Voltage doubling for heater circuits

[bigfathairyvika]

Hi All,

My workshop radio's detector/1st if valve is starting to degrade.
The valve is a Brimar 7R7 double diode pentode which uses a 6v heater as do all the of valves in the circuit.
In order to give myself more of a choice in replacement I have the idea getting a few 14R7 ( far cheaper ) and adding a simple voltage doubler to its heaters.

Will this cause more problems than it cures?
I'm thinking maybe hum induced on heater supply or some other problem.

Mark

[VT FUSE]

I think I would be looking at making an adaptor to take an EBF81 B9A valve or even the EBF83 which is equally happy at mains voltages as many know.

[broadgage]

It can be done, but needs a bit of experiment or calculation to get the effective voltage on the heater correct.

Presuming that we are talking about a standard voltage doubler consisting of two diodes and two capacitors.

With 6.3 volts input, the output will with an infinite capacitor, be twice the peak value of the input. 12.6 multiplied by 1.4 or say 17 volts, less the drops in the two diodes, say about 15.5 volts.

In practice it will be less because the real world capacitors wont remain at the peak voltage but will partially discharge during each cycle.

A basic voltmeter wont record this voltage accurately.

It would be simpler to obtain a valve with a 6.3 volt heater, or obtain a 12.6 volt heater supply in some way.

[Argus25]

Assuming you heaters are currently running from 6.3V AC in the normal way, then a voltage doubler with diodes & capacitors to generate DC would be of little utility.

The better method is to wind a small auto-transformer with a ratio of about 1:2.1 (to allow for the transformer losses). In other words a nearly center tapped winding with the 6.3V applied to the tap and 12.6V acquired across the whole winding. Just off the top of my head, for 1 valve heater like this with less than 2W requirement, a small laminated iron core with a cross sectional area of about 1cm^2 would be fine or possibly a bit bigger, and fill the bobbin space with a wire size such that the final DC resistance is of the whole winding is in the range of maybe no more than 0.5 to 1.5 ohm, and the turns numbers will probably be enough that the core isn't running a very high peak flux density. You can experiment by checking that the off load current of the transformer is less than a few 10's of milli-amps. If you like I could post the equation for the flux density, better to keep it below 1.5T

[bigfathairyvika]

Ok, thanks for the ideas.
I think I will use duncans psu program to simulate a voltage doubler.

I really don't want to wind a transformer to do this as I have a spare 6-0-6 transformer hanging about which I could use as a last resort ( Could be useful in another project ). Although I don't have any iron cores about I do have a large box full of ferroxcube stuff.
But I do understand that a transformer would be the best way to do the conversion. I'm not upto all the calculations but would probably wind and try.

Shoving a b9a in between all the other octals just won't look right!

I'll update when I've tinkered a bit.

Mark

[Al (astral highway)]

Originally Posted by bigfathairyvika

Ok, thanks for the ideas.
I think I will use duncans psu program to simulate a voltage doubler.

I really don't want to wind a transformer to do this as I have a spare 6-0-6 transformer hanging about which I could use as a last resort

That's a surprising conclusion! I think as others have said, making a voltage doubler isn't ideal for this situation. It's also really complex compared to your alternative...

If you have a 6V transformer, why not just use that? Ta da!

Use both windings in parallel if you want. The 0.3V nominal deviation from 6.3 won't make any discernible difference in the role you describe. They were designed and manufactured with tolerance both ways and it's not a power device. It could run a little lower than 6V and would be just fine.

Or use the 6-0-6 secondary as an auto transformer?

[G6Tanuki]

For step-up-autotransformer service it doesn't *need* to be specified as a 6-0-6 transformer; it just needs to be an 'autotransformer' with a freely-available centre-tap so it works as a 1:2

(you're not using the mains side so the primary:secondary ratio is irrelevant)

Pretty much any "Wall-wart" transformer with a centre-tapped secondary will do the necessary: you're only talking about a few Watts. Just remember to insulate the unused primary-terminals to stop yourself getting a nasty bite if you happen to touch them!

I've used the autotransformer trick in the past to power a couple of the 25.2V-heater versions of the 6146-family RF PA valve (all I had available) when the original design used the 12.6V-heater variant.

[bigfathairyvika]

If I use the 6-0-6 transformer , I'll be using it as a 12v supply direct to the valve but where is the fun in that?

If I use some diodes and capacitors ( which I have plenty ) then the transformer can be used in another project.

Also cropped up this afternoon is a boost regulator I found which with a diode and capacitor I can set to exactly 12.6v.

[Sideband]

Nothing wrong with this sort of experimentation if valves become obsolete/hard to obtain or just priced out of reach but putting things in perspective, the 7R7 is still available so why not just obtain an original....it will probably outlast the set! It's not as though they fail every couple of years. Chances are you'll never need to replace it again.

[Ed_Dinning]

Hi Mark, I have used the small auto transformer trick several times for both valves and CRT's.
There is no danger to the main transformer id the VA rating of the valves is the same at either voltage.
You could even change all the valves to 14- types and just use one auto transformer.

Ed

[bigfathairyvika]

Nice one Ed.
Changing the lot had crossed my mind...
I shall have to try the auto transformer trick.
I've heard of this sort of thing using only one side of a transformer but never understood it.

[G8HQP Dave]

Has anyone mentioned that rectifying and smoothing will mean you need approximately double the VA rating for the heater winding? You can get away with using this trick for one valve, but if you intend eventually replacing all of them with 12.6V valves then the heater winding may get too hot.

[Refugee]

I agree.
There used to be a data sheet in the Farnell catalogue before the internet to assist with designing SLA chargers using transformers.
It clearly stated that you must de-rate by 50%.
I once warned a penny pinching boss about this. The job bounced and there was a bill for damaged floor coverings to be paid. He learned to listen to me after that.

[Ed_Dinning]

Hi Gents, the beauty of the autotransformer is that it is compact and does not put a significantly increased load on the original transformer.

Ed

[Radio Wrangler]

If you're using 6-0-6 secondaries of a transformer as an auto transformer to double 6.3v, then remember that the primary pins will run with about 240v across them.

So you might as well apply mains to its primary and use the secondary to power the 6.3v valve. It'll take a couple of watts of load off of the set's mains transformer and give it an easier life.

David

[bigfathairyvika]

Have I confused a few people?

Original valve 6.3v heater.
New valve 12.6v heater.

btw: I've just setup a test circuit with two transformers, 0-12 into a 12-0-12 ( 12-0 ) and taken the ouput from the 12-12 and I do get 25.3v ( roughly double the 12.85v )
But, this has roughly one amp running through the 12v side.
Should the second transformer be a higher x-0-x voltage to obtain higher winding resistance thus reducing the standing current?
I do only need 150ma for the replacement valve, but pulling an extra amp from the heater windings isn't going to make for a happy transformer.

Actually... For all this trouble I could just use my second test transformer to power the new valve!!!!

Oh I wish I was 30 years younger then I wouldn't be up at 5am testing transformers!

Mark

[Radio Wrangler]

Sorry, I should have said use a small mains transformer with 6-0-6 or dual 6v or single 12v secondary to power the 12.6v valve.

Put mains on the primary, power the single valve alone from the secondary, and your set's transformer has an easier life.

Regarding your 30 years comment, I've just notched up another one, and I feel it!

David

[trobbins]

Mark, I believe your heater winding is being used for multiple valves?

When you say "degrade", that doesn't refer to increasing hum does it? I think you are just saying that it would be great to have the opportunity to use a 14R7 instead of a 7R7 for one of the valves.

You could use a capacitor input doubler to get circa 12-14VDC for just powering the 14R7 heater. That would mean the 6VAC heater and 14VDC heater have a common terminal by default.

That would allow the heaters to still be referenced to ground (eg. one side of the 6V heater grounded, or a humdinger, or the heater is 3.15-0-3.15 with CT grounded). If hum becomes a concern (given the 14VDC heater would now have an asymmetric voltage to ground), then you could DC elevate the heaters, or change from humdinger back to grounding the common heater terminal.

[bigfathairyvika]

Yup, the heater winding ( single transformer does heater, b+ etc ) drives five valves altogether. ( 7R7, 6V6, 6X5, 6SL7, 6K8 )

And yes, I was thinking "hum" or "buzz" , in that putting a diode in circuit would introduce extra noise into the heaters of the other valves.

The current heater winding is not center tapped but one side is grounded.