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EDY19

Mon Feb 27 2006, 08:39PM

Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408

Im looking to wind a ~50VA transformer to convert audio from a amplifier (~50W@4ohms) to a signal that can be used to modulate the grid of a VTTC for audio Modulation. The output needs to be around 300V or so, so how do I go about figuring the number of windings to get a impedance on the primary of 4 ohms so the amp can power it safely?

Hazmatt_(The Underdog)

Mon Feb 27 2006, 10:13PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

If you don't have the core, I wouldn't bother because its not like a 60Hz core at all.

It might be easier to order it from

rather then trying to make one.

Steve Conner

Mon Feb 27 2006, 10:44PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Hazmatt is right, but a 60Hz core will sort of work. It won't be particularly hi-fi though. I would get an ordinary 120v (or better still 240v) to low voltage (about 24v?) transformer, and feed the amp into the low voltage winding via a 4 ohm series resistor for protection. A toroidal transformer might give more treble than a regular E-I type.

...

Tue Feb 28 2006, 01:14AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

So long as there is no dc bias through the core you should be fine with a normal mains one; and EI one will work but a toroidal one will sound better.

As to the number of turns; the exact amounts will depend on the core, but the ratio of primary/secondary would be calculated as voltage in/voltage out.
You 50w amp with a 4 ohm load is giving 14 volts, so if you want 300v out you will about a 1:20 ratio.

Probably the easiest way to figure it out would be to buy a mains->24v transformer (so you do not saturate the core at low frequencies), unwind the mains coil (getting a rough idea of how many turns were there) and then rewind it with smaller wire with either 25% or 150% more than what you took off... (for 120/240v mains)

Or if you are too lazy to count the primary turns;
remove the primary
hook the 24v winding up to 15vac (variac)
wrap 1 turn of wire through the core
measure the voltage of that winding
then take 300v/volts across the winding, and use that many turns

EDY19

Tue Feb 28 2006, 02:25AM

Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408

I don't know too much about transformer design, but would it be worth a try to wind one on a ferrite core? I know they are designed for higher frequencies, but tesla coils don't make the best low frequencies anyway. I have a flyback core, as well as a 8 inch long ferrite rod. Would either of these be better than an iron core?

...

Tue Feb 28 2006, 03:14AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

I wouldn't recommend it as ferrite has very crappy low frequency response. Most (if not all) audio transformers use iron; they just use powdered iron or very thin iron to minimise eddy losses.

just as long as you have extra winding on the transformer to prevent if from saturating you should be fine with normal iron cores.

EDY19

Tue Feb 28 2006, 11:28AM

Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408

So maybe a powdered Iron toroid would be best- large enoguh to handle 50VA- Ill keep my eyes out

Sulaiman

Tue Feb 28 2006, 04:15PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

A transformer that can efficiently operate at 50Wrms,
over the entire audio range.
Very big and expensive !
Look for a cheap/used Valve output transformer
(they are still in demand)
You can make lots of compromises though ....
1)
As far as I'm aware you cannot get a plasma bass speaker
so you only really need mid- and high- frequencies
which means a MUCH smaller/cheaper transformer.
2)
I wouldn't bother about the very high audio frequencies
(because I can no longer hear them)
so a mains power transformer,
which usually work up into the kHz range would do.

I would go for a toroidal mains transformer, 10VA should suffice
lower secondary voltage = greater transformation ratio.
(10VA at 60 Hz is like 50VA at 300 Hz w.r.t. volts/turn)
(A normal E I laminated one is also good enough)
Put a simple high-pass filter at the PA input, above 300 Hz
(e.g. 47nF capacitor in series for 10 kOhm load/input)
And more importantly, to protect the amplifier,

put a resistor (3 to 10 Ohms, 5 to 20W) with about 100uF NON-POLAR capacitor (the kind used for crossovers etc.) in parallel

a 3 to 5 Amp fuse

all in series with the amplifier output.

With such a large inductive load (the transformer) I also reccomend;
At the amplifier TWO 1 to 5 Amp 50V minimum diodes
one from -Vsupply to the output (band/cathode to output)
one from the output to +Vsupply (band/cathode to +Vsupply)

EDIT:
the diodes should be 100 V minimum for a bit of safety margin.
You may find a LINE transformer you could use,
they are usually 5W or so, and are designed for audio,
they are usually atached to speakers that are fed by large PA systems.(shops/offices/public buildings etc.)

EDIT EDIT:
If you are modulating the hv dc of a VTTC two problems;

The anode voltage could flash-over to the audio side.
-an earthed inter-winding screen would be best
-separate primary and secondary windings could be ok
-at least EARTH the transformer core/frame and the audio PA

SORRY, I'VE MADE A BIG OVERSIGHT !!!!!
If the modulation transformer secondary is actually in the anode circuit it will have to carry the d.c. anode curent,
which would saturate it.
A single-ended valve output transformer would be Ideal
a small mains transformer would be saturated/useless.
What is the VTTC anode voltage, current, frequency?

and it will be subjected to voltages across and currents through
at VTTC frequencies/voltages/currents etc.
Since it will be driving the grid the insulation problem is the same!
but there should be no saturation.

EDY19

Tue Feb 28 2006, 08:45PM

Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408

The transformer only modulates the grid, so it does'nt have to carry all the current of the MOT.

Hazmatt_(The Underdog)

Tue Feb 28 2006, 10:48PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

did anyone look at the links I posted?

They both have what you're looking for. Or you could contact Hammond directly, its your option.

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