OT difference ?

[daz]

if you have 2 OT's, one a 4k 40 watt and the other a 3.2k 50 watt, all things being equal what would the differences be between them sonically. What would each of the 2 differences contribute to or take away from the amp's tone?

[MWJB]

Difficult to say as things are most unlikely to "be equal"...it will only take an hour for you to swap & see?

[daz]

I already have A/B'd two like that. But thats not going to tell me what i want to know because all things AREN'T equal since they are 2 different manufacturers. So i don't know what differences i can contribute to the difference in specs and what to the difference in manufacturers. The marstran is a copy built to to match an old marshall OT Brian dissected. thats what supposedly gives it the great tone some say about it. What i'm asking is IF that magic isn't there for me, i will have wasted $135 unless the higher wattage or higher primary resistance make some positive difference over what i now use. I'm simply trying to determine whether it's worth chancing $135 or not. If it at least helped the tone in some small way at least it won't become a doorstop, and i can't afford $135 for a doorstop. I'm also considering a magnetic components "classic tone" OT because it's well under 1/2 the price and yet recieves some very good praise. In fact, i read that 2 rock uses them !

[Steve Conner]

There is a problem with that "all other things equal", like others said, they're not and never will be. There are about 20 other design parameters in an OT and the designer can set them however he likes, but he only tells you two of them. The other 20 are his secret recipe that he charges you the hefty margin for.

So when you say some OT is "a 3.2k 50 watt one" then that tells you about 10% of what there is to know about it. You probably wouldn't understand the other 90% anyway, it's all voodoo that only old, grizzled transformer designers can perform. Choice of material for the core, wire gauge and fill factor, interleavings, and so on.

Having said that, you can probably count on the following...

A bigger, heavier one will have more iron and more copper. That means less power loss, and bigger bass. And a larger power rating usually means a bigger, heavier unit, so we could guess the 50 watt OT might make the amp a little louder and chunkier sounding.

The 3.2k impedance will get a different sound out of the power tubes compared to the 4k. I don't know any more about this, because every EL34 amp I've ever used had a 6.6k primary, and I always thought 3.2k was really low. It would be like running one of my 6.6k amps with an 8 ohm speaker in the 16 ohm jack.

Some people argue that no modern production OT can ever be a true clone of a vintage one, because they didn't have grain-oriented silicon steel back in those days, and even when it first came out, amp builders might not have used it because of the price premium. But now GOSS has taken over and you can't get transformer laminations made out of anything else! The guys over in the Pickup Makers forum agonize over what kind of steel alloy their pole screws are made of, and I bet the kind of steel alloy used in an OT core is just as important as that.

Paper bobbins don't sound any different to the modern nylon ones. Not in my 3dB Club anyway

[daz]

well, you see, what i was trying to accomplish by wording it the way i did was simply to keep replies from being all about how different materials and all that which is going to change things. In other words, when i said all things being equal what i meant is if i buy a OT thats say 3.2k and 50 watts, how are those two things going to affect the tone over an identical OT form the same manufacturer made with the same materials and design with the only changes between them whatever needs to be done to make one 4k/40winstead of 3.2k/50w. You pretty much got to that part in your replay eventually, but if you weren't sure how those things would sound different, no one here probably will. What i DID get from your post is a 50 watt should be a bit beefier sounding which is good, but that 3.2k should not sound as good, or at least thats what it seems you were saying.

[pdf64]

Steve, the OT in my Selmer T&B works out at about 3k5 primary - it's an old 60s one with a gz34 - but even so I'd have thought that your 70s (?) one would be similar?

daz, 6L6 type p-p amps tend to have a primary around 4k. So as the power rating is a bit lower at 40watts, I'd assume that the 4k/40watt OT was modelled on a 6L6 amp (like a very early Marshall or 5F6A).
There's a chart on p9 of
http://www.mif.pg.gda.pl/homepages/f...93/6/6L6GC.pdf
showing various parameters plotted against load impedance, which indicates that distortion is maybe 9% rather than 8%, for a load of +/-1k off optimal. Ok it's for 6L6 rather than el34, but the same principle (ie it doesn't make much difference) should apply.
But it doesn't say anything about frequency response - maybe trawl through the data sheets or the RCA book etc, to see if there are any words of wisdom.
Maybe the guy in a current thread, who's taken 2 tubes out of his 6 tube peavey, could advise whether that changed the tone any, as that would make a bigger difference to the working load impedance than the difference between 3k2 and 4k. As all else would be equal in that case.
Remember that the optimal load will vary with the operating conditions (eg cathode or fixed bias, plate voltage) and with what you're optimising for, eg lowest distortion, max power, or a compromise.

Is your amp still cathode bias? If so, 40W compared to 50W shouldn't make any difference in theory, as you'll only be putting out about 30W - as I understand it, the power rating is primarily based on the capacity of the magnetic material to resist saturation, which will only occur when pushed beyond max power / the lowest rated frequency. So a 40W OT should have plenty of headroom on a 30W amp at low E.
Which is why switched mode power supplies can get away with such small transformers - they're working at kHz, rather than line frequency.

[Steve Conner]

Steve, the OT in my Selmer T&B works out at about 3k5 primary - it's an old 60s one with a gz34 - but even so I'd have thought that your 70s (?) one would be similar?

Yeek! I could be wrong on that, I'll check it out and get back to you.

[MWJB]

3.5K (+/- a couple of hundred ohms) per tube primary Z is pretty typical for P-P fixed bias EL34 amps, sometimes even as low as 2.5K.

[dai h.]

around there seemed to be typical for a Marshall 50W 784-139 (EL34s). Clones seem to be around 3.4 - 3.5k. Vox AC50 might have been 3.5k too IIRC (might be misremembering).

[Wakculloch]

My T&B OT measured 5.4K.
Wattage ratings are usually governed by the steel and the point at which it saturates, so unless you are going to purposely underspecify to achieve some sort of effect, or overspecify to avoid saturation, there should be no difference.
I find it hard to believe anyone can hear a difference between 3.5 and 4K, as i cant hear much change when doubling the load, but look forward to being wrong.
From what i can recall when stripping Marshall OT's (only 3 plus a Laney), they were all wound to the same pattern as outlined in the April 55 issue of Wireless World, ie 1/4 pri, 1/2 sec, 1/2 pri, 1/2 sec, 1/4 pri, and i wouldn't be surprised if most British made OT's are wound this way.
I dont know how American ones are wound, but would be interested to find out.

[MWJB]

"I find it hard to believe anyone can hear a difference between 3.5 and 4K" Indeed, I think anyone would be hard pushed, if that was the ONLY difference.

"as i cant hear much change when doubling the load, but look forward to being wrong." Well, typically twice the rated load tends to sound a little darker/bassier, half the rated load brighter, maybe airier? Though there are usually other factors at work, different speakers for different loads etc, subtle differences even when the same speakers are wired differently (e.g. 4x8ohms wired for 8ohms sound different to the same 4x8ohms wired for 2ohms on matched secondaries).

However, these differences are largely really apparent in quick change over/AB testing...to determine the differences listening to an unknown amp would be very difficult indeed.

[Steve Conner]

From what i can recall when stripping Marshall OT's (only 3 plus a Laney), they were all wound to the same pattern as outlined in the April 55 issue of Wireless World, ie 1/4 pri, 1/2 sec, 1/2 pri, 1/2 sec, 1/4 pri, and i wouldn't be surprised if most British made OT's are wound this way.

Wireless World! :-D

My local uni library, last time I was in there, had bound volumes of it going back to the first issue in 1919.

[MWJB]

"they were all wound to the same pattern as outlined in the April 55 issue of Wireless World, ie 1/4 pri, 1/2 sec, 1/2 pri, 1/2 sec, 1/4 pri, and i wouldn't be surprised if most British made OT's are wound this way." Admittedly, I'm largely ignorant once you get under the end bells, but isn't this an "interleaved" winding? Tweed bassmans had interleaved OT windings, tolex Fenders & some tweeds (5E8A) were straight wound...or, so I heard...

[daz]

Wow, a ton of replies overnite, thanks. I guess it will be a crap shoot. Not sure i want to chance almost $150 on that. I'll have to think more on this or maybe try a magnetic components classictone OT at less than 1/2 the cost. Heck, if 2 rock uses them they gotta be pretty good i would think. by the way, any thoughts on sozo caps ? I was thinking about replacing my mallories with them if and when i order a OT.

[Steve Conner]

any thoughts on sozo caps ? I was thinking about replacing my mallories with them

If you want my $.02... Don't go there! Just believe that the only things that matter about a capacitor are its value, voltage rating, and whether it's electrolytic or not. It'll make your life a whole lot easier and save you money.

There have been some real good debates on capacitor tone here in the past, dig them up. My personal belief is that there is no difference, but it's the easiest thing in the world to convince yourself that there is one.