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 21st Mar 2017, 9:12 am   #1
Diabolical Artificer
Dekatron
 
Diabolical Artificer's Avatar
 
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 7,638
Default Elevated heater winding w/out separate tfmr?

Hope I have this right. Sometimes we need to float or elevate a valve's heater supply for instance with a cathode follower or cascode so as not to exceed htr/k V. This is normally done by using a centre tapped htr winding and connecting the tap to a potential divider to raise it say to 100v.

AFAIK you can't do this by using a winding on a mains tfmr that has say the HT winding and one or two other htr winding's, you need to use a separate "filament" tfmr. This is where I'm a bit foggy on the subject, I presume by connecting DC to one winding it would effect all the others.

If this is the case is there an easier to elevate, float a htr supply without using a separate tfmr? I had thought of using say a 1:1 tfmr to isolate the elevated htr supply from the main tfmr and the vague thought popped into my head of using opto isolator's.

Before I expend any brain power to this train of thought, I thought I'd run it pass you chaps as I might be talking out my hat.

Andy.
__________________
Curiosity hasn't killed this cat...so far.
Diabolical Artificer is offline  
Old 21st Mar 2017, 10:23 am   #2
GMB
Dekatron
 
GMB's Avatar
 
Join Date: Aug 2003
Location: near Reading (and sometimes Torquay)
Posts: 3,086
Default Re: Elevated heater winding w/out separate tfmr?

Opto-isolators do not convey significant power, so forget that.

A small transformer works.

A modern solution would be a small switched-mode power supply. You can get isolating PSUs that convert one low voltage to another.

Another thought is that if all the heater ratings are about half what is needed you can bias the entire heater supply to half way up.
GMB is offline  
Old 21st Mar 2017, 11:14 am   #3
G6Tanuki
Dekatron
 
G6Tanuki's Avatar
 
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 13,953
Default Re: Elevated heater winding w/out separate tfmr?

There's no reason why you need to use a *separate* transformer - if your main transformer has multiple LT windings you can 'lift' one without any problems. The 'lifted' winding neither knows nor cares whether it shares a transformer-core with other windings.

Consider a classic fullwave HT supply using a directly-heated rectifier. The rectifier's heater-winding is happily 'lifted' above ground by the HT voltage!
G6Tanuki is offline  
Old 21st Mar 2017, 11:15 am   #4
MotorBikeLes
Nonode
 
Join Date: Dec 2012
Location: Kirk Michael, Isle of Man
Posts: 2,346
Default Re: Elevated heater winding w/out separate tfmr?

It was common practice (probably still is) to fit the little blue RS transformers (mains in, 6.3 or whatever out) to an oscilloscope where the transformer's CRT htr winding had gone leaky. I think they were rated at about 1.5KV and leak tested at 5KV. Never had a failure. Not sure what voltage you want, but that should do. They are small.
Les.
MotorBikeLes is offline  
Old 21st Mar 2017, 11:15 am   #5
ITAM805
Nonode
 
ITAM805's Avatar
 
Join Date: Jul 2010
Location: Folkestone, Kent, UK.
Posts: 2,172
Default Re: Elevated heater winding w/out separate tfmr?

Hi Andy, I'm currently working on a Klemt M40, they do it from the HT
Attached Thumbnails
Click image for larger version

Name:	IMGP1018 (Large).jpg
Views:	103
Size:	79.0 KB
ID:	139589  

Last edited by ITAM805; 21st Mar 2017 at 11:23 am.
ITAM805 is offline  
Old 21st Mar 2017, 1:50 pm   #6
kalee20
Dekatron
 
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,061
Default Re: Elevated heater winding w/out separate tfmr?

If it's for a temporary situation, just use batteries for the relevant heater!
kalee20 is offline  
Old 21st Mar 2017, 5:29 pm   #7
G8HQP Dave
Rest in Peace
 
Join Date: Sep 2008
Location: Solihull, West Midlands, UK.
Posts: 4,872
Default Re: Elevated heater winding w/out separate tfmr?

Quote:
Originally Posted by Dr Wobble View Post
Hope I have this right. Sometimes we need to float or elevate a valve's heater supply for instance with a cathode follower or cascode so as not to exceed htr/k V. This is normally done by using a centre tapped htr winding and connecting the tap to a potential divider to raise it say to 100v.
Heater circuits should never be left to float, but elevation is fine. This does not require the presence of a secondary centre-tap but it does require a separate secondary. This may or may not require a separate transformer - in most cases you can just use another secondary on the main transformer.
G8HQP Dave is offline  
Old 21st Mar 2017, 7:21 pm   #8
julie_m
Dekatron
 
Join Date: May 2008
Location: Derby, UK.
Posts: 7,735
Default Re: Elevated heater winding w/out separate tfmr?

Or even make a new winding by poking a length of 20-gauge enamelled transformer wire (or even 7 / 0.2 insulated hook-up wire, if you're that desperate ) through the gaps either side of the bobbin and pulling it taut. Start by winding on 20 turns, power it up, see what comes out, then add or subtract turns as required to get the desired voltage. Remember the voltage will sag once you start to draw current, and by more than you would expect just from Ohm's Law because there is also a phantom resistance due to imperfect coupling of magnetic energy -- but once you start getting close, you can safely put the valve into circuit and measure the voltage under real-life load conditions.
__________________
If I have seen further than others, it is because I was standing on a pile of failed experiments.
julie_m is offline  
Old 22nd Mar 2017, 6:46 am   #9
Diabolical Artificer
Dekatron
 
Diabolical Artificer's Avatar
 
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 7,638
Default Re: Elevated heater winding w/out separate tfmr?

Thanks all. not sure where I got the idea that you have to use a separate ( I know, doesn't look right, see - https://www.grammarly.com/blog/separate-seperate/ ) tfmr, have been using an RS htr tfmr.

So no worries, I was talking out of my hat. As I'm winding the tfmr it's not a prob Julie. Thinking about it scope tfmr's sometimes have a htr winding with more insulation for CRT htr's with cathode's operating down at -1kv or similar.

Apologies Dave, if floating is the wrong word. What would a winding as described above at minus Xv be? Sunk, descended, lowered ?

Andy.
__________________
Curiosity hasn't killed this cat...so far.
Diabolical Artificer is offline  
Old 22nd Mar 2017, 8:12 am   #10
trobbins
Heptode
 
trobbins's Avatar
 
Join Date: Oct 2009
Location: Melbourne Australia
Posts: 898
Default Re: Elevated heater winding w/out separate tfmr?

Quote:
Originally Posted by Dr Wobble View Post
What would a winding as described above at minus Xv be? Sunk, descended, lowered ?
Perhaps 'elevated to a negative voltage' is most appropriate. Either way will shock you the same

Elevation (positive or negative) may also assist hum ingress in audio circuits, especially for valves that have relatively low resistance between heater and cathode (ie. down in the low Megohm region, rather than up in the 100's of Megohm), which can occur with age or just bad luck.
trobbins is online now  
Old 22nd Mar 2017, 11:53 am   #11
G8HQP Dave
Rest in Peace
 
Join Date: Sep 2008
Location: Solihull, West Midlands, UK.
Posts: 4,872
Default Re: Elevated heater winding w/out separate tfmr?

Negative bias?

Floating heater supplies have no bias at all (not even zero bias) so can take up whatever voltage they will. If you are lucky a bit of leakage somewhere will set a sensible voltage. If you are unlucky the heater supply can reach a rather large voltage which if you are really unlucky might disrupt valve operation. The general rule is: nothing inside a valve envelope should be allowed to float.
G8HQP Dave is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 10:47 am.


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.