Exceeding mod xfmr power ratings....how far?

[KM1H]

I read somewhere that BCB iron can be run substantially above its ratings as long as the low frequencies are rolled off below 300Hz. The BC iron is rated down to 50Hz I believe? I cant find that article or post.

Are there any documents on this with curves or whatever? I have a 500W RCA that Id like to get about 750W from and Ive no interest in those low frequencies.

Carl
KM1H

[WA1GFZ]

Look at how a transformer saturates as the frequency is reduced. Then consider IR losses

[k4kyv]

I read somewhere that BCB iron can be run substantially above its ratings as long as the low frequencies are rolled off below 300Hz. The BC iron is rated down to 50Hz I believe? I cant find that article or post.

But why waste a good broadcast quality modulation transformer to run space-shuttle audio?

First of all, you will need a reactor, unless you  have one of those  rare BCB mod xfmrs designed to carry DC through the secondary.  Then consider the original working voltage of the transformer, and compare that to the voltage you will be running.  Some BCB transformers are tremendously conservatively rated.  In my HF-300 rig, I use a mod xfmr taken from an RCA 250 watt broadcast transmitter.  I have run the final at 1 KW DC input and hit 150% positive modulation peaks with no problem.  The only rating listed on the nameplate of the transformer is 2500 volts DC maximum.  That is exactly what I run to the modulator, and the final runs at 2 KV.  Since voltage is what toasts transformer insulation, I think I am still running the transformer within ratings.  Plus, I don't normally piss-beat the transformer to that degree.  I have an identical spare transformer that has been sitting on the shelf for over 30 years without ever being called into service.

Back in the 60's I modulated a KW with a little Electro-engineering mod xfmr that came out of a 250w Gates transmitter with no problem.  I added a spark gap across the primary and set the whole thing on insulation so that the core was floating.

Most of my mod xfmr blowing experience was when I was experimenting with high  level speech clipping with a splatter filter between the mod xfmr and final.  When I took out that crap, I never blew another mod xfmr.

I have never heard anything about suppressing the lows making it any less likely that a modulation transformer would crap out.  I would take it with a grain of salt.  Chopping off the lows at 300~ is kind of like slopbucket and Daylight Shifting Time; it is alleged to perform miracles and solve the world's economic and energy crises, reduce the crime rate and prevent the spread of AIDS.

[ke7trp]

I think the point of the Freq cut is to lower the current through the Transformer. Extreme lows and extreme highs are going to stress the trans.  I have an 80 HZ Cut in my Audio rack and run that.  It does seem to take the load of the iron.  99% of you guys cant hear 80HZ and lower anyways.  300 is cutting way to much out. I would think 150 and below can be cut.  Also 8K or higher can be cut.  Your not playing music.  Since obtaining a Real time spectum analyzer and installing it in my audio chain, I have discovered that my voice has very little energuy above 8K. 

I had a box that ran a 250 watt Thermodore mod iron with Two 810s with STIFF volts at 2650.  That has to be in the 500 to 650 watt range.  It died.

Clark

[WA1GFZ]

Excessive voltage and temperature rise kill a mod transformer. Saturating an iron core does nothing as long as there isn't excessive current to heat it up.

[ke7trp]

Run an audio sweep from 20 to 20K and see if yours lives   Your mod current will go big at the top and bottom.

I think alot of them are killed by people not loading the PA high enough. They thing its a volumn knob for power. When you dont load up enough, the volts go high right away and arc trans out

C

[KM1H]

There is a huge difference between an 8 pole xtal filter with steep skirts at 300 hz and a plain old audio filter with a 3dB/octave or slightly more roll off. That will still give sufficient bass at 150 Hz and reduce the useless stuff below that. Rolling off slowly above 3500 or so and sharply above 5 kHz doesnt lose anything of consequence and reduces the TX BW as well as IM products.

Yes, I agree that voltage is the killer and wont be exceeding those specs.

You can easily hear the effect of audio iron in a receiver. Replace the OEM lightweight with one rated at 2X the power or more and hear the low end improvement. A single 6AQ5/6V6 really can sound good.

Carl
KM1H