UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > General Vintage Technology > Components and Circuits

Notices

Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

Closed Thread
 
Thread Tools
Old 6th Mar 2021, 9:33 am   #1
Diabolical Artificer
Dekatron
 
Diabolical Artificer's Avatar
 
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 7,639
Default EHT PSU Question.

If you have a butchers at the attached schematic, page here - http://www.chambonino.com/construct/constwire2.html you'll see he's stacked several PSU's/rails on top of one another to get 2.5kv DC (and 750v DC).

Questions - 1) He's using 1000v rated bridge rectifiers, surely there's a problem exceeding the BR's PIV rating further up the stack/at the top - 2.5kv DC + AC peak or is that 2.5v DC + AC ripple?

2)He's stacking different AC RMS rated windings, IE 3 x 185v RMS & 5 x 260v AC RMS - is there a problem mixing different DC rails from a voltage sharing angle as regards the caps? He's using 100k sharing R's on all caps/rails

3) Is this a better way to implement a EHT PSU? Say one 1170v AC (or two) winding/s with one or two bridge rectifiers, that or series connect the various windings, rectify in one go, cap stack the same. What are the advantages/disadvantages of each approach?

4) From a safety POV would it be better to split the supply in two, EG +/- 1250v? 1250v still lethal but not as bad as 2.5kv. I think there'd be an issue with phase splitter & other stages, EG Vg2 750v, 813 cathodes would be at - 1250v, so Vg2 would be -500v with respect to ground/chassis mains earth = problem with OP of IP stage being referenced to 0v/gnd? Think I may be getting a bit lost there...

5) Lastly, there's no choke on the 2.5kv rail, only on the 500v rail... I've read about putting the choke in the ground so the choke isn't floating at EHT.If one did this, IE choke in the ground, could you also use a choke at 500v? Again,may getting muddled there...

TFL, Andy.
Attached Thumbnails
Click image for larger version

Name:	EHT PSU schematic.jpg
Views:	126
Size:	41.2 KB
ID:	228366  
__________________
Curiosity hasn't killed this cat...so far.
Diabolical Artificer is offline  
Old 6th Mar 2021, 10:07 am   #2
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,803
Default Re: EHT PSU Question.

The bridge rectifiers should be OK, but if metal cased or with a matal heatsink underside he may be overstressing the insulation to ground.

He's certainly pushing things too far as far as transformer primary or screen to secondary insulation rating goes.

With four 813s in push-pull pairs, he's going to have to arrange flashover protection in case of lack of speaker loading or disconnection (Guess how I know this! I was a schoolkid at the time)

These sorts of things are definitely lethal. Even a broken ground connection can put high voltage in places you don't expect it.

RF-capable power valves and the self-resonance of audio band designed output transformers are also another interesting aspect. Serious use of stoppers is essential and they can fail spectacularly.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is offline  
Old 6th Mar 2021, 10:17 am   #3
Ed_Dinning
Dekatron
 
Join Date: Jan 2004
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 8,173
Default Re: EHT PSU Question.

Hi Andy, unless he has special high insulation transformers I suspect that is a combustion event waiting to happen. I may last a few minutes, but should have a lot of individual fusing with the fuses having a suitable voltage rating.
A correctly made transformer may be cheaper than a fire

Ed
Ed_Dinning is offline  
Old 6th Mar 2021, 11:39 am   #4
Diabolical Artificer
Dekatron
 
Diabolical Artificer's Avatar
 
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 7,639
Default Re: EHT PSU Question.

Quote:
The bridge rectifiers should be OK
Even with 2.5kv sitting on the cathodes?

Quote:
Hi Andy, unless he has special high insulation transformers I suspect that is a combustion event waiting to happen. I may last a few minutes, but should have a lot of individual fusing with the fuses having a suitable voltage rating.
high insulation in terms of pri/sec insulation or high insulation enamel wire Ed,like PTFE?

Re fuses would one rate the fuse for Vac, in this case 260v? I suspect he's using 240v rated jobbies. T3A would be very expensive I think,here's rated for 3kv - https://uk.farnell.com/siemens/3ne93...tag/dp/3450548, a snip at £657 not inc VAT. A chap on Youtube who builds whacking great EHT supplies for TX's uses 1r resistors as the EHT fuse.

If I built such a supply I'd have protected OP valves, Ik goes over Ix and a relay opens the nuetral to the mains tfmr as well as some of dumping the power stored in the caps perhaps. Keyed mains sw, time delayed HT/EHT,what else would one do?

I see he never completed the amp/s, selling such a beast to your average guitar player let alone having one of these on stage would be.... interesting.

Andy.
__________________
Curiosity hasn't killed this cat...so far.
Diabolical Artificer is offline  
Old 6th Mar 2021, 11:53 am   #5
ChristianFletcher
Heptode
 
ChristianFletcher's Avatar
 
Join Date: Dec 2003
Location: South Yorkshire, UK.
Posts: 900
Default Re: EHT PSU Question.

Looks exciting!
__________________
Every Silver Lining Has Its Cloud https://youtube.com/channel/UCvBpiuUUnErJlNBm6DWb3Ww
ChristianFletcher is offline  
Old 6th Mar 2021, 12:04 pm   #6
Diabolical Artificer
Dekatron
 
Diabolical Artificer's Avatar
 
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 7,639
Default Re: EHT PSU Question.

Quote:
Looks exciting!
Indeed.

Looking at fuses just now,finding a 2.5kv DC rated fuse at less than an map is problematic,most EHT types being rated for 100A ish. The only fuse I have at that kind of voltage is a 5kv 500mA jobbie, it's just a normal 1 1/4" cartridge type, no HRC or anything.

A.
__________________
Curiosity hasn't killed this cat...so far.
Diabolical Artificer is offline  
Old 6th Mar 2021, 12:47 pm   #7
JonSnell
Hexode
 
JonSnell's Avatar
 
Join Date: Oct 2019
Location: Weymouth, Dorset, UK.
Posts: 474
Default Re: EHT PSU Question.

Why not us a PD500 as a fuse, it has the advantage of a slow start as well.
Take a look at the early valve CTV Decca series, they used the PD500 to shunt the 25kv, should be good in series.
1 1/4 fuse may not hold 2.5kv at any current, could flash over.

Last edited by JonSnell; 6th Mar 2021 at 12:49 pm. Reason: More info added
JonSnell is offline  
Old 6th Mar 2021, 2:35 pm   #8
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,803
Default Re: EHT PSU Question.

Quote:
Originally Posted by Diabolical Artificer View Post
Quote:
The bridge rectifiers should be OK
Even with 2.5kv sitting on the cathodes?
Yes, the whole bridge in the case of the top ones is floating up to 1.kV above ground, but the reverse voltage on its diodes should be OK. The risk is in the diodes and terminals breaking down to any metal mounting or heatsinking.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is offline  
Old 6th Mar 2021, 4:43 pm   #9
Ed_Dinning
Dekatron
 
Join Date: Jan 2004
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 8,173
Default Re: EHT PSU Question.

The problematical insulation is from any of the various secs to both Pri and frame and will to some extent depend on the position of the transformer in the "chain".
Depending on which bit blows up first then there could be 2KV across a fuse when it lets go, a bit much even for an HRC type

Ed
Ed_Dinning is offline  
Old 6th Mar 2021, 7:04 pm   #10
Diabolical Artificer
Dekatron
 
Diabolical Artificer's Avatar
 
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 7,639
Default Re: EHT PSU Question.

Quote:
Why not us a PD500 as a fuse
Not sure I'm with you there Jon, a PD500 is only rated for 1.6mA. in series it wouldn't pass enough current, as a shunt...? will have to look into this more.

Quote:
... the whole bridge in the case of the top ones is floating up to 1.kV above ground
right David, so we are only interested in the voltage across the diode, which is only 367v DC across two diodes in series. I was under the impression that one diode would see -367v AC peak + 2.5kv DC, but that's wrong, each diode sees 735v ... -367v AC + (+367v DC). Is that right,does the AC and DC voltage sum?

Quote:
The problematical insulation is from any of the various secs to both Pri and frame and will to some extent depend on the position of the transformer in the "chain"
If the tfmr was a toroid inside an insulated container,this would alleviate this somewhat no? Obviously all tfmr terminals would have to be as far apart as poss. Can you expand on what you mean about "in the chain" please Ed?

As far as I see it the main potential weak points are - diodes, sharing R's and caps going SC, the last two being less of a potential problem as it's unlikely all would SC at once. A diode going SC could result in HV DC being connected to a tfmr sec winding then shorting across to the pri wndg and hence to ground/mains earth with possible melting of wndg's causing heat hot enough to set fire to insulation.

I've been reading up a bit about fuses, fusing...AFAIK DC is more problematic because it will maintain an arc longer than AC because AC at some point reaches 0v. Therefore DC needs more space/length between point A & B for the arc to extinguish. Looking at fuses available at Farnell etc a 2.5kv fuse rated at around 1A are not available, also EHT rated fuses are very expensive. With this in mind and riffing on Jon's idea in post #7 about using a valve as a fusing element, could one use a thyratron? If there was a sense resistor say as part of the voltage sharing chain, that if current over X amount is exceeded, causing Vgk more negative in a triode thyratron,thus slamming the thyratron shut, as it were

Whilst looking at EHT type fuses I saw various odd types like semiconductor types as well as liqqued filled fuses. I wonder if a 4" length of 1A fuse wire in a tube filled with high tempco oil might fit the bill? It would obviously need to be tested. This talk of homemade EHT fuses might be scary, but after watching Dave Jones test "proper" fuses recently, resulting in a wide range of blowing times,it dents your faith in fuses as protection.

Andy
__________________
Curiosity hasn't killed this cat...so far.
Diabolical Artificer is offline  
Old 6th Mar 2021, 7:44 pm   #11
G6Tanuki
Dekatron
 
G6Tanuki's Avatar
 
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 13,953
Default Re: EHT PSU Question.

It's a variant on a design I have used in the past: series-connecting the outputs from a pair of 240:415v "Panel transformers", with each tranny having its own bridge and smoothing-capacitors.

Got me 850V@0.5A for a HF linear-amp, with a convenient half-voltage tap for supplying the screen-grids. Personally though I wouldn't stack more than two transformers this way: my worry would be that the secondary insulation of the 'top' transformer [both secondary-to-primary and secondary-to-core] would potentially [pun intended] be overstressed.

I like my gear to be reliable!

I used 6-amp wire-ended 'fast recovery' diodes for the bridges, mounted transversely in the gap between two small single-sided PCBs.

For fusing, I used a sacrificial 10-Ohm 1/2-Watt resistor mounted stand-off using a couple of the old ceramic 'fishbone' beads on each lead.
G6Tanuki is offline  
Old 6th Mar 2021, 8:28 pm   #12
Ed_Dinning
Dekatron
 
Join Date: Jan 2004
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 8,173
Default Re: EHT PSU Question.

Hi if you look at your circuit, the further you move away from the earthed (say centre tap), the greater the common mode voltage applied to each successive transformer and the greater the insulation stress to pri and ground.
If each individual sec is then fused for safety it may help to prevent a cumulative breakdown.
A fuse is a highly sophisticated device (Chinese excluded) so it would be not advised to make you own (a good cop out for the insurance company in case of a fire)

Tanuki's circuit with only 2 transformers in use is fine, as these devices are normally rated to at least 2KV . Extrapolation is not to be advised

Ed
Ed_Dinning is offline  
Old 7th Mar 2021, 3:42 am   #13
trobbins
Heptode
 
trobbins's Avatar
 
Join Date: Oct 2009
Location: Melbourne Australia
Posts: 898
Default Re: EHT PSU Question.

Given the transformers used by Champ were custom, I'd be pretty sure they were specced and tested for that application - ie. a secondary to primary (and core although that is less severe) of some working voltage sufficiently in excess of 3kV, and probably sitting around a rating of 5kV. As such, you can't go down that path unless you arrange an equivalent custom transformer. And that doesn't even touch the working voltage rating for the obvious custom output transformer.

With respect to the B+ supply fusing, that takes quite some more assessment to appreciate. Firstly, each 260Vac winding has its own 1A T fuse (presumably IEC compliant), but the energisation has a soft-start, so initial charging of each 1000uF would be sedate, and would not get near the 1A T capability. So the focus may likely be on that 1A T fuse and a B+ fault, and what conditions could force a 260Vac winding current to greater than the fuse time profile allows. Also the max operating winding current is not known, only that it has a 750mArms continuous capability.

A common type of fuse that may get someway to a DCV rating and with some semblance of HRC capability could be as used for microwave appliances. Certainly better than using a 250VAC glass 3AG with no DCV rating, and microwave applications have ratings down at the 1A end.

But the various forms of protection presently used, and could be used, should really have some flesh added to what they can do and when and how, as this perhaps needs to be 'design first, then test' rather than 'test first, then redesign' for someone wanting to clone that type of amp.
trobbins is offline  
Old 7th Mar 2021, 1:03 pm   #14
Diabolical Artificer
Dekatron
 
Diabolical Artificer's Avatar
 
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 7,639
Default Re: EHT PSU Question.

I'm winding my take on this tfmr as of time of writting, but my tfmr is a 1.5KVA toroid. All windings are over specced, EG 18 AWG for the primary wndg with a 5% more turns than needed. That was covered i two layers of kapton tape, then one layer of Nomex paper impregnated with Ultimeg 2000/372 varnish. On top of that is a shield,covered in another of impregnated Nomex with lastly one layer of Kapton tape.

The secondaries are 25 AWG, good for 1.7A, though I've designed for 800mA, each is 304v AC RMS unloaded, which gives 379v DC at full load - 398v @ 200mA. I won't be going as high as 2500v, 1500v only, so four secs. Each sec has more insulation as it goes "up", the first two windings are insulated by two layers of Kapton tape rated for 4.5KV, the others will be insulated with DMD tape, rated at 10KV.

I tested pri/sec insulation at 10KV, insulation resistance read as 150,000M. I left it at 10KV for a minute using an Ionisation tester listening for insulation breakdown, all kushti. I've three secs on so far. This tfmr is only for the EHT/g2 supply. I thought about sealing the whole thing in an ali enclosure filled with sand, terminals on porcelain standoffs.

I'll take on board your various comments, thanks again, Andy.
__________________
Curiosity hasn't killed this cat...so far.
Diabolical Artificer is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 4:20 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.