Too much iron?

[JJGross]

Hi,
Is there such a thing (besides weight factor) as too much transformer, specifically power transformers? Obviously there would need to be a heavy dose of voltage attenuation if I'm trying to drop 500v down to 290v, along with all the cooling issues that would involve. That's not the 'too much' part I'm wondering about.

Reason I ask is that I have several old hi-fi and organ tranformers laying around just begging to go into new guitar amp builds. These things were all used in circuits with far more tubes than I'll ever put in a guitar amp. After all, the Hi-Fis also powered radios and the organs pushed whole tone banks with sometimes dozens of tubes in them. I'm sure the amperage available in some of these things is through the roof. IIRC though, trannys will only deliver what's being drawn from them 'downstream' so would that only really mean that these amps would have overhead to burn?

Any help or advice is appreciated, especially from other salvage-happy builders in the group.

Cheers,
- JJ

[Jana]

Big iron is good. I like big iron. Big iron makes big boom.

Nothing wrong with using iron that is over rated for the application, I do it a lot, not that I am an expert or anything. You won't get the bouncy, saggy, pogo stick feel of an old, underdesigned fender, but what you will get is a tight and solid feel. Some like it, some don't. Build an amp and try it, if you don't like it, well, nothing lost, just swap out the transformers.

[Ronsonic]

The only thing with the old organ amps is they fed heaters for like a zillion tubes so you may find the heater supply a bit high. Otherwise big iron good. I likes me some big iron.

Over in one tube hifi forum I was in we used to joke about the weight to watts ratio. I built one stereo amp that does about 8W per channel and weighs almost 90 pounds. This is considered light and suspiciously efficient in those circles. Transformer input, interstage transformer, choke loaded driver tubes, big chokes in the power supply filters. No electrolytic caps either.

[ampgeek]

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.

[jaysg]

have a mint Ampeg V-4 OT lying around. It's a 100 Watter intended to match up to 4 X 7027'ish tubes.

Any chance you can measure the primary impedance of that?

[ampgeek]

Jay,
I measure 108 ohm total and equal 54 ohm side-to-center.

Never tried to measure impedance before. Do you have a technique for that? Measure the voltage drop of a defined AC current maybe?

Dave O.

[Jana]

http://www.radioremembered.org/outimp.htm

[Structo]

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?

[Jana]

You won't get more power with a larger OT. You will probably get more *perceived* power because the low end will be tighter and clearer. But will it raise an amp from 30 watts to 50 watts? No.

[Analog Assassin]

Well, Cesar Diaz said he put 100-watt transformers in SRV's 50-watt amps (Super Reverb, among others) so that gives you an idea what it does. It gives that big bottom sound. I like that sound, I don't like mushy tones.

[ampgeek]

You might be able to squeeze out a few more watts with an "oversized" OT if you define max power as the point of distortion I think. I would be surprised if you could perceive the difference though.

Which...was sorta-kinda my question earlier in this thread. It is my understanding that at some threshold current you saturate the OT core at which point you distort. I was wondering if that phenomenon might be responsible for the characteristic JCM800 tone. In which case, I might not want to use the V4 OT in that build.

I see a couple of JCM800'ish suitable OTs at Weber IIRC. One for higher headroom and one for an earlier break-up. Any thoughts/knowledge on the differences between those two?

Conversely, any experience doing a JCM800 style circuit with an oversized OT?

Thanks for the link Jana! I am familiar with how to determine the ratio but I think that I considered the question verbatim as Jay doesn't request ratio. Probably should have asked myself why but...I am only an amateur!

I will borrow a variac from the lab next week and run it through it's paces and post what I learn.

Thanks again,
Dave O.

[Jana]

You can also do it with a signal generator. Another way is to use a filament transformer, either 6.3 volt or 12 volt. Make sure that you apply the voltage on the primary side though, if you put 12 volts on the secondary side it will step it up quite a bit! The main point is that you accurately measure the voltage on each side so that you can calculate the ratio.

I have experience with the plexi/jcm800 circuits. It really depends on what you want out of the amp. If you are going for the traditional plexi/early jcm 800 circuit... the low gain preamp and pushing the whole amp... an OT that is sized accordingly is the preferred way to go. Keep in mind though that the plexi OT is a bit beefy anyway when compared to a similar wattage amp such as a fender.

For a higher gain amp, an amp that has some overdrive going on in the preamp, I prefer a more beefy OT and more filtering in the power supply. Not necessarily to reduce hum and chances of motorboating, etc. but more so to give a more solid response from the amp. Don't know exactly how to explain it. The distortion is tighter, more defined, more solid in the low end without getting flabby/farty. I have been known to use a 100watt OT for a 50 watt amp on more than a few occasions. If the OT is a 100watter of, let's say, 2000 ohm primary with 16, 8, 4 ohm taps, well, it is also a 4000 ohm transformer with 32, 16 and 8 ohm taps. The 32 ohm tap isn't very useful, but the others are. This is going to be one solid transformer for a 50 watt amp!

I also paint my transformers before I use them, lol. But you guys knew that already, right?

[Ronsonic]

Jay,
I measure 108 ohm total and equal 54 ohm side-to-center.

Never tried to measure impedance before. Do you have a technique for that? Measure the voltage drop of a defined AC current maybe?

Dave O.

The impedance ratio is the voltage ratio squared (voltage ratio = turns ratio). With nothing connected to the primary or secondary apply a measured AC voltage to the secondary. You can use a sig gen or just a heater transformer. Yes that will step the primary voltage up a lot. Be careful. If you can't safely handle 100-200 volts of AC you've got no business messing with this stuff in the first place.

So apply, for example, a measured 7VAC to the secondary leads in question. Then measure the voltage at the primary, plate to plate for a push-pull tranny. Let's say you get 140VAC. That means a turns ratio (equals voltage ratio) of 20. Square that, you've got 400. Multiply by the nominal impedance of the secondary you were using, let's just say that was the 8 Ohm tap and you've got 3200 as your primary Z.

Yes, I picked values I could do in my head while typing. In the real world you'll want a calculator. We do it by applying a voltage to the secondary so that we get values large enough to easily measure accurately. I don't like applying wall current, directly or with a variac to the primary because of the huge current available. If something goes wrong it will be really bad without adding fuses and/or GFI, so it's easier to do it the other way. Using a sig gen is also possible if it throws enough voltage to measure well and easily. With that you have to be sure your voltmeter is not loading down the circuit at all.

[ampgeek]

No worries Ron! I have numerous amp builds/repairs and lots of high voltage engineering experience under my belt. Thanks for the concerns though.

The amp gig is a hobby and I don't have a lot of instrumentation in the garage (e.g., signal generators, other unique stuff). Just the nomal DVM's and a TV7A/U tube tester.

As I mentioned, I am very familiar with the ratio and how to measure it. But...that wasn't the question Jay asked. I took the question verbatim and "spun" off try to figure out how to measure the impedance of just the primary side (if there is such a thing!).

In the spirit of the topic, I am trying to decide whether or not to use an oversized OT in a JCM 800'ish build. And..based on what Jana offered, will probably give it whirl.

Any thoughts/experience to offer on that one?

Thanks again,
Dave O.

[Ronsonic]

No worries Ron! I have numerous amp builds/repairs and lots of high voltage engineering experience under my belt. Thanks for the concerns though.

The amp gig is a hobby and I don't have a lot of instrumentation in the garage (e.g., signal generators, other unique stuff). Just the nomal DVM's and a TV7A/U tube tester.

As I mentioned, I am very familiar with the ratio and how to measure it. But...that wasn't the question Jay asked. I took the question verbatim and "spun" off try to figure out how to measure the impedance of just the primary side (if there is such a thing!).

In the spirit of the topic, I am trying to decide whether or not to use an oversized OT in a JCM 800'ish build. And..based on what Jana offered, will probably give it whirl.

Any thoughts/experience to offer on that one?

Thanks again,
Dave O.

I'm at a bit of a loss, so let me repeat the parts I thought were important.

The impedance ratio is the turns ratio squared. The impedance ratio times the nominal load is the primary impedance.

This overwhelms every other factor. It makes sense if you think about it. The more perfectly the primary Z is a reflection of the load Z the more efficient the transformer is. To simplify yet another step, any primary load that is not reflected secondary load is a loss. You'll notice that output trannies are efficient, they don't get hot much - nothing like a power tranny anyway.

To quote you "try to figure out how to measure the impedance of just the primary side (if there is such a thing!)"

There is such a thing, but to calculate it is A: really damn complicated and B: doesn't tell us anything useful. It's like calculating the gear on a bicycle without having the wheel diameter.

Just as an example, In a push pull tranny the currents through each half of the primary attempt to cancel the reactance of the other. So what's the impedance of that?

The Radiotron Designer's Handbook, I believe it's chapter seven is the go to reference for all things in audio transformers.

Honest, the reflected load Z is enough. I've repurposed a lot of iron in quite a few builds. This works. Yes, if you're fudging a lot, connecting different taps to get a correct impedance, etc, it can go wrong. But if it's pretty close, it's pretty close.

I'd check the impedance match and if the thing is big enough, and the JCM800 had plenty big iron already, go with it. Output trannies really can hardly be too large.

Normally my first question for a situation like this is, what tube complement was it originally used with? Which tubes do you propose?