Structo wrote:Maybe you guys can clear something up that a friend asked me.
We were discussing output transformers and I was talking about matching the primary impedance with the type output that an amp has.
He stated that the power of an amp can be increased by using a different OT.
For example you have an amp with a 2x6L6 output.
It has a smallish OT that is rated at best around 40 watts.
He thinks if you put a bigger iron OT (one that has a higher power rating) that has the same primary impedance and the proper secondary taps to match to a speaker load, that it can increase the power output.
I didn't think so but couldn't back up what I thought with the math.
Isn't a higher power rated transformer one that simply has a larger gauge wire so it will handle the increased current and voltage (Power)?
OR is it true that you can actually increase the output power by using a bigger OT?
You can increase undistorted power if the transformer is the limiting factor and it sure can be. You do get the larger wire gauge with lower resistive losses, you also get more iron which can help transfer more power with lower hysteresis losses and distortion.
The iron is a part of the magnetic circuit. And more is, uh, more. Usually, you won't get anything significant, but it can make a difference. Now the original tranny may have been very well chosen for the desired sound and even if it looks feeble can actually deliver all the power the amp is really capable of. Way too much iron can lose some efficiency. I don't think that's a practical concern in most amps.
Anyway, yes you can get some more clean headroom. Good example to compare and contrast, Fender Bassman and Bandmaster. Almost identical power sections but with more iron in the Bassman for more bass and a touch more output before breaking up.
ampgeek wrote:Any general thoughts on oversized OT's?
I am itching to build a 50W JCM-800'ish (2204) amp and have a mint Ampeg V-4 OT lying around. It's a 100 Watter intended to match up to 4 X 7027'ish tubes.
I am clear on how to match speaker impedances of this OT to 2 X EL34's but not so clear on how, tonally, it might end up.
Don't some amp circuits' characteristic tones rely on a certain amount of OT "core saturation" (not sure if that is the right phenomenon)?
Might I not get, all else being equal, that characteristic JCM-800 tone with an oversized OT?
TIA,
Dave O.
I got caught up in the impedance calculating and didn't look back for this post.
You can use this output for 2xEL34 by reallocating the output taps, connecting a 4 ohm load to the 8 ohm taps, etc. That's such a huge lump of iron, it'll probably do fine. It would be even more awesome as a 2203, if you've got the power iron.
JCM800s had some pretty big iron. I don't think saturation was a part of the formula.
No need to be at a loss Ron! Like I keep saying, I took Jay's request to measure the primary impedance literally without, initially, asking myself why (which I also admit in a previous post). My bad!
Technically, we measure turns ratio and then calculate the rest. Done it a few times with rogue OT's, matched them up to correct speaker loads and understand it fully......well....from a practical perspective anyway. We don't, literally, measure impedance.
Good input on the original question. I greatly appreciate it.
I just joined this forum. Hi to Bob H, if you happen to see this.
Measuring primary impedance is not hard. But you can tell more about output transformers by feeding them a 1kHz square. If the transformer has insufficient primary inductance the flat of the output will have a slope indicating low end attenuation/distortion. If it has large overshoots on the leading and trailing edges, it has excessive leakage inductance. The ringing of the overshoots is connected with winding self-capacitance.
Its best to do this with the transformer in an amp as its normally used except with the signal injected after any tone controls, the feedback disconnected, and a resistive load.
wscrane wrote:I just joined this forum. Hi to Bob H, if you happen to see this.
Its best to do this with the transformer in an amp as its normally used except with the signal injected after any tone controls, the feedback disconnected, and a resistive load.
So what causes the abnormalities you are talking about? Is it age, usage, poor manufacture at birth, wear and tare, etc?
Thank You
The thing it comes down to more than anything else is suitability for a particular application. But of course some things are better than others. Just for us it's a little harder to pin down "better."
wscrane wrote:I just joined this forum. Hi to Bob H, if you happen to see this.....
Hola Amigo,
For those that don't know Mr. Crane, meet the finest amp Guru east of the Rio Grande... maybe west as well. YOMV of course, but this guy is a vacuum tube wizard that would have Gandolf green and gasping in envy. If that were not enough, he was "The Guitar Player" in the legendary Santa Fe Motor Kings... I can only say good things about him!
Welcome Woody, great to see you making contributions already!
Hey mates,
I did some testing on the Ampeg V4 OT that I mentioned earlier and determined that the 8 ohm secondary tap winding ratio is 20:1.
So....inductance ratio is 400:1 and, thus, an 8 ohm load would reflect 3.2K ohms back to the primary. The 4 and 2 ohm seconday taps ratio similarly.
Seems to make sense as a pair of 7027's at 450'ish volts in PP is "looking" for ~6K ohm so, I assume, a quad would like to see ~3K ohms.
The 7027 plates in the V4 are running closer to 550 V but...all of the numbers above seem to be in the same ballpark in spite of that.
Seems like a pair of EL-34's (ala JCM-800) would match pretty well to this OT without having to "re-allocate" the secondary windings. They like to see ~3.4K in that circuit IIRC.
wscrane wrote:Bob, thanks for the intro but you forgot to mention how I saved the world from the giant robot and invented the repelatron skyway et cetera.
Arggggggggggggghhhhh!. I was supposed to cover that part but obviously didn't get to it quickly enough.
Folks, it's true, every word of it. Woody is just too humble to tell you about the whole story. You should have seen it!
