Do output transformers "saturate"

[pdf64]

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I'm not so sure putting a Vibrolux OT in a Super is such a good idea.
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I’d have thought that too, but Fender were seemingly happy to mix and match a beefy, higher voltage power supply with the little OT, eg AA1069 Bandmaster Reverb and AB763 Vibroverb use the Super Reverb spec 125P5D mains transformer with the Vibrolux spec 125A6A output transformer.


https://el34world.com/charts/Schemat...b763_schem.pdf
https://el34world.com/charts/Schemat..._schematic.pdf
https://el34world.com/charts/Schemat...1069_schem.pdf
https://el34world.com/charts/Schemat...a763_schem.pdf

[Chuck H]

Also, Fender used the 125p1b in the Princeton Reverb with four preamp tubes and a pair of 6v6's. And also the AA764 Champ with only one preamp tube and one 6v6 ?!? Not that this is an over voltage issue but the current demands are very different. I didn't measure the filaments on the AA764 Champ I had on the bench a few years ago but, with a hugely over rated filament winding capacity and a 117V primary spec I expect they were probably high. I may have access to that particular amp since it's local and thinking on it now I should probably correct that for the health of the tubes.

[Mike K]

Well, despite that Mercury Magnetics does not deliberately rust their transformers themselves, or that oxide layer of a lamination isn't exactly the same thing as rust, so the article doesn't actually say anything, the name Mercury Magnetics and their transformers has now been imprinted to your subcobsciousness. Mission of subliminal marketing achieved.

Funny.


But you know what got them in my subconscious? I heard an amp that sounded better than anything I'd ever heard. When I found out more about it I found out they were using that brand of transformers.


Maybe that other horsehockey works for some people, but give me something that sounds amazing and I'm a believer.


WTBS I will 100% say, with confidence, just putting a MM on your amp will not always make it the best thing you've ever heard. Even though I've had good success with them, they were instances they weren't always my cup of tea. If you can't accept that, or that from any transformer winder, then you're cutting yourself short.

[Mike K]

I’d have thought that too, but Fender were seemingly happy to mix and match a beefy, higher voltage power supply with the little OT, eg AA1069 Bandmaster Reverb and AB763 Vibroverb use the Super Reverb spec 125P5D mains transformer with the Vibrolux spec 125A6A output transformer.


https://el34world.com/charts/Schemat...b763_schem.pdf
https://el34world.com/charts/Schemat..._schematic.pdf
https://el34world.com/charts/Schemat...1069_schem.pdf
https://el34world.com/charts/Schemat...a763_schem.pdf

5C1, 5F2A both use a very similar (if not the same spec by most modern reproduction) PTs and use different size OTs. I assumed it was due to the tone control on the Princetons and the fact that most had 10" speakers, although at some point, and I'm not sure on the details of this, Fender had 8" speakers in those.



Again, the original article was claiming they were limiting bass power via the OT to preserve speakers. To me that makes sense as the average current flow would be higher for the lower frequencies, and thus cause heat issues if the voice coil of the speaker was the limiting factor. But it doesn't necessarily make sense that they are saturating - we should look at the specs and see if the primary inductance was limiting bass for all operation. Although as Tim said, we'd have some dynamic effect of bass cut as power increased, further protecting the speaker at high power output.

[35L6]

Helmholtz , you are right about gain , I was actually thinking about max power . The highs and bass resonance will run out of headroom before the mids . If the load line was above the knee the mids woul d run out first . I was looking for an explanation for a difference in tone between 2 OTs at full power without saturation . Of course there would be heavy clipping at this point so I don't know how audible it would be . For those of you that measure a lot of OTs , how close to rated impeadance are they ? In other words do 8k OTs all measure at 8k or is there some spread ?

[g1]

I’d have thought that too, but Fender were seemingly happy to mix and match a beefy, higher voltage power supply with the little OT, eg AA1069 Bandmaster Reverb and AB763 Vibroverb use the Super Reverb spec 125P5D mains transformer with the Vibrolux spec 125A6A output transformer.

Ok so it should be a safe combination.
So I'll rephrase my question about the comments made by Hunter and Baier.
If you wanted to get the 'creamy compression' of a Vibrolux from a Super, and see that the Vibrolux uses a smaller OT, and lower voltage PT; which transformer change would you think more likely to get you there?

[Mike K]

Ok so it should be a safe combination.
So I'll rephrase my question about the comments made by Hunter and Baier.
If you wanted to get the 'creamy compression' of a Vibrolux from a Super, and see that the Vibrolux uses a smaller OT, and lower voltage PT; which transformer change would you think more likely to get you there?

Personally I would always opt for low voltage, high current PT, the large OT and modify the preamp to give me the bass and presence I desired. But I'm not a Fender purist married to old schematics...

[Helmholtz]

If you wanted to get the 'creamy compression' of a Vibrolux from a Super, and see that the Vibrolux uses a smaller OT, and lower voltage PT; which transformer change would you think more likely to get you there?

I think the main reason for power stage compression (apart from clipping ) is yielding screen voltage and to some lesser extent the sagging of other supply voltages.
So I vote for the PT.

Can't see how an OT could cause compression.

[Helmholtz]

The lower -3dB corner frequency of an OT is the frequency where the inductive impedance (reactance) of the OT equals the reflected load, i.e. when Ra = 2*pi*f*Lp.
With single ended amps Ra is the reflected load and Lp is the primary inductance.
With PP class (A)B amps Ra = Raa/4 and Lp is the inductance of one half-primary.
The higher Ra the more L is needed for good bass performance.

The tricky part is using the „correct“ inductance, as this is not a fixed number.
An LCR meter might read something like 2H, indicating a somewhat poor bass response.
As the test voltage of typical LCRs is typically below 1Vrms, this corresponds to an output in the mW range.

At voltages corresponding to a medium output, L might rise to a maximum of say 40H and especially with smaller OTs will significantly drop towards full output level.

Using a 50Hz voltage of 230Vrms across one half-primary (corresponding to an output of 50W) I measured an average inductance of 25H with Fender BM and SR OTs.
Vibrolux OTs measured around 8H.

With an Ra of 1050 Ohm, this means a corner frequency of 21Hz for a Vibrolux OT and around 7Hz for the BM/SR OTs.

I don't think a speaker will notice the difference in a guitar amp .

Draw your own conclusions about the credibility of some people.
Often the obvious makes much more sense: Leo used smaller transformers as well as less or lower power speakers for cheaper and lower power amps.

[Mike K]

Interesting measurements, but it seems there is more to this. Apparently the OTs in question are 28W, not 40W. So indeed they are probably different than what you are measuring, unless you happen to have an original 5E7 Tweed Bandmaster 3x10 with a Triad 1848 which in use ran 3 x 8R in parallel into the "4 ohm" tap. I have no idea what that gave for the primary. I've seen pictures, the OT is physically very small looking.







And the information that was iterated by Hunter and Baier is not from either. Apparently it is contained in "The Tube Amp Book" by Aspen Pittman. We'd have to have that book to see where the actual source could be traced.

[g1]

And the information that was iterated by Hunter and Baier is not from either. Apparently it is contained in "The Tube Amp Book" by Aspen Pittman. We'd have to have that book to see where the actual source could be traced.

Well that makes more sense. I had wondered whether they might have been reading Gerald Weber.

Once again, I have no beef with the idea that a smaller OT may reduce the bass response, but am irked that they all blatantly disregarded the lower power supply capability (when they compared the Vibrolux to Super).
As it is the power supply that sets the limit on the power output.

[catalin gramada]

A response of a Partridge 100w who get in partial saturation at 40cps no more than 110w and a Hammond in same circuit 130w at 40 cps.
Old tests I did for a project, not remember the details ...but at 400cps remember get same output.both 1.8-1.9k or so...and out of rust. I still remember the Partrige get a clean sine till 80w around.at 40cps.then start to get in saturation. But don't think is a practical inconvenience: who play at 40cps dimmed at max output ? At 100cps no sign of saturation anymore for its nominal output.

[Mike K]

Well that makes more sense. I had wondered whether they might have been reading Gerald Weber.

Once again, I have no beef with the idea that a smaller OT may reduce the bass response, but am irked that they all blatantly disregarded the lower power supply capability (when they compared the Vibrolux to Super).
As it is the power supply that sets the limit on the power output.

Had not heard of him (surprisingly).


I think you're 100% right. Power supply voltage = headroom and power output. If you want an amp that distorts more easily, then that's an obvious place to change.


I think when we look at the Tweed BM and Pro, I'm betting that choke before plate (not a choke input as there is a cap before) and the fact that the screens and plate are tied to the same node has a big impact on the "creamy" distortion and compression. But the OT is dinky...



The funny thing I found is I was on the Victoria website looking at their clones of these amps. They rate them at 28 watts. I did the calcs based on the B+ but it's hard to say what they'd actually produce given the choke before the plates. I'm guessing that is going to drop a massive amount of voltage when trying to go from an idle of 90mA to a full signal of 300+mA!

[Helmholtz]

From the Baier article: "Now the 40 watts of low end that the tubes are delivering will never get to the speaker because it’s saturating in a smaller OT."
As well as: " This basic transformer architecture has remained unchanged to this day. The Blackface Vibrolux, Vibroverb, and Pro Reverb all share this smaller OT, and it is a key ingredient to the vibe of these amps. (The part that Fender produces today is literally identical to the 1950s specifications)."

^^The upper statement referring to BF amps is wrong.

The nominal output of a Vibrolux Rev. is 35W and that's exactly what my own amp measures before clipping.

My measurements show that this transformer could even handle 50W power input at 50Hz without significant drop in relative output.
It might overheat if that power is sustained, though.

The main difference to the SR OT is transformer efficiency/losses.
So the smaller OT might deliver 4W less to the load.

The OT did show signs of beginning saturation at my extreme test conditions, but that doesn't limit signal amplitude.

[Mike K]

A response of a Partridge 100w who get in partial saturation at 40cps no more than 110w and a Hammond in same circuit 130w at 40 cps.
Old tests I did for a project, not remember the details ...but at 400cps remember get same output.both 1.8-1.9k or so...and out of rust. I still remember the Partrige get a clean sine till 80w around.at 40cps.then start to get in saturation. But don't think is a practical inconvenience: who play at 40cps dimmed at max output ? At 100cps no sign of saturation anymore for its nominal output.

Interesting. Yeah 40 Hz is not of much use to guitar players. And even so with a 100W amp you're typically amplifying the high and mids much more than the low end. For a bass player, maybe of interest. That would certainly cause some (unpleasant) distortion on the low E or B strings.


I did a little reading last night and it seems the high end is probably actually the more interesting part for guitar amps. A lot of testimonials talk about things I've heard like "blurriness" or "fizz" on the highs. Less note separation, etc. Almost everyone reports a stronger and clearer low end as well on "better" iron.

So what I read was the primary inductance was largely academic and more is always better (hence maybe corroborating what Merc told me is that they try to maximize this for every design). That is not to disagree with what Helmholtz said, but agree, that it changes with load and our ears can be sensitive enough to hear fairly minor changes in the frequency response.

I watched a video that showed some square wave tests for high fi audio output transformers and they were clearly distorting at 10kHz. And these were apparently very good designs. That's probably on the optimistic end for a guitar tube amp, but a tiny bit does happen there. Perhaps this "blur" or "fizz" are results of these type of square wave distortions at more realistic guitar frequencies e.g. 5-7k.