Power Transformer ratings?

[Blair]

Hi guys,

Here is a good question for the harvesters out there. I am one:)

If you pull a power transformer from an amp, find the schematic. (Let's use Fisher X202-C in this case). I add up the tubes. There are 10.

For the 6.3V winding, there are 4 X 7591 tubes and three #47 bulbs.

.8A X 4 + .45A(bulbs) = 3.65A

The drivers and various other tubes are in series on a 25.2V winding. There are 6, so .9A

Add up the current for the four power tubes based on the darashwet and a few mA for the drivers, and it is around ~325mA.

How "on the edge" did manufacturers make these transformers typically? For example, 3.65A? Wouldn't they want to go slightly higher to say 4A?

I'm not too worried because I can use 6L6 tubes with the. 3.65A winding, but I have always wondered how tight the relationship is between what the tube filaments draw and what the power transformer can generate in old gear.

Thanks,

Blair

[Geek]

Heh, depends on *who* made it

If it was vintage Hammond, 3.65A means it can handle 4A. If it's a Collins, it means it can handle 6A. If it was a Dynaco, drawing 3.65A from a winding that should be 3.5A is norm

Cheers!

[Blair]

Ha!

That's my fear. Just like the 6L6 amp I just built. Runs hot despite being under current on all filaments.

Hopefully the fisher is better.

I have a tiny transformer that came from a 2 x 12AX7 phono amp. I want to use it for a preamp. Any idea how much current it might deliver?

Thanks,

Blair

[Geek]

I have a tiny transformer that came from a 2 x 12AX7 phono amp. I want to use it for a preamp. Any idea how much current it might deliver?

Well, we'd need to know some things first, like HV secondary DCR, weather or not it's CT'd, its unloaded voltage (across the whole winding, CT'd or not) and same for the heater winding and what the primary DCR is.

Cheers!

[soundbrigade]

I just ordered a pair of power transformers for a coming project. For the B+ (500-600V @ 130mA) this was what I was ordinated:
depends on the filter arrangement, simply said I would supply a
tapped winding of 520-550-580 V 350 mA.
Right now this feels like overkill, but better be safe than greedy.

Some people recommend a doubled I-rating for, especially, heaters. I usually go for 50% extra when I order a transformer.

[Blair]

Hi Magnus,

That's how I do it if I buy a new tranny. I usually go 50% over current capability vs. need.

Gregg,

Is there an equation based on those #s to calculate the current? I would like to build an SRPP or something like it, but the 12AX7 pair in the phono pre it came from is not likely to draw more than 10mA or so.

Thanks,

Blair

[Blair]

The secondary is around 130V and the DCR is about 415 ohms. No CT.

Thanks,

Blair

[Geek]

Hi Blair,

OK, so for a 10% voltage drop (13V), I=E/R or 31.3mA. There's eddy currents, losses and other rubbish that get in the way, so I would declare your HV secondary maximum 20mA current draw.

When you do the same thing for the heater winding, rule of thumb with those is to take the 10% voltage (either 0.63V or 1.26) drop divided by the winding resistance and divide that result by 5, lest you risk over VA'mping the core before cooking wire.

Cheers!

[Blair]

Cool,

Thanks Gregg! That should do for an SRPP 12AU7 preamp. I usually bias them around 4-6 mA.

Blair

[Ty_Bower]

There's some good info in this thread:
http://www.diytube.com/phpBB2/viewtopic.php?t=1465

...including some charts on calculating winding capacity as a function of "unit resistance" (ohms dc resistance per volt). I've used that chart on some unknown transformers, and the results were what I expected.

[Blair]

Awesome Ty!

This is very helpful!

Blair

[Blair]

OK,

So this is great info for the dumpster transformer!

How about chokes? You see them listed all the time of fleabay with unknown voltage. I build lots of preamps, so noting too high for voltage. I've never seen a choke for tubes rated for less than 3-400v. Is most magnet wire rated for around these voltages? I assume they use a thicker varnish on the higher voltage chokes?

Something like this?

http://item.mobileweb.ebay.com/viewitem ... 0576002241

Thanks,

Blair

[Geek]

Well, unless they've figured out how to bend physics a wee, I'd say it's rated for 5... maybe 10mA?

Cheers!

[Blair]

Hi Gregg,

I was just using that one as a quick example. I found one the other day that is .4H at 200mA or so. The thing holding me back was the unknown voltage specs.

Thanks,

Blair

[Geek]

Voltage ratings are a real crapshoot.

After having quite a few "deals" turn into so much smoke on me, I usually order chokes fresh from a Hammond distributor.

Any particular current, voltage and henry rating you seek? I may have a good one for you in my surplus bucket.

Cheers!