OT, the bigger the better? And which one to choose?

[Steve Conner]

Interleaving means mixing the primary together with the secondary. For instance, recipe #2 in RDH4 says:

Wind one-quarter of the primary.
Then stop and wind one-half of the secondary on top of it.
Wind the second quarter of the primary, bring out the center tap and then wind the third quarter of the primary.
Wind the remaining half of the secondary.
Finally, wind the last quarter of the primary.

This would be called "once interleaved" and has one-quarter the leakage inductance of the "straight wound" transformer used in the Tweed Deluxe, which would be recipe #1. The RDH4 has recipes for up to 16x interleaving.

DC imbalance could be caused by mismatched tubes, or it could be caused by an unbalanced PI where the two outputs clip with different duty cycles. I remember seeing one amp where, when you cranked it right up, the blue glow in one power tube would get brighter and the other would go out. Maybe it was broken, but it seemed to sound pretty good. I bet money that this happens to some extent in all classic amps.

Saturation works on the average DC imbalance, not instantaneous. The OT is designed to handle one half-cycle of signal at full power and the lowest operating frequency without saturating. So in that sense, yes, it "cancels through time".

A while ago I did an informal test of some guitar amps I had around, and none of them showed signs of OT saturation when running full power at 84Hz, low E on the guitar. Even at 42Hz, low E on the bass, the waveforms were still remarkably clean. Remember the classic 50W amps were probably intended to be played with a bass guitar plugged in one channel and a lead guitar in the other.

Cheap SE OTs are half saturated all the time. Putting a bigger OT in a SE amp can blow your socks off, however the extra bass risks trashing the speaker.

[frus]

Interleaving means mixing the primary together with the secondary. For instance, recipe #2 in RDH4 says:
DC imbalance could be caused by mismatched tubes, or it could be caused by an unbalanced PI where the two outputs clip with different duty cycles. I remember seeing one amp where, when you cranked it right up, the blue glow in one power tube would get brighter and the other would go out. Maybe it was broken, but it seemed to sound pretty good. I bet money that this happens to some extent in all classic amps.

Thanks for the great explanation!

That's what my amp does too. It's a 5F4 knockoff, so it has a concertina PI. And it's got 6L6's so their bias voltage is approximately equal to half the max-amplitude the PI is capable of delivering, i.e. the PI goes into overdrive at roughly the same time as power tubes. I scoped the power tubes output, and it shows that the 6L6 that gets signal from the PI plate clips with much wider duty cycle as you say, than the cathode PI output tube (which remains "clean" longer, so its waveform is narrower). You can distinctly hear the raspy distortion when this happens (at any frequency, I was using the sinusoidal signal generator). As you slam the PI with more signal, they even out, distortion gets less raspy, and then the cathode output takes over and has wider duty cycle (distortion increases again). The blue glow really goes out in one tube and increases in the other

Long tail PI does that too, but only when the PI starts overdriving and outputs become unbalanced (and that's when the power tubes are already in deep overdrive)

On the other hand, I had a pretty mismatched set of power tubes (over 20%) and noticed that bass becomes mushy just before the overdrive sets in. I scoped the output and then installed a trim pot instead of the plate PI resistor, and matched the outputs. The mushiness audibly decreased. I guess from your explanation, that the OT became saturated because of mismatched outputs.

Saturation works on the average DC imbalance, not instantaneous. The OT is designed to handle one half-cycle of signal at full power and the lowest operating frequency without saturating. So in that sense, yes, it "cancels through time".

That's what I meant, but I'm not a native english speaker, couldn't formulate it right, thanks!

[Austin]

Interesting read! Couple questions came to mind after reading this,

I realize that nobody uses class B for guitar amps, but isn't it true they could be made to perform well? And isn't it true you could get more power, say a 100 watts from a 50 watt sized output transformer this way? How does the high idle current in a class A pp stage affect the opt? Could it contribute to a loss of bass, (like on a se type where you get more bass response with less idle current) especially with the smaller opt? Thanks

[Alan0354]

Hey Steve, thanks for the reply. So which brand make OT with interleaving for guitar amp? Is Mercury Magnetics interleaving?

[JoeM]

Interesting read! Couple questions came to mind after reading this,

I realize that nobody uses class B for guitar amps, but isn't it true they could be made to perform well? And isn't it true you could get more power, say a 100 watts from a 50 watt sized output transformer this way? How does the high idle current in a class A pp stage affect the opt? Could it contribute to a loss of bass, (like on a se type where you get more bass response with less idle current) especially with the smaller opt? Thanks

According to some posts here a few years back, some Music Man amps are very close to Class B.

http://music-electronics-forum.com/t2478/

[frus]

Interesting read! Couple questions came to mind after reading this,

I realize that nobody uses class B for guitar amps, but isn't it true they could be made to perform well? And isn't it true you could get more power, say a 100 watts from a 50 watt sized output transformer this way? How does the high idle current in a class A pp stage affect the opt? Could it contribute to a loss of bass, (like on a se type where you get more bass response with less idle current) especially with the smaller opt? Thanks

If I understand correctly, you still have to have a certain amount of iron to reproduce low frequencies at a certain power level. So 50W transformer would have more distorted output if you slam it with 100W of the same (low) frequency.
Concerning class A, I don't think idle current makes a difference, as long as DC in both halves of the primary is in balance, as Steve kindly explained

[frus]

Hey Steve, thanks for the reply. So which brand make OT with interleaving for guitar amp? Is Mercury Magnetics interleaving?

I think their policy is not to make the "best" OT possible, but to make them the same as they were made in classic amps
So if you buy say a Partridge replica OT for Hiwatt, you'll pay more than double than you would for simple tweed Fender non-interleaved OT of the same power rating

[Alan0354]

If I understand correctly, you still have to have a certain amount of iron to reproduce low frequencies at a certain power level. So 50W transformer would have more distorted output if you slam it with 100W of the same (low) frequency.
Concerning class A, I don't think idle current makes a difference, as long as DC in both halves of the primary is in balance, as Steve kindly explained

Let me see whether I get this correct. DC in push pull doesn't matter because the current goes in at the center tap and comes out equally ( suppose to) from the two ends. The way the primary is wound is such that the DC current from center tap travel in opposite direction ( one side clockwise, the other side counter clockwise). So the magnetic field generated by the two side is equal and opposite. They cancel each other out.

Then Steve said saturation happen only if the DC current on both side are not balance( one side larger than the other side). So you have net magnetic flux through the coil. But in reality, the imbalance should be quite small compare to the capability of the OT, it should not have that big an effect. Is this true?

[Alan0354]

I think their policy is not to make the "best" OT possible, but to make them the same as they were made in classic amps
So if you buy say a Partridge replica OT for Hiwatt, you'll pay more than double than you would for simple tweed Fender non-interleaved OT of the same power rating

The bottom line is whether there is any improvement of sound quality for guitar amp using interleave vs the cheap winding the whole primary at one time?

[Steve Conner]

It's purely subjective, but I think many guitarists would feel that a highly interleaved OT let through too much high end for their taste, or at least a high end that was too extended and smooth.

If the guitar amp has a feedback loop around the OT, then a certain amount of interleaving is needed just to reduce the excess phase in the loop and keep it stable at high frequencies. The 5E3 has no feedback, so it can get away with a straight wound transformer.

A PP OT doesn't have much capability to resist DC imbalance, because it wasn't designed to handle it. That is one of the main selling points of PP over SE after all: the OT can be made smaller for the same output power and LF bandwidth because it doesn't have to handle DC. The improvement is achieved by stacking the core without an air gap, which allows the same inductance to be made with less iron and fewer turns. However, it's this very same air gap that allows the core to handle a DC bias.

Many classic OTs used a kind of compromise where the laminations were stacked in little blocks of the same orientation, rather than alternating with every one, to give more of a "distributed air gap". My 1960-vintage Crown stereo amp has this, and so did the old RS Universal OT and the Mercury "reissue" of it.

An extreme example the other way would be the new toroidal OTs, with an unbroken magnetic circuit of grain-oriented steel that saturates hard with only a few tens of milliamps unbalance. Hi-fi guys like them, but they never caught on for guitar amps, I wonder why?

[frus]

The bottom line is whether there is any improvement of sound quality for guitar amp using interleave vs the cheap winding the whole primary at one time?

well for instance I've built an 18W marshall but used Weber bigger, non-interleaved OT. It had too much bass and too little treble. Tone control didn't seem to do much. I then put a GDS/Heyboer OT which is wound like the old OT's that 18W used (and is a bit smaller), and it sounds just right

I'm sure you could change other components to achieve the same frequency response, but that would mean..... research *shudder*

[shocki]

In a German guitarplayer magazine I found a workshop where they were upgrading a Deluxe Reverb OT with something beefier. Actually they were using a Tweed Pro OT like this one Outputtransformer for: FenderŪ Tweed Super 5F4, Pro - Output Transformers

It seems the amp sounded just before but with more clean headroom (they wrote something about 28W) and tighter bass. I am not shure If they use 6V6 or 6L6 though. Don't have the arcticle here right now.

Especialy with the Deluxe Reverb the bass gets really farty at higher volumes (no matter what speaker) which I don't like. I think a upgrade could really help. I played a Valvetech Superlux which is a Deluxe Reverb Clone. Had a way bigger OT and didn't have this problem. Otherwise it sounded pretty similar to my Deluxe Reverb. I played both next to each other.

[JoeM]

In a German guitarplayer magazine I found a workshop where they were upgrading a Deluxe Reverb OT with something beefier. Actually they were using a Tweed Pro OT like this one Outputtransformer for: FenderŪ Tweed Super 5F4, Pro - Output Transformers

It seems the amp sounded just before but with more clean headroom (they wrote something about 28W) and tighter bass. I am not shure If they use 6V6 or 6L6 though. Don't have the arcticle here right now.

Especialy with the Deluxe Reverb the bass gets really farty at higher volumes (no matter what speaker) which I don't like. I think a upgrade could really help. I played a Valvetech Superlux which is a Deluxe Reverb Clone. Had a way bigger OT and didn't have this problem. Otherwise it sounded pretty similar to my Deluxe Reverb. I played both next to each other.

Unless other circuit components are the same, especially the amp's filtering, B+ voltages etc., how can you be sure the OT is the only thing contributing to the less farty bass? I'm not saying a larger OT doesnt help, but Fender's can get farty bass at high volumes. There's several threads on this, even on the old Ampage board.

[Alan0354]

Unless other circuit components are the same, especially the amp's filtering, B+ voltages etc., how can you be sure the OT is the only thing contributing to the less farty bass? I'm not saying a larger OT doesnt help, but Fender's can get farty bass at high volumes. There's several threads on this, even on the old Ampage board.

From the little experience with tube amp I have, I notice slight change in component make a difference, no one component stand out, just a matter of how much you want to make the component work into the circuit. I personally is born CHEAP, I don't believe in using expensive component unless it is truly justifiable. If there is a reasonable way to work around it, I would. That's the reason I take a lot of interest in OT as they are one expensive component. The other is the speaker, but that's another topic.

[JoeM]

OK, but there's a point where going larger doesn't help, and in some amps the original amp's character gets lost. I had put a Deluxe Reverb OT in my Princeton. Definitely a difference, but an improvement? Didn't sound like a Princeton, so the original OT went back in. So for some amps, the original (presumably smaller OT) gives the amp it's characteristic sound. (at least in part) We're not talking HiFi here.