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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. |
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21st Mar 2017, 9:12 am | #1 |
Dekatron
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
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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.
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21st Mar 2017, 10:23 am | #2 |
Dekatron
Join Date: Aug 2003
Location: near Reading (and sometimes Torquay)
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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. |
21st Mar 2017, 11:14 am | #3 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
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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! |
21st Mar 2017, 11:15 am | #4 |
Nonode
Join Date: Dec 2012
Location: Kirk Michael, Isle of Man
Posts: 2,346
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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. |
21st Mar 2017, 11:15 am | #5 |
Nonode
Join Date: Jul 2010
Location: Folkestone, Kent, UK.
Posts: 2,172
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Re: Elevated heater winding w/out separate tfmr?
Hi Andy, I'm currently working on a Klemt M40, they do it from the HT
Last edited by ITAM805; 21st Mar 2017 at 11:23 am. |
21st Mar 2017, 1:50 pm | #6 |
Dekatron
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
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Re: Elevated heater winding w/out separate tfmr?
If it's for a temporary situation, just use batteries for the relevant heater!
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21st Mar 2017, 5:29 pm | #7 | |
Rest in Peace
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Re: Elevated heater winding w/out separate tfmr?
Quote:
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21st Mar 2017, 7:21 pm | #8 |
Dekatron
Join Date: May 2008
Location: Derby, UK.
Posts: 7,735
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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.
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22nd Mar 2017, 6:46 am | #9 |
Dekatron
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
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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.
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22nd Mar 2017, 8:12 am | #10 | |
Heptode
Join Date: Oct 2009
Location: Melbourne Australia
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Re: Elevated heater winding w/out separate tfmr?
Quote:
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. |
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22nd Mar 2017, 11:53 am | #11 |
Rest in Peace
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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. |