Two Heater Windings

[TIMBO]

Hi guys, I have a PT that has two heater windings and I don't know if they have the same current capacity.
Neither have a CT.
Both sets of wires are the same diameter.
Both measure the same resistance.
Both measure about 7v.

Is there a way of calculating there current.

[sluckey]

I don't know of any calculations. I would just load it up with a string of tubes and keep adding tubes until the voltage drops to about 6.3v. Then let it cook for several hours, checking on it about every 30 minutes. If it survives, call it good and add up all the tube currents IAW the tube manuals. Repeat for the other winding.

[TIMBO]

Thanks sluckey. That was what I was thinking.
At this point I was going to use one winding for a EZ81 (1A) and the other for 2X6gw8 (1.4A) and 2x 12AX7 (.6A) and see how that fairs.

I know the amp this PT came from had 1x6v4,2x6m5, 1x12ax7, 1x6be6 and another 1x6***
So I would assume one winding for the rectifier and the other for the rest.

[adamG]

Hi Timbo,

I hope this might be helpful.
Just convert the formula.
I use it for my PT's wiring. It works.

Regards,

Adam

[jjasilli]

Using Ohm's Law: a 12ax7 drops 6.3 volts across its heater drawing 300 mA.  What is the heater resistance?  V = R X I.  6.3 = R X .3.  R = 6.3/.3 = 21 Ohms.


Building on sluckey's suggestion, instead of tubes, I would hook a winding up to a 100 Ohm R (of adequate W).  Measure the voltage across the R.  If it's 6.3, then it can handle at least 5 12ax7's. Whatever, you now have useful info to complete the analysis.

[sluckey]

Using Ohm's Law: a 12ax7 drops 6.3 volts across its heater drawing 300 mA.  What is the heater resistance?  V = R X I.  6.3 = R X .3.  R = 6.3/.3 = 21 Ohms.


Building on sluckey's suggestion, instead of tubes, I would hook a winding up to a 100 Ohm R (of adequate W).  Measure the voltage across the R.  If it's 6.3, then it can handle at least 5 12ax7's. Whatever, you now have useful info to complete the analysis.

Say what???

100Ω across 6.3v will draw .063 amps. That's pretty useless info.

[Lindsay]

Timbo,

Text me the wire diameter.

Lindsay

[PRR]

> Say what???

One 12AX7 is 21 Ohms. What is five 12AX7? Obviously 105 Ohms. Nearest standard value, 100 Ohms. 

I think he is *stringing* us along.

[sluckey]

Yep. Right down a rabbit hole.

[jjasilli]

Whoops: it's parallel not series.  It seems you should be able to put x number of 20 or 22 Ohm R's in parallel until the 7V supply drops to 6.3. There's got to be a way to do this without using tubes.

[sluckey]

Whoops: it's parallel not series.  It seems you should be able to put x number of 20 or 22 Ohm R's in parallel until the 7V supply drops to 6.3. There's got to be a way to do this without using tubes.

Sure there is. But in the world of tube amp tweakers, it just makes more sense to me to use tube filaments as a load.

[adamG]

Hi Guys,

The question was :"Is there a way of calculating there current."
So, this transformer was built with precise diameter of wires ref. to its maximum power.
If Timbo converts the formula, He'll know the currect and the maximum taps' power load, right?
IMO, voltage drop is another issue.

Regards,

Adam

[TIMBO]

Thanks for the input guys, I have just about got the heaters wired so i'll be able to load the windings as sluckey suggested.
I thing the wire gauge is around 21 awg.

[adamG]

I thing the wire gauge is around 21 awg.

Are you sure of 21 awg. I seems like a small transformer, right?
If so, then you can load this winding with 1,066A.
I assumed that the heater winding is on the top of the transformer coil. Then, the factor 0,7 is ok.

Regards,

Adam

[jjasilli]

Adam, your formula's may be good, but I can't read the handwriting.

[92Volts]

It's not a safe assumption that hookup wires are the same diameter as windings. Durability is a big factor. For an extreme example, it would be crazy to make a customer handle/solder 32AWG, though it would handle a small tube amp's HV current!

There are also different ratings for power distribution vs. chassis wiring: https://www.powerstream.com/Wire_Size.htm

For short distances in a device, 21AWG can handle 9A.

More loss is acceptable in a transformer (you can compensate with extra windings) than hundreds of feet of wire in a building or further outdoors. On the other hand, windings are poorly cooled in a transformer... Without knowing a lot more than I do, I'm hesitant to make (or trust) any estimate of transformer current capacity based only on winding size, let alone the size of the exposed hookup wires.

[PRR]

The current rating for wires out in the breeze (even in chassis) is *not* same-as their rating when wound-up tight in a transformer.

So anybody looking at ordinary wire tables: don't.

Chassis wiring is not a precision design task. You can usually fit a plenty big wire.

In transformers you have hundreds of yards of wire wound-up in an inch of space. It is always a compromise, and sometimes a tough one. The wire gauge is carefully determined. Then "fudged" because you can't buy (or afford to buy) #23.7, or because you want full layers but a given gauge makes an awkward part-layer (the next winding will sit crooked). With all of this the designer may have tossed-in the buck for a larger core than strictly necessary, and now has a slack winding. You can back-engineer from gauge to a fair estimate of current.

No transformer burns-up instantly on mild overload. You can do the math (right way round), load it with dummy resistors, and see how it smokes.

[TIMBO]

Thanks guys.
My findings.....
Wall voltage is 246v
Valve lineup 1xEZ81, 2x6GW8 and 2x12AX7
Green wires - 2x6GW8 and 2x12AX7 - 3.3v/wire (CT)
Black wires- EZ81 - 6.8v (no CT)
All valves connected in parallel
Green wires - 1xEZ81, 2x6GW8 and 2x12AX7 - 3.24v/wire (CT)
Black wires - 1xEZ81, 2x6GW8 and 2x12AX7 - 3.08v/wire (CT)
The wall voltage here can creep up to 250v so I'm thinking using the GREEN tap would be the best option for the heater supply to ALL valves.

There does not seem to be any stress on PT.

[sluckey]

Just a thought and it's not strictly necessary but sounds good to me   

If I had those two filament windings I would use the smaller one with no center tap to heat the EZ81 and use the other winding for all the other tubes. Then in the "unlikely event" of a heater/cathode short in the EZ81, you will not put B+ on the other tube filaments, putting the PT at risk if you have the other filament winding CT connected to ground. Like this...

     http://sluckeyamps.com/18w/18w.pdf

[TIMBO]

Thanks sluckey, My only concern is by having the separate supplies, EZ81 on one winding and the rest on the other, is they all can see a much higher voltage than 6.3v.
So if the heaters are around 10% overvoltage is this acceptable??

[sluckey]

Is to me.

[jjasilli]

Agreed.  Up to 7V is considered ok.  5V is the minimum.  (See, the Valve Wizard).  At your discretion you can drop a volt using power resistors or a diode.


I also agree that it was easier to spec the filament windings using the tubes in this build.