output transformer experiment findings

[ALBATROS1234]

so i am sure most of you will cringe at the least ,some will cuss and stanp their feet but i did a what if experiment.i was playing around with my new build and decided to try another ot i have the was in an old radio to see what it sounds like. very similar, not bad was the finding. i also had a transformer which i am fairly certain is an ot and tryed that. pretty much the same result,decent tone slight volume change slight tone change. so i start thinking in my little pea brain, i wonder what is really the difference between an output trans and a normal step down type. i test dc resistance on my 3 ot transformers as well as some step down types i have. i found one which was in the ballpark, an old(1950s/1960s) step down transformer with either 120v or 277v primary and a 24v secondary. well you know where this is going. the shocking thing.....it works and even sounds good, virtually the same as my others i just tested.granted this is just a simple 2 tube single ended with a sub 300vdc feed but it actually works and sounds as well as the one i bought. i know i know you guys have just lost what little respect you were begining to start to consider having for me but i had to. and even after leaving it on and playing on it for an hour it neither got warm nor waned in performance.

questions, i have gone over it in my head but i cant figure any way this could be damaging to my tubes or components and i do know about air gaps and impedance , eddy currents ,core saturation, and hysteresis , but i must ask those more educated than myself on the topic of tube amps, can i do any damage to anything? to me it seems if it works it works and my feelings are cool i have another ot to play with.

[shooter]

 

can i do any damage to anything

we can, and probably have

glad you're running, If all your idle DC voltage checks fall in the good section of tube charts, it probably works.
save up your pocket change and look into multi secondary taped PT, Hammond or edcor, forgot.

PA OT impedance can be simplified for me:), pick one, say 5k, your plate volts is 500vdc, I=500/5000
.1A/500 ohms(Rk)=50vdc(Vk), your bias for SE, (you'll have to ask a PP guy for thier math) .
 find a tube that likes that and you are cooking with gas

[trobbins]

Some quartz halogen downlight transformers have dual 120V primaries, which then allow them to be used as a PP output transformer.  That type of transformer has also been used for SE output stages, as a 50W rating means they can cope with a fair bit of primary DC current and still manage a decent AC swing.  There have been a few builds over on the AGGH website if you search:
http://www.guitargear.net.au/discussion/index.php?board=8.0

For diy guitar amps they are a great way to re-cycle transformers being thrown out.  Frequency response is fine, and the primary winding is certainly designed for high voltage use and sufficient inductance to support high signal level at 50-60Hz.

[ALBATROS1234]

I was just surprised that it sounded so good. I figured it would make noise because it is in fact designed to do virtually the same thing as a proper output transformer( substantially lower voltage)and the dc resistance measurements were quite close. What damage could be done, output tube? In what way? I am just curious. I am sure the more powerful the amp the more likely to damage stuff. I am only messing with 6 and a half watts. I get that without a load how you may damage the it. But the volume and tone is virtually the same as with the other I bought as well as the one I pulled out if a 1960s radio and the other one I posted about a couple weeks ago which is likely another radio type ot.

[PRR]

A self-bias amp is hard to hurt the tubes.

> designed to do virtually the same thing as a proper output transformer

Similar idea.

A power transformer must work good at 50/60Hz and (for rectification) several harmonics. It has a steady power input and heat may be a problem.

An audio power transformer works down to similar frequency and up to much higher frequency.

With many simple winding techniques, the power transformer is likely to be "flat" to the top of the audio band, >5KHz. With other winding techniques (especially dual-winding) it may fall badly above 500Hz.

The real limit is voltage divided by frequency. If you go past this, there's trouble. A power winding may be good for, say, 100V at 50Hz. This will also be good for 200V at 100Hz, perhaps 400V at 200Hz (though insulation breakdown may limit re-rating this way).

If you apply too high voltage at too low frequency, the core saturates. Distortion gets very high. Inductance almost vanishes.

In power transformers, distortion is a minor thing. The very low impedance of an Electric Company sucks-up the distortion. A few small transformers on a power line cause no real trouble. (The big transformers on the street are not worked too close to saturation; other factors tend to tune-out the distorted waveform.)

In audio from expensive (for the Watt) high-impedance tubes, distortion is a big deal. A "100V 50Hz" winding probably should not be worked over 30V-40V on 50hz audio or the deep bass will be colored with harmonics. (Which is not necessarily bad for small bass amps which can't really pump 50Hz air: put some hair on it and the ear will re-create 50Hz from the overtones.)

OTOH, audio is not steady 230V, but more soft than loud. Frequently the heat in an audio transformer is totally insignificant.

A more significant hassle is Turns Ratio. Tubes will "work" mis-loaded but Watts output will be down. A 6V6 biased 300V 40mA will do well with a 7K load. Put the 2K OT from a 110V radio on it, power is less than half. (Which is not an amazing change of loudness.) Use a 400r OT from a transistor radio, power output will be way down.

> 120v or 277v primary and a 24v secondary

120:24 is 5:1 voltage and 25:1 impedance. 8r load reflects as 200r, which is much too low to be happy with the usual audio tubes.

Are you USA? 277:24 is 15:1 voltage and 133:1 impedance. 8r load reflects as 1,066r. This superficially looks excellent for a 25L6/6Y6 at 110V and 100mA. At 110V the peak plate swing is say 80V so the RMS is 50-60V, 1/5th of the 60Hz rating, so THD may be low. As could be expected, since I think this may be a 40VA core and you would only be asking 2Watts of audio through it. In SE, the DC in the core puts it at a disadvantage, but 20:1 over-size covers that well.

SE and self-bias amps tend to not go crazy with bad loads. They idle at all the power they can stand. They divert some of that to a load for output. If the load is bad, they are in no worse shape than just idling. Big push-pull fix-bias amps CAN get in trouble, because they will suck more current to cover the load, and if bad, they can bust a gut trying. Less of an issue because few handy power transformers are remotely suitable for the very high voltage primary needed. A 600V 100W 6550 amp has 800V AC across the primary at full roar, and we would expect a 2400V AC rating to keep THD low. They don't let me play with 2.4KV transformers.

[ALBATROS1234]

very infomative sir thanks, i am in the usa, new orleans to be precise and i was using the 277:24v winding so maybe this is why due to your explanation it seems to work ok. i figured that the only reason it was operating decently was low overall power.