3 quick transformer questions

[gary h]

Hi all,

I've just received the transformer for my filament supply. It's rated at 8V in parallel configuration. With no load (DC reg boards and tubes) I get over 10V.

Is this because the 8V rating is for a 115V primary and mine sometimes being over 121V boosts the secondary relatively, or will this come down once the full load is connected? I doubt the latter and don't want to stress the regulated DC circuits over their input tolerances?

What is the best way to fix this? Resistors before the primary or after the secondary? How do I calculate this in order to get a useful selection of resistors? I assume I'll need about 10W?

Why do I get 3V between the matching phase dual secondary terminals? Shouldn't they be just above 0V? I shorted out the transformer secondary briefly while I was probing with the DMM. Can this cause serious damage? It was on for a couple of seconds.

That's more than three, huh? Whoops, there's another.

thanks,

gary

[Iain McNeill]

You're getting 10V for both those reasons. If your mains measures 121VAC (and that's still in spec! I always design for 130Vmax) then your 8Vnominal at 115V becomes 8.4V.

The primary and secondary windings will have resistance which will drop some volts as you start pulling amps. You can measure these easily and factor them into the equation.

You can use resistors to drop the volts, 1 ohm drops 1V at 1A which is probably all you need.

However, what about running a DC filament. The rectifier diodes will drop around 0.6V each regardless of current and then you can filter and get all that mains rubbish off your filaments.

In any case make sure whatever you use is rated for 1W minimum - I'd use 5W devices for longevity.

Measuring between windings will give strange results due to leakage currents and the high impedance of the meter. I wouldn't pay much attention to this. As for shorting the windings, not recommended but you'll be fine for short periods. The winding resistance will limit current and most transformers are short tested as part of UL safety testing.

Why didn't your fuse blow?

[gary h]

Iain,

thanks for the info. I like the diode idea. The transformer is first feeding a pair of dc regulating boards which work nicely up to 12-15 Volts but produce more noise and heat the higher the AC goes. Ideally I want to be feeding them 7.5 V to get my 6.3. (My line stage uses 4 6SN7s.) I have 4 10W 0.47uF resistors to dial this in but without loading the circuit with the tubes they don't do anything, my DMM still reads 10.1V. (I don't understand this, shouldn't the voltage drop be reflected even without any current?) As for the fuses, I haven't added them yet as I am still connecting the entire PS together on my desk to test it before I solder and fasten everything together.

Thanks again for the help. I take it I will need about 5 Ohms of resistance to get 2.5 V down at 2.4 amps?

gary

[Tweeker]

There is no voltage drop without current. V=I*R

uF is a measure of capacitance.

2.625 (2.7 is close enough) ohm 20+ watt resistor (15 watts dissipated, it will get hot) or series/parallel resistor network could be used as a dummy load to simulate the heaters.

[Tom Bavis]

Originally posted by gary h
Iain,

thanks for the info. I like the diode idea. The transformer is first feeding a pair of dc regulating boards which work nicely up to 12-15 Volts but produce more noise and heat the higher the AC goes. Ideally I want to be feeding them 7.5 V to get my 6.3. (My line stage uses 4 6SN7s.) I have 4 10W 0.47uF resistors to dial this in but without loading the circuit with the tubes they don't do anything, my DMM still reads 10.1V. (I don't understand this, shouldn't the voltage drop be reflected even without any current?) As for the fuses, I haven't added them yet as I am still connecting the entire PS together on my desk to test it before I solder and fasten everything together.

Thanks again for the help. I take it I will need about 5 Ohms of resistance to get 2.5 V down at 2.4 amps?

gary

1 Ohm will drop 2.4V at 14.A. But that includes the transformer winding resistance too - you might not need ANY extra resistance.

I'd use the secondaries in series and connect the 6SN7s in series-parallel. Half the current, half the percentage loss in the rectifiers.

[Iain McNeill]

Originally posted by Tom Bavis

1 Ohm will drop 2.4V at 14.A.

Not sure what you mean here Tom - possible typo?
Anyway, you're right. Once the transformer is loaded the voltage may be just fine. Gary, You need to calculate or measure the loaded output voltage. Tweekers suggestion will work to measure it.

If you have regulators already then just make them beefy enough to handle the power dissipation and you're done. Most series regulators I've used need at least 2V to guarantee regulation. You say yours prefer ~1V? Maybe a simple LM317 or emitter follower stage would give you better noise performance? I'm surprised the regulators aren't better than you describe.

edit- Oh I see you say they work fine to 12V. You're good to go then! No wasteful resistors necessary

[Tom Bavis]

Did I really type that? Of course I meant "1 Ohm drops 2.4V at 2.4A"...

[gary h]

Thanks guys,

so much of this stuff is relatively esoteric (to me that is) that I forget the simplicity of Ohm's Law. I'll run the whole circuit tonight and report back with my results.

I'd like to better understand the series/parallel configuration better. I'll search the forum but if anyone has time to burn I wouldn't mind some pontification. The transformer powering the heaters gives 16 V up to 2.7 A in series and 8 V at 5.4 A in parallel. Does series/parallel require the center tap?

thanks,

gary

[Iain McNeill]

Many will try to deny it in subtle ways but we are all governed by the laws of physics. Yes, it is that simple! V=IR is true everywhere.

Regarding the series/parallel question. That is a bit more complex. The power loss aspect is true and this is why our electric co's distribute power at 100,000's of volts - a megawatt is only ~10A! You can tolerate a lot of cable resistance with this arrangement.

In a multi-tube amp design I believe there are other considerations. I'll admit to being a solid-state kinda guy here and bow to experience but I think you need to consider what happens if one of the filaments goes open/short in a series/parallel design. Seems to me the fail condition is to put excessive power in the other tubes filament possibly causing a domino effect failure mode. Don't know what this does to your speakers.

Any experienced vacuum-state guys care to elucidate?

edit_ no center tap needed here - that's a balanced audio thang.

[hemgjord]

I think the "series/parallel" configuration Tweeker refers to is nothing more fancy than a number of resistors connected in series and/or parallel to achieve the combined resistance and power rating to correctly simulate your heaters.

/Olof