1000W High-pressure bulb ballast as tube amp power transformer?

Thread Starter


Joined Feb 10, 2020
Let me say first that I read the earlier post about this that stirred plenty of philosophical debate, brow-beating, non-apologies and the like. But what I noticed is the question may have be asked innocently incorrectly and certainly wasn't answered in sufficient detail for me not to ask something similar.

I purchased a used Xtrasun Growlight Convertible Ballast. It's meant to ease the start of high-wattage, high-pressure bulbs. Inside the heat-sink box is a massive transformer, a massive 26uf/500v cap and starter. My question is not "can I turn this skateboard into a spaceship" (though I have seen a DeLorean turned into a time machine and a flying skateboard), rather is it possible to use this as a power transformer in a vacuum tube amplifier? I don't want to use it as the output transformer to power speakers, but to provide (or help provide) the power supply for the HT part of a circuit.

I've built a couple of tube guitar amps repurposed from vintage tube gear, so I'm familiar with the general principles. I also was a certified hydro-turbine operator in another life. That said, I don't know the answer to my question. It looks to be a similar electrical path. I noticed a reference to autotransformers in the other post, and I admit I should remember more about how those differ from others--but don't. If my query is insulting to you, please don't respond. It's not meant as a joke, the pulling of a leg, the tweaking of the nose, etc. but a serious search for knowledge from those more informed than I. Hopefully someone might be kind enough to enlighten me on the whys or whys not. Thanks in advance.


Joined Sep 7, 2010
Searching this item , this is apparently what's inside the convertible ballast ... I think "convertible" means it can run on either 120 or 240 AC

This is probably a 1:2 transformer ....

Investigate the coils , what resistance how many turns ... feed in AC to one coil (being careful) and see what output you get on other coils


Joined Jan 15, 2015
It all depends. The bulbs I seem to recall were run at something like 235 to 250 volts. If you are using amps having tubes that require about a B+ of 250 VDC you may be able to build a decent power supply. Just keep in mind you will need good filtering. Measure the output and see what you have. Look at the data sheets for the tubes and see what the plate voltages are.


Thread Starter


Joined Feb 10, 2020
Thanks to both of you who've replied. I believe it is a 1:2 transformer, as suggested, and it looks like the one in the photo. Is there any issue with it being an autotransformer?


Joined Jul 10, 2017
Thanks to both of you who've replied. I believe it is a 1:2 transformer, as suggested, and it looks like the one in the photo. Is there any issue with it being an autotransformer?
An autotransformer has a single tapped winding and can be configured as a step up or step down voltage transformer. There will be a safety issue if it is an autotransformer because there will be no isolation form the mains supply.
The trasnsformer in the picture appears to have two separate windings.
If the capacitor is connected to a single winding, then this may be a constant voltage transformer (CVT).

In general, the ballasts have a high open circuit voltage and limit the current once the lamp lights,

I used a CVT to power a solid state amp and I had to abandon it because the hum from the transformer was annoyng. I got great bass though.


Joined Nov 6, 2012
This is a BALLAST Transformer designed specifically to initiate an ARC by creating
a high voltage, at a very limited current, and after the Arc is established,
it acts as a "CURRENT REGULATOR".
The Bulb it is designed to work with doesn't care what the voltage is,
but it MUST have the current limited to a specified range for proper operation.
It's virtually the same thing as a Florescent Lamp Ballast.
It is designed to act as a "Filter Choke Coil" more than it is a Power Transformer.
Even if you decided to completely re-wind the whole thing,
the Iron Core is designed to be very "Lossy" on purpose,
and would be a poor choice for a Power Transformer Core,
and would run extremely HOT.

A "Musical Instrument Amplifier" is a strange animal.
It is designed to be really low fidelity, on purpose.
The power supply IS DESIGNED TO "SAG" under load, on purpose.
This is the same reason for continuing to use a "Rectifier Tube" in these amps,
the Tube Rectifier will "Sag" under load, creating additional distortion,
and a power "Compression" effect.

Designing a Musical Instrument Tube Amplifier is an "ART" not a science.
None of the "proper rules" apply.
For instance, most Musical Instrument Tube Amplifiers have very little,
or even absolutely zero, feedback circuitry, again, on purpose.
Most of these amps also run the tubes with very low Bias Current for the same reasons.
They are designed to have the "Right Kind", and the "Right Amount", of distortion.
I would never attempt to build one,
especially when they are so cheap and plentiful.
You can also Electronically Emulate any popular Amp and Speaker Cabinet Combination
with your computer, and the plentiful software, designed specifically for this purpose.
Then you can use standard High Fidelity Amps and Speakers, but get the
"Distortion Style" of your choice, (simply described as "Tone" by musicians).


Joined Aug 21, 2017
It are not very much of uses such lamp except the short-term use. And any short term use ought be improper to speak about good power efficincy factor. Thus when I am using those lamps (for them are damn good against building mold infection) I am using just the wife`s hot iron or some nichrome spiral of according resistance, let the nominal current via lamp is right. Just one night with beaten outer colb and mold is dead forever. Of course, for human that exposition dose is lethal, thus one must swith on and immediately leave the house. And got back only after fundamental aeration off the ozone.
Another way is to use a parallelized "long lamps" chokes, often some 10 to 20 parallely gives the necessary current. Just take an ammeter and read it. If current is too small, switch more chokes and if too large, cut some chokes off.


Joined Aug 21, 2017
Oh, sorry, I misread Your question. However over the planet there are much of those bulbs but nowhere to pick a chock. Seems Your problem is cardinally vice-versa, so the answer for Your case - this transformer probably may have an air-gap, regulating the current, and resonant capacitor producing it more stable. So, the use for audio amplifier is very non-probable. Simply the voltage in output may be current-dependable. Reccomend to measure the V(out)=F(Amperes). If stable - then useable.