Some questions about output transformers...

[hesamadman]

Since most of the literature I have read focus on pre amp circuitry, I am lacking in the power amp side of things. Im going to try and word these questions without being too confusing.


It is my understanding that the output tubes are setup throughout the circuitry to have a certain amount of wattage. So lets say that the output tubes are producing 20 watts. Where does the wattage of the OT come to play? If an OT is a 20 watt OT, does that mean that it is only rated for that many watts? And the actual amount of watts are directly related to the tubes?


Maybe a good scenario would be for me to ask this. If we had a schematic of an amplifier that was 30 watts. Like an AC30. What would be the result of using a 15 watt OT? What would be the result of using a 50 watt OT?


Sorry of these questions seem silly. Trying to wrap my head around this.

[Paul1453]

Well I can tell you from experience, when you put more Watts into an OT than it is rated for you will get smoke and a dead OT and maybe damage other amp components.   

I don't think you can hurt an OT by not giving it enough Watts, but you may not like the sound quality it produces at that level.   

[hesamadman]

Well I can tell you from experience, when you put more Watts into an OT than it is rated for you will get smoke and a dead OT and maybe damage other amp components.   

I don't think you can hurt an OT by not giving it enough Watts, but you may not like the sound quality it produces at that level.   

Thanks for that info Paul. Im assuming the wattage is just a wattage rating....

[lord preset]

Large mismatches can lead to smoke but if an output transformer is slightly undersized it will saturate/compress as it reaches its limit.  If you look at a Bandmaster OT vs a Super Reverb OT you will see that the SR is much bigger despite being roughly the same wattage on paper.  And the SR has considerably more headroom. The AC30 OT is an interesting case.  It is nominally a 30 watt OT but it is much bigger than say a Hot Rod Deluxe OT which is supposed to be 40 watts.  I used the Hot Rod OT in an AC30 circuit and it worked fine but was not as loud as a real AC30.  I guess the point is that for output transformers size matters and ratings can be a bit fluid.  You can assume a 30 watt rated  OT will not melt in a 30 watt design.  A 15 watt might or might not depending on how conservatively it was designed.

[hesamadman]

I used the Hot Rod OT in an AC30 circuit and it worked fine

On a side note I used a JTM30 OT in an AC30. Had some extremely beefy gain but it had WAAAY to much bass. I could not get rid of it. I even put 500pf CC everywhere in the preamp and it was still to bassy. Of course when I swapped the OT (while the 500p CC were still installed) it was VERY harsh. Tuned it back and got it sounding great with the correct OT.

[HotBluePlates]

... Where does the wattage of the OT come to play? If an OT is a 20 watt OT, does that mean that it is only rated for that many watts? ...

An output transformer's power rating (wattage) is the rated power through-put, at a specified bandwidth (frequency range from low to high), for a specific amount of distortion (caused by the transformer, by magnetization and ultimately saturation of the core).

A transformer is simply 2 or more coils of wire wrapped around a hunk of iron (the core). The wire is only so-thick, which sets the maximum amount of current which can flow through the coil before the wire melts (generally more than 500mA for the primary winding, several-A's for the secondary). The wire has insulation on it, as well as insulation between windings, which sets the maximum voltage which can be placed on the coils before punching through the insulation and shorting coil-to-coil, coil to core or coil-to-case. Most transformers are high-pot tested to withstand several-thousand volts, because inductive flyback during use could be that high.

If you just multiplied these maximum possible currents and voltages to get a "wattage rating" for the transformer, they would look like they can withstand 1,000w or more. So that isn't what sets the rating.

But you already know intuitively that transformers rated for more power are bigger and heavier. The bigger transformer has a bigger core. And the bigger core can accept more magnetizing energy to pass from primary to secondary, which also means more power throughput. Just compare a reverb transformer (0.4 pounds for 3.5w) to a 60w output transformer (8 pounds).

But bandwidth matters, mostly how low the transformer can go. Lower Hz means bigger and heavier core. That 60w OT above does full 60w down to 30Hz, and weighs 8lbs. Compare to Hammond's 50w Bassman replacement which does 50w only as low as 70Hz, and so only weighs 4lbs.

You can cram more than rated power through a given transformer, assuming the power supply & circuit will allow that to happen. You could use a 20w OT rated down to 30Hz and pass 50w through it. But when you put more-than-rated power through the transformer, it won't pass all that power at as-low a frequency, and sooner or later the core will saturate and add its own distortion. It would go as-low and with less distortion if the core were made bigger, which is exactly why the Hammond 60w OT linked above is double-weight of Hammond's own 50w Bassman replacement.

There is some upper limit for every core for how much magnetizing energy it can handle (power to be transferred from primary to secondary). As you approach and exceed that, the transformer doesn't pass any more power to the secondary, but just saturates and distorts more.

In general guitar amp OT's were smaller (less weight) than a same-power OT for full-range hi-fi because it was cheaper for the amp maker (smaller, lighter core) and because bass-shave and increased distortion were acceptable to the manufacturer's cost-performance equation.

[Paul1453]

Well I can tell you from experience, when you put more Watts into an OT than it is rated for you will get smoke and a dead OT and maybe damage other amp components.   

I don't think you can hurt an OT by not giving it enough Watts, but you may not like the sound quality it produces at that level.   

Some of the other guys have given you some better insight on OT usage.

I've only burned up 1 OT so far.  Thank God.
It was a little unknown Philco I paid less than $5 for including shipping.
I suspect that OT is 5W or less, it seems to work well with the 6AQ5s 4.5W output.
It definitely could not handle a single metal 6L6's output.   

What that taught me, was to be leery of giving OTs more watts than they are rated for.
If I had a 30W OT that I had paid $50 or more for I don't think I would be willing to risk putting that in a circuit designed for 50W.
As some of the others have pointed out, that 30W OT might work in a 50W circuit but I can't afford to take that chance.
I will only use an OT that is rated at equal or more Watts than my circuit is designed for from now on.

You can certainly do what you want with your OTs, but they can and will burn up if you give them more than they can handle.   

[jjasilli]

If an OT is a 20 watt OT, does that mean that it is only rated for that many watts? And the actual amount of watts are directly related to the tubes?
Yes. 
What we have is the Law of Conservation of Energy.  Watts are a measure of energy.  The vacuum tube sucks (draws) el energy from the wall outlet, (AC rectified to DC), & converts it to a useful boost of signal.  Because of the the Law of Conservation of Energy, if there are 20W on the primary side then there are 20W on the secondary side -- minus system losses but trannies are very efficient in this regard, so we simply consider them to be 100% efficient with the same Wattage on ea side.

But you must also keep Amperage, i.e. Current Draw, in mind.  W = V X I; I = W/V.  Given 20W & 400V on the primary side: 20/400 = 50mA. Given 20W & 280V on the primary side: 20/280 = 71mA.  So you need to know that the tranny is rated to handle the Current it will Draw in a particular application.

Maybe a good scenario would be for me to ask this. If we had a schematic of an amplifier that was 30 watts. Like an AC30. What would be the result of using a 15 watt OT? What would be the result of using a 50 watt OT?
This question covers 2 areas: operation & tone.

Good quality trannies, e.g., Hammonds, are considered capable of handling up to 50% more current than the spec sheet states.  If the tranny is under spec it may run warm, hot, even scorching hot.  But if it's all it's metal (no paper), this may be tolerable.  However at some operating point it will fry -- either catastrophically, or prematurely over time.


As the tranny's coils get hot, their resistance goes up.  This causes a voltage drop, which is good if you like Sag.  Small OT's tend to impress saturation or compression upon the signal.  Large OT's will not contribute sag, saturation or compression.  An extra large tranny will do no harm operationally, but is wasteful in terms of price, space & extra weight.

[Paul1453]

Maybe those Philcos weren't 5W after all. 
After working fine for a week with the 6AQ5, it appears I may have killed another OT. 
Luckily these were very inexpensive.

The little OTs primary resistance has gone from around 410 ohms to 82 K ohms.

It started up fine this morning, and I even had my low gain 12AU7 in the preamp.
I had run my guitars with humbuckers, and even my bass through it yesterday with no problems.
Tried a single coil strat copy on it this morning, should have been even less input signal but a different frequency range.

Turned up some gain and things got scratchy and output dropped to nearly nothing.
No smoke this time, but I'm pretty sure this OT is dead now too.
It now drops over 80% of the B+ before the output tube. 
The current didn't spike, and I still have a tiny bit of output from the speaker.
Now I'm guessing those Philco's were 1-2 Watt OTs and had been performing well above it's specs for over a week.

A word to the wise, match your OT's wattage to your circuit carefully.

[HotBluePlates]

I disagree with some of the perspectives provided.

I've tinkered amps since the mid-90's and have never had a burned transformer (though I do typically run things conservatively). Usually when a transformer does burn up, there is operator-error involved somewhere else.

Possible examples include:
Component failure or accidental short-circuit which removes output tube bias (massive plate current, usually killing a tube)
Short-circuit of B+ (usually blows a fuse, unless not installed or over-rated)
Rectifier failure or mis-wiring (may only kill components, but could burn up PT)

But let me tell about some notable errors I've made which did not damage a transformer:
- Put EL34's in an amp with a PT not rated for the heater current -- Insulation on heater wires melted instantly at switch-on, but PT unscathed.
- Goofed bias circuit in a 6V6 amp -- Normal ~22mA idle current rocketed up to 150mA before I could turn the power off; there was hum from the speaker but no damage to tubes or OT/PT.

Everyone should also realize that what Fender (for example) used as an OT in a 20w amp would be considered a ~5-8w OT by a hi-fi manufacturer. Aside from tuns ratios, the only difference between the old-school Fender OT and a modern hi-fi OT is core size/weight (and material in some cases).

[DummyLoad]

richard and i have melted down 3 edcor and 2 hammonds. all were SE OT's. one of them was after installed a 1A FA fuse.


depends mostly on the winding wire size. most OT can take several hundred mA for some time. exceed that time, winding get's hot, varnish melts, wire burns it two, catastrophic failure.


the OT above were 10W or 15W models that richard was running with 6L6GC very hot with around 475V B+. i have personally fried one reverb OT experimenting with MOSFETs.



--pete

[Paul1453]

Usually when a transformer does burn up, there is operator-error involved somewhere else.

Possible examples include:
Component failure or accidental short-circuit which removes output tube bias (massive plate current, usually killing a tube)
Short-circuit of B+ (usually blows a fuse, unless not installed or over-rated)
Rectifier failure or mis-wiring (may only kill components, but could burn up PT)

Operator error is a definite possibility in my case, but could it possibly have something to do with single coil pickups too?

I have been messing around with this circuit extensively over the past week.
Swapping in different preamp tubes, 12AX7, 12AT7, 12AU7, even the Hammond 1/2 AU7 1/2 AX7 tube.
No problems to report with any of the changes.
The circuit functioned properly at startup with the single coil guitar and the mixed gain Hammond tube.
Swapped to the 12AU7 and it was still functioning well at low gain.
Voltage at 250VDC Current at 30mA.
No spike in current or drop in voltage noted when problem started.
I had the single coil guitar sounding as I went from like 2 on gain to 5.
That is when the problem began.  Like I said no current spike or voltage drop noted, and no smoke.
I was watching closely as I turned the gain up.
The problems you mentioned should have shown in voltage or current readings, correct?
Tried a different pre tube and then the output tube no change.
Checked circuit voltages and found the B+ no longer making it through the OT like it should.
Checked OT's primary resistance and found it had increased greatly.
Maybe just a very old OT that finally gave up?

I'd be interested to hear your ideas about what might have gone wrong, HBP.   

OK, thinking more on the single coil question. 
I forgot that I had been using my Ibanez JEM which has a single coil pickup in the center position exclusively until I broke a string.
No issues then, and shooter has verified using different freqs. with no adverse effects on OT.
So I think my idea of high frequencies from a single coil pickup affecting the OT is out the window now.
It just seemed strange that I had used Humbuckers and even a bass on this circuit with no problems, both up to full gain.
Pull out my son's Jay Turser single coil junker and the OT dies.
Maybe just a very old OT that finally gave up, is seeming more likely to me.
Of course it could still be operator error.   

[shooter]

have something to do with single coil pickups

I have - on purpose - inputted a square wave twice spec, other than a very annoying sound the tubes/trannies were fine. Also did freq sweeps from 20 to 20k as sine, triangle and square, no problem.  This was all done on a prototype PSE EL84, set up with proper *DC specs*  since I'm lacking in fixed bias, bets are off for that.

[jjasilli]

I too had a tube short-out internally in my 100W PA amp which had the trannies scorching hot, presumably for hours.  Smelled pretty bad too.  After shutting down the amp, it took overnight to cool down to warm to the touch.  To my amazement the trannies still work.  (I since installed fuses for the power tubes.)


My point above was that excessive heat could shorten the life of a tranny, even if it doesn't fail catastrophically.

[HotBluePlates]

My point above was that excessive heat could shorten the life of a tranny, even if it doesn't fail catastrophically.

That is a fair statement.

depends mostly on the winding wire size. most OT can take several hundred mA for some time. exceed that time, winding get's hot, varnish melts, wire burns it two, catastrophic failure.

Agreed.

... could it possibly have something to do with single coil pickups too? ...

I don't see how a (usually weaker) single-coil could be more dangerous to the OT than a humbucker.

...
Voltage at 250VDC Current at 30mA.
No spike in current or drop in voltage noted when problem started.
... and no smoke.
...
Checked circuit voltages and found the B+ no longer making it through the OT like it should.
Checked OT's primary resistance and found it had increased greatly.
Maybe just a very old OT that finally gave up? ...

Possibly just an OT with one foot in the grave & another on a banana peel...

Because you're using a regulated bench power supply, I'd expect voltage to stay rock-steady, especially if the failure is an open-circuit (no voltage to the output tube would be the OT going open-circuit). I'd expect that current (for the output tube, or total B+) would drop, since the open OT takes away you single biggest current draw for the B+.

I imagine the open could happen deep enough in the coil that smoke wouldn't be obvious. The only time I had an open transformer winding was in a PT which arrived to me that way; I found & repaired the break only a few turns into the affected coil. I will say I wasn't sure if I was gonna be successful with the repair, but figured the PT was dead anyway so I had nothing to lose by attempting the repair. It worked and that PT has held up for years since...

[Paul1453]

You are correct again, Sir.

The current did drop once the problem manifested.

B+ still on pin 6, but very low on pin 5 from the OT.

[Paul1453]

Killing little unknown old Philco OTs is not as bad if you learn something. 
Which I think I did.   

I had one more little unknown wattage OT to use with my SE 6AQ5 circuit.
The problem was this one is 3.6 K ohms load with 8 ohms for speakers.
The little 5" speaker I was using is only 4 ohms.  My 6AQ5 wants 5K load at 250V.
That one 4 ohm speaker was not going to work with this OT.

Then I saw 2 little 2.5" speakers in my junk pile.  2.5W and 3.2 ohms each.
I put the 3 speakers in series, with my biggest one 1st (don't know if that matters), and got 10 ohms speaker load.
10 x 450 = 4.5K through this OT.  I figured, close enough to the 5K the 6AQ5 wanted.
I tried it out and it seemed to work pretty well.

It changes my plans for converting this crappy little SS amp into a better sounding little 6AQ5 tube amp.
I mounted the little speakers pointing out the sides of the housing.  It would probably sound better with holes in the sides. 
But for now, they are mounted on the same wood support as the 5" speaker.
They are off the side walls about 1/2" not screwed directly on the sides of the housing.
Tried it out again, and I'm pretty happy with how it sounds.  A 12AX7 preamp is a bit too much for it this way.
A 12AU7 seems to work much better.  This OT is slightly smaller and will fit better in my limited housing space.

I think I learned a little bit more, about matching speakers/OTs/output tubes.
Did I get it right, or at least close enough this time?