5v heater from 6.3v filament

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Sleeper

Well-known member
Joined
Jun 6, 2004
Messages
649
Location
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Hi, I'm wondering if there's a simpler way to drop a 6.3vac (2.5Amps) heater winding down to 5v (2amps) for a 5Y3 rectifier...
I'm hoping I don't have to place yet another parts order so I'm hoping a big resistive divider isn't the only way to do it.
I tried something a bluebird told me. Thought I'd come across some deep tube secret.
that would be to place two diodes facing opposite directions on one of the windings, makes sense- diode drop, but unloaded I'm only dropping about 1/4 volt that way.

Hmmmm.

Love to hear if there's some other way to do it.

Kelly
 
You have to be careful here. The heater of the 5Y3 is also the cathode. If you are using this as a B+ power supply rectifier, the heater windings will be at B+ potential. Obviously you don't want this voltage sitting on the heaters of the other tubes you may be using.

Usually the power transformer has a seperate isolated 5V winding specifically for the rectifier heater. If you are using a seperate 6.3V winding or transformer for this supply, you can drop a volt or so across a series wirewound resistor.
 
Hi Bill, I have 2 - 6.3v windings, one for the tube heaters and one for the rectifier heater...
couldn't find a 6xx rectifier that could do the 100ma I need for this supply.

I thought a series resistor would just drop current not voltage???
don't I need a divider to drop voltage?
I want to drop as little current as possible.
been trawling posts and plugging numbers into online voltage dividier calculators but this is just confusing me more.
rats. I'll have to take a fresh look in the morning.
Kelly
 
[quote author="Sleeper"]couldn't find a 6xx rectifier that could do the 100ma I need for this supply.
[/quote]
An EZ81 (a.k.a. 6CA4) might be enough.
I thought a series resistor would just drop current not voltage???
don't I need a divider to drop voltage?
You need a resistor in series that drops 1.3V when 2A current flows through it so that there are 5V left for the heater. (Hint: Ohm's law)
Code:
     I=2A
||---->-------+
||            | <--
||            R    1.3V
||6.3V~       | <--
||        heater   5V
||            | <--
||------------+

HTH
Matthias
 
Thanks Matthias
you helped me to see what I was doing wrong there...
good coffee helps to.
I get a .65r at just under 3 watts, use a .75watt standard 5watter and I'll drop a little more and be fine.


I'm not sure why I decided against the 6ca4, it does look quite good...http://www.drtube.com/datasheets/6ca4-rca1960.pdf
I was looking for an octal base, plus I have a bunch of 5y3s here-now- that's often a key design factor for me, trying to use what I have.

Thanks.
Kelly
 
I'm not sure what you are building here, but did you consider a plain old silicon rectfier, like a 1N4007? If you do want to use the tube, the isolated 6V winding should be fine for what you want to do. a 0.5 ohm resistor should give enough voltage drop to give you 5V drop across the filament.

I would use a 5W resistor just to be on the safe side long term. This should only be dissipating slightly over a half watt, but it's good practice not to run a resistor near it's maximum rating. If you can find a 2W ceramic wirewound, it would be a good choice too.
 
just converting an old webcor tube reel to reel into a fender tweed type amp. 2- 12xx7 2 6v6's I kinda want the sag.
I was thinking I'll try making a silicone bridge with 4007's in an octal can, I have a few of those left over from building T4b's.
I know weber amps makes a drop in silicon sub for 5y3... called the copperhead, or something like that. more stuff to look into, like the
Thanks for confirming the calculations Bill.
all the best
kelly
 
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