all times!

Keith was applying the same logic by not presuming your circuit was as you'd expect from a generic schematic.

Safety is No. 1 in DIY- there are so few of us, we need to keep the population numbers up without any unwanted accidents!


How's it sounding now?

Mark

gary o said:

I cant see how the HT and LT can interfear with each other,

Click to expand...

Really... I wasn't being obstructive...

Three wires. +200V on one. 0V on the middle one, +6V on the third one.

Now lift the ground, and it will -VERY POSSIBLY- want to drift towards the middle point, since it only has connections to points between 6V and 200V. WITH NOTHING BELOW IT.

Damage can very possibly ensue if ANYTHING in  the low-voltage part of that circuit is polarized. If you think I'm just being obstructive, please wait until a few other people have considered the same information.

Keith

Keith sorry I dont think yr being obstructive It seemed to simple to me maybe because im lookin at it now and its in my head Im forgetting you all cant see it Il try and think of a good way to discribe it before i post again and, I have lots of basic electronic knowledge missing & am trying to learn thats why I ask, I usually try and ask questions later, in fact thats what I have done and its working in this case

Thanks for advice and safety concerns

gary,

I think you've probably seen this in guitar amps for "Standby" functions- this usually works okay, but there are problems I've seen with this when someone has modified/bodged the output stage to have a variable "bias" supply- an LT supply from a separate rectifier off the main transformer. They forget to take into account that there is still a path for current to flow back through the LT supply (often damaging the components it works it's way through!)

Again, no point speculating without schematic!

Mark

Not seen the Guitar amp thing Iv draw a crappy plock diagram its basically a  homemade 436 type limiter and home made supply that has separate ht neg & pos from a transformer and LT neg and pos from another transformer in the same PSU box, both negs go to ground I just wanna switch off the ground connection at the limiter end thus HT & LT has no ground neg connection so limiter dont work does that make sense

never posted a pic this way so I hope its visable.....thankyou for your patience

Wouldnt let me add pic .......second go hope this is readable

Quoting Keith
"Now lift the ground, and it will -VERY POSSIBLY- want to drift towards the middle point, since it only has connections to points between 6V and 200V. WITH NOTHING BELOW IT."

+1

Lifting internal ground references on circuits with 300dc voltages........IMO could and probably will end up with component failure.....even
with a loud spectacular display
........where safety glasses lf you go ahead .....back EMF can be very destructive !!!

I dont understand but am willing to learn and even the hard way glasses on.... as I said Ive already said Its wired the way I discribed and all is well , cant see how there can be a problem as I say not trying to argue just trying to understand......as I said its 2 separate transformers not taps from one transformer....one Ht one Lt and they just share a common ground maybe Im misleading with my discription....anyway its up and running all voltages are normal.

Thanks for all your help.

Aha I think Im understanding this just spoke to tech friend on the phone who found a way to get at least some of this into my brain about the differential 280 ish volts that may find earth thru me i wont be the first time  but I dont want it to be my last.

Thanks once again.

gary o said:

Wanting to switch 290VDC 12.6VDC & 6.3VAC on my vari mu limiter all with single pole switch by switching the earth.....is this a bit naughty.......it runs from separate  PSU that has a master switch on it

Click to expand...

I vote for trying to solve the hum problem with a single fixed arrangement.  Even with the diagram I'm not 100% clear on what you're trying to do - switching between AC & DC on the filaments only?  Why involve the high voltage winding?

The filament switching would seem to require 2 poles.  Have you tried keeping the most sensitive filaments on DC and leaving the others AC via separate filament winding with hum pot or CT floating set up?   

Hiya Lasso not switching between AC & DC...& the the high voltage is the for the compressors HT....anyway not to worry Ive given up on the idea, Im starting to understand what the guys are saying .....I basically have a tube compressor that requires 12.6 v and 295v as usual  & also 6.3 ac....all from from a external PSU that may be left on as it supplies other pre amps and such normaly id just turn off mains supply to transformers in PSU.....So I was simply gonna disconnect both negatives with one pole of a switch at compressor end leaving PSU on and with the other pole of the switch, switch off the 6VAC, it all works fine but I realize its not safe so will use triple pole switch

My AC floating happily with no pots no hummm, I been quite supprised how silent my DIY runs I used to think it all had to be DC for heaters but as you say sometimes only sensitive parts of circuits benefit from DC for heaters....its good news 

Cheers.

Sounds good. I've yet to have a problem with straight non-floating AC heaters except for the SRPP type circuits. 

Is this thing gonna be racked and ready for action anytime soon?  Be sure and post some pics.

yeah i found that in the only SRPP build I did .....I usally leave mine floating but have had good results grounding one side or both side via resistors and the hum pot thingy too

lassoharp said:

Sounds good. I've yet to have a problem with straight non-floating AC heaters except for the SRPP type circuits. 

Is this thing gonna be racked and ready for action anytime soon?  Be sure and post some pics.

Click to expand...

Ha wont be racked up my little studio isnt yr usual sort of thing.....lots of boxes all connected together all DIY is on going I never declare anything finished  so its a bit of a mess......however its up and running and is in service theres two new boxes both projects I started years ago but shelved theres the 436/Chiswik vari type compressor & a federeral AM864U vari mu.....Im just doing some listening tests at the mo                                       

OT but sorta related.

I decided to try saving time on a recap once (and only once) by experimenting with method.  Can filter caps with multiple sections.  Being really really smart that day, I left the can in place, and disconnected the negative of the old can cap, then proceeded to to simply parallel new caps off of the positive leads, going to ground.  Much simpler, right?  Until you touch the case (negative) of the disabled can cap, and it bites you.  'Cause there's enough leakage through the old cap to still build up potential on the disconnected negative side. 

I've tried to be less brilliant since then.