Primary impedance for OTs - output and tone

[bent765522]

All else being equal (and I guess it never is..), how does primary impedance on output transformers affect output level and tone?

I’m considering putting an oversized output transformer in my Princeton reverb build.. something like a 20w 6.6k deluxe reverb OT or oversized Princeton OT at around 8k (I’m looking for a little more bandwidth, not necessarily more volume or headroom although that would be ok).

Basically my question is how would a 6.6k vs an 8k or 8.5k OT differ in output / clean headroom / tone, assuming they were rated at the same wattage?

Thanks!!

[pdf64]

All else being equal, 6k6 would probably have a lower clean power output and higher level of 3rd harmonic at high power output.
For better bandwidth, a beefy lamination stack and interleaved windings are beneficial. So heavier and more expensive.

[R.G.]

This is one of those questions that seems like it should have a simple, straightforward answer - but doesn't, really.

Transformers all by themselves don't have impedances. They have ratios from primaries to secondaries and side effects, the inevitable inductances, resistances and capacitances that go with physical objects.

The impedance label on an OT only has meaning when you specify the load impedance it drives. A 6.6K to 8 ohm OT only presents the tubes a 6.6K load when there is actually 8 ohms on its secondary. Put a 16 ohm load on the secondary and the tubes see about 13.2K plate to plate. Or if it's a multi-tap output, putting 8 ohms on the 16 ohm tap causes the tubes to see about 3.3K. Changes in output, headroom and/or tone then result from the output tubes operating at a different loading.

A given pair of tubes will have a broad, flattish curve of power output versus loading. The tube manufacturers specified a plate to plate loading for the maximum power their tubes did. Output tubes also have a broad, flattish curve of distortion versus loading, with a minimum at a certain loading. The maximum power and minimum distortion happen at different loadings. A single pair of 6L6s for instance, will have a distortion minimum somewhere near 6.6K plate to plate, and a power output maximum near 4K to 4.4K. The hifi types want their 6L6s to run at 6.6K, guitar amp makers want to run them at 4K-ish. But the user can then affect things by odd-loading the OT to "show" the tubes a different impedance.

The watts rating of an OT is also subtle. Power transformer watts rating is based on how hot the insides of the windings get. For output transformers, the manufacturer of the OT will specify watts for the intended impedance ratio, which includes the expected tubes for the OT, at least roughly. They will then make at least some attempt at making sure it doesn't get too hot. OTs are generally not limited by heating as much as power transformers are.

Given all that, for the same tube set and output load, how changing from 6.6K to 8K-ish may be perceptible. Probably 8K to 8.5K will not. The change in output and clean headroom are intertwined with how the change in impedance happens to fit or not with the tubes. Likewise, the change in tone will depend on where the loading happens to hit the tubes' distortion maximum/minimum curve. Different speaker loading (i.e. double/half the nominal impedance on the secondary) will have a far bigger effect. And given the issues with human hearing being logarithmic to volume, changes in loudness for a 2:1 power change will be just barely perceptible.

What are your target tubes?

[R.G.]

Simul-typing!

All else being equal, 6k6 would probably have a lower clean power output and higher level of 3rd harmonic at high power output.
For better bandwidth, a beefy lamination stack and interleaved windings are beneficial. So heavier and more expensive.

That's a really good - and much shorter!! - answer.
I'm guessing that the OP has EL84s or similar, which usually want to do best-ish at 8K-ish. 6.6K would run them at lower power. I can't remember from the curves whether the second or third harmonic predominates as loading changes, so you're probably right about that. And yep - bigger, more stack, more copper, more labor in interleaving and more expensive are the way to get higher bandwidth.

[bent765522]

thanks for the replies! this is very informative and will take some time for me to consider all this. I guess I'll just need to try a couple of OTs and see how they sound.

R.G.I'm using 6v6 tubes in my Princeton Reverb. The volume level is good (I use this amp for studio work with the volume typically on 5 or 6) but I've always wanted just a little more punch in the low end (I use it with an external 1x12 cab @ 8 ohms). I might try a deluxe reverb OT and see how it sounds.. I also have an old blackface bandmaster OT 4k:4ohm which I understand would give me 8k:8ohm with the 6v6 tubes in my Princeton. I guess anywhere between 6.6k and 8.5k will work and I'll have to try a few and see how I like them.

thanks for the info! much appreciated.

[pdf64]

All of the OTs mentioned are kinda lofi, low spec type. Perhaps not quite as low spec as those in equivalent Gibson models of the same era.

[pdf64]

… I can't remember from the curves whether the second or third harmonic predominates as loading changes …

Me either!
I was just working on the basis of low order harmonics predominating and it being push pull, so there should be a fair degree of cancellation of even harmonics

[Stevem]

The limitation of the OPs amps frequency response is more from his speaker then any other part of the amp.

[Fischerman]

I recently built a 15W 2xEL84 Lightning clone using the alleged Matchless transformers built by Heyboer and sold on amppartsdirect. The 15W 2xEL84 OT has a primary impedance of 3800 ohms. Less than half of what most 2xEL84 amps are and about the same as their 30W amps (that OT is 3700 ohms).

The amp sounds bigger than any 2xEL84 amp I’ve ever heard. It rivaled my 30W Rocket.

Purely anecdotal but there’s one anecdotal data point for you.

[TUBEDUDE]

I recently built a 15W 2xEL84 Lightning clone using the alleged Matchless transformers built by Heyboer and sold on amppartsdirect. The 15W 2xEL84 OT has a primary impedance of 3800 ohms. Less than half of what most 2xEL84 amps are and about the same as their 30W amps (that OT is 3700 ohms).

The amp sounds bigger than any 2xEL84 amp I’ve ever heard. It rivaled my 30W Rocket.

Purely anecdotal but there’s one anecdotal data point for you.

I wonder where the knee falls in that load line. Hopefully the plate voltage is lowish.

[Fischerman]

I recently built a 15W 2xEL84 Lightning clone using the alleged Matchless transformers built by Heyboer and sold on amppartsdirect. The 15W 2xEL84 OT has a primary impedance of 3800 ohms. Less than half of what most 2xEL84 amps are and about the same as their 30W amps (that OT is 3700 ohms).

The amp sounds bigger than any 2xEL84 amp I’ve ever heard. It rivaled my 30W Rocket.

Purely anecdotal but there’s one anecdotal data point for you.

I wonder where the knee falls in that load line. Hopefully the plate voltage is lowish.

It’s just under 350v, I don’t consider that low for EL84s. But it runs the screens differently, from an extra lower voltage node instead of straight off the other side of the choke.

[Colossal]

I recently built a 15W 2xEL84 Lightning clone using the alleged Matchless transformers built by Heyboer and sold on amppartsdirect. The 15W 2xEL84 OT has a primary impedance of 3800 ohms. Less than half of what most 2xEL84 amps are and about the same as their 30W amps (that OT is 3700 ohms).

The amp sounds bigger than any 2xEL84 amp I’ve ever heard. It rivaled my 30W Rocket.

Purely anecdotal but there’s one anecdotal data point for you.

I wonder where the knee falls in that load line. Hopefully the plate voltage is lowish.

It’s just under 350v, I don’t consider that low for EL84s. But it runs the screens differently, from an extra lower voltage node instead of straight off the other side of the choke.

I've often wondered about this. I've seen two versions of a 2xEL84 Lightning schematic; one showing that the screens are choke fed, and the other, adopted by Trinity Amps, showing the choke in the usual position, but the screens fed after an additional 22k dropping resistor. If I recall correctly (and that may not be the case), the original amps did indeed have choke fed screens, so this would imply the screens had pretty high voltages. I remember reading a discussion on this topic on the Trinity forum. That 22k screen dropping resistor origin may have come from an erroneously documented schematic. But at 3800 for 2xEL84s, you'd think you simply have to drop the screen voltage. Maybe the originals used much more robust tubes....

With the OT primary being at 3800, this puts the load line well across the max dissipation curve.

[WhopperPlate]

IMG_4370.jpeg

[bepone]

3800 is not correct for 2 EL84s, it is kinda low even for one..
so speaker 8 ohm must be inserted to 4 ohm tap from the amp, then the load line will be ok (7600 ohms aa)

[Fischerman]

I wonder where the knee falls in that load line. Hopefully the plate voltage is lowish.

It’s just under 350v, I don’t consider that low for EL84s. But it runs the screens differently, from an extra lower voltage node instead of straight off the other side of the choke.

I've often wondered about this. I've seen two versions of a 2xEL84 Lightning schematic; one showing that the screens are choke fed, and the other, adopted by Trinity Amps, showing the choke in the usual position, but the screens fed after an additional 22k dropping resistor. If I recall correctly (and that may not be the case), the original amps did indeed have choke fed screens, so this would imply the screens had pretty high voltages. I remember reading a discussion on this topic on the Trinity forum. That 22k screen dropping resistor origin may have come from an erroneously documented schematic. But at 3800 for 2xEL84s, you'd think you simply have to drop the screen voltage. Maybe the originals used much more robust tubes....

With the OT primary being at 3800, this puts the load line well across the max dissipation curve.

FWIW, I ended up using the extra node for the screens but used a 10k dropper instead of 22k. That put the screen node at about 290vdc with 350vdc plates at idle. About 10.2vdc on the 120ohm cathode resistor.

I’m now building a 15W Liverpool in a sister chassis using a custom Fender Deluxe OT that is 6k6 at 2/4/8 ohm secondaries but my intention is to run it 4/8/16 ohm. This kind of puts it at the other end of the spectrum wrt OT primary impedance for 2xEL84s.