Just a throwaway thread, power transformer as an OT?

printer2

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Three years and I finally got covid, over most of it and other than no energy to do anything, I AM BOARD! Sleep, answer forum posts, watch some TV, not much else. So I look over and see three smaller power transformers on the cabinet. They came out of console radios, powering SE amplifier sections with either 6V6 or 6K6's. The idle mind thinking, "The secondaries are all center tapped, they have 6V and 5V windings. Wonder what they can do as a OT? Not that I am short real OT's, I have picked up so many old radios that I have more OT's than I want types of amps. The smallest one is larget than the Hammond Princeton Reverb replacement. Will they have any high frequency response or not?

Now the question is in my head I will have to find out at some time. I did take a smaller Hammond transformer with a pair of 120V windings going to a lower voltage (can not remember what, maybe 9V?) and unwound the secondary to get the (hopefully) right ratio for some 12BK5's I have. Just came across a chassis I have with a single 12AX7 with a mosfet PI going to a pair of 6AQ5's into a 70V line transformer. Maybe I should compare the power and frequency response out between the current transformer, the modified Hammond, the vintage power transformer and a real output transformer? Sounds like a real interesting project. And more than a few hours work. Hope this tired feeling leaves me soon, not like I need time to think up more new projects. More than enough ideas as it is. But that darn curiosity does nag me...
 

Stevo Bambino

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I do not have any insights on the transformers, but I got the bat plague a year and a half ago. I had it pretty bad. The tired feeling did not go away for more than a year. My lungs still have not fully recovered. They keep filling up with fluid. I went from jogging 6 miles every day to maybe a mile on a good day. My sense of smell and especially taste have not fully returned yet either. I can taste salt and smell smoke and that is about it. At least I will be able to smell it if my house is on fire. Hopefully you did not get it that bad.
 

printer2

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I do not have any insights on the transformers, but I got the bat plague a year and a half ago. I had it pretty bad. The tired feeling did not go away for more than a year. My lungs still have not fully recovered. They keep filling up with fluid. I went from jogging 6 miles every day to maybe a mile on a good day. My sense of smell and especially taste have not fully returned yet either. I can taste salt and smell smoke and that is about it. At least I will be able to smell it if my house is on fire. Hopefully you did not get it that bad.
I think I might have gotten away lucky although while having it I was in pretty rough shape pain-wise. I have a permanent neurological pain condition that I baby all the time so it does not run away on me, because of it I could not hold a guitar for long until I made a 3 lb guitar. A normal Flu can wreck havoc on me because of it so I had been really careful and the reason it took me this long to get it. Hope you recover completely, I can tell you these long term health issues are no fun.
 

NickK_chugchug

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My understanding is the output transformers have an air gap in the core whereas power transformers don’t (or at least heavily distributed in the case of a torroid). The air gap reduces saturation which causes distortion.

If you have a well balanced push-pull amp you may be able to get away with it.

I did have a look at winding my own power and output toroidal transformers at one point.
 

alexhael

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I have used a filament transformer for an output transformer. It was a 115/230v to 8v 12va.
Used with a pair of 6V6's pp 9K to 16Ω speaker. Didn't get hot, sounded okay.
Like you, I just wondered if it would work. Haven't properly mounted it yet though.
 

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dougsta

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My understanding is the output transformers have an air gap in the core whereas power transformers don’t (or at least heavily distributed in the case of a torroid). The air gap reduces saturation which causes distortion.

If you have a well balanced push-pull amp you may be able to get away with it.

I did have a look at winding my own power and output toroidal transformers at one point.
Push pull OT's don't normally have an air gap as the DC is balanced but I have some PP OT pulled from vintage gear that do have an air gap, so what gives?
I'm thinking the OTs are dual purpose for SE and PP use, ie just SE OT's with the primary having a CT added for PP use. They are also wound as 1/2 primary - secondary - 1/2 primary, ie in 3 sections.
Power transformers are wound pri - sec in 2 sections, modern PT also use split bobbins for increased isolation with the sections wound side by side causing poorer regulation.
Tips to get better PP OT balance are to wind turns numbers in 5 sections as 1/4 pri - sec - 1/2 pri - sec - 1/4 pri.
The inner and outer 1/4 pri sections are connected in series and then another series connection the 1/2 pri section, this being the CT. This gives better balance as the length of wire is shortest for inner winds, the above method distributes the lengths of wire used across the winds better. Another trick is to use different gauges for the inner and outer winds, using heavier gauge wire on the outer wind to reduce resistance caused by a longer wire length for the same number of turns.

Doug
 

NickK_chugchug

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Push pull OT's don't normally have an air gap as the DC is balanced but I have some PP OT pulled from vintage gear that do have an air gap, so what gives?

That's could be down to production costs - if they could get and the same transformer core got used for both SE and PP but they laid more windings for the SE?

If the amp was a parafeed design, you could use non-gapped OT transformers as the cap prevents DC on single ended.
 

NickK_chugchug

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Three years and I finally got covid, over most of it and other than no energy to do anything, I AM BOARD!
Fingers crossed your recovery speeds up. I went down with C19 on the 19th Dec.. the same day as my video job interview.. by the next day I was flat on my back and when the guy called to say you got the job - I sounded like Barry White and warmed death. My wife had mild symptoms but didn't test positive and she was back on her feet in no time.. I was still coughing over 3 weeks later and it took a while to get back on my feet properly..
 

printer2

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The interleaved windings is done more to reduce leakage inductance and winding capacitance rather than having the same dc resistance from what I remember although I have never really dived into audio transformer construction. Too lazy to wind my own I guess.

If using a transformer in parafeed would you not need an inductor also? That would double your transformer mass I would think.
 

2L man

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I did install 230V/33V toroid power transformer to Princeton circuit which I have used for many kind overload testing. Using 8 ohm speaker the OT impedance comes only 390 ohms. Volume is low and very distorted after vol setting 2 or so. The loadline is almost upright and low volume sound is quite good. So power toroid which should not handle any DC current component produce at least some output power is SE circuit.

This is the loadline when anode sweep set to produce 10% 2nd harmonic distortion and power is only 0.22W

Max power estimate is 1.7W and 2nd harmonic distortion estimate is 24% and there OT does not be main distortion source and at least thats how distorted the max output sounds :) Possibly there comes lots of core saturation based distortion as well?

HiFi builders have used EI- and Toroid-core PT as OT but often they use very low impedances and low anode voltages. Power transformers do not function very well when anode voltage sweep comes much higher that the Mains Vpp rating is.
 

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printer2

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I did install 230V/33V toroid power transformer to Princeton circuit which I have used for many kind overload testing. Using 8 ohm speaker the OT impedance comes only 390 ohms. Volume is low and very distorted after vol setting 2 or so. The loadline is almost upright and low volume sound is quite good. So power toroid which should not handle any DC current component produce at least some output power is SE circuit.

This is the loadline when anode sweep set to produce 10% 2nd harmonic distortion and power is only 0.22W

Max power estimate is 1.7W and 2nd harmonic distortion estimate is 24% and there OT does not be main distortion source and at least thats how distorted the max output sounds :) Possibly there comes lots of core saturation based distortion as well?

HiFi builders have used EI- and Toroid-core PT as OT but often they use very low impedances and low anode voltages. Power transformers do not function very well when anode voltage sweep comes much higher that the Mains Vpp rating is.
I have been reading a little in Radiotron-Designer's Handbook 4th Ed, starting on p-206, in there it said with a low impedance driving the transformer that the THD would be low. So triode operation would be better for the hifi guys than us. Still under the weather so just skimming the chapter rather than having the information sink in. Not like I will be designing transformer.


OMG. Steve, got my smell back today. Something really rotten smelling under the week of piled up dishes.
 

dougsta

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I have been reading a little in Radiotron-Designer's Handbook 4th Ed, starting on p-206, in there it said with a low impedance driving the transformer that the THD would be low. So triode operation would be better for the hifi guys than us. Still under the weather so just skimming the chapter rather than having the information sink in. Not like I will be designing transformer.


OMG. Steve, got my smell back today. Something really rotten smelling under the week of piled up dishes.

Sense of smell was the worst for me.

Patrick Turner (RIP) did some great work to create his website, here is an internet archive due to a broken link of a page worth a read:


Lots more here on his site now preserved.

 

KahunaSapien

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Although i havent tried it myself the 55W iron core power transformers for 12v downlights work for single ended output guitar using the 12v winding as the secondary. They apparently have a higher leakage flux which kind of wirks like an air gap to prevent saturation. Also they can be sourced cheap secondhand when people upgrade to LED lights.

A 3-5 watt SE output stage (EL84?) is only 10% of the power rating and not going to saturate the core significantly

The sticking point may be the turns ratio between the 120 and 240 Vac versions
 

Bendyha

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Kevin O’Connor’s book, (the only one I own) The Ultimate Tone Vol. 5, devotes a chapter to dumpster projects, and covers quite well the use of P.T's as O.T's, along with some other neat ideas..
Yes, it is a fascinating "I wonder if it would work" project, but like you, I have a heap of old O.T's lying around, and so probably will never get around to actually trying it.
 
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