1977 Fender Twin Transformers

[solderhead]

Now that I'm paying attention, I should have said Fig 6. That's what I get for posting early in the morning before coffee has kicked in.

What's interesting about this is that if you look at Trace D in the diagram, the second IMD curve, you'll see that H&K chose their 43% operating point that lies just to the right of the upward break of the knee in the IMD curve. That is to say, they tried to optimize a balance between good power output and the nadir of the IMD curve. Fender OTOH, went for power and you can see that curve D breaks sharply upwards disproportionately to the increase in power. This is what makes me think that Fender was more interested in power than low distortion numbers. Of course all of this attention to distortion levels in the power section goes right out of the window when you add cascading preamp distortion to the mix.

[solderhead]

What about the (rare) Vox amps that had the screens connected to taps on the OT ? I think they were produced in the late 60's or early 70's and had solid state preamps.

If we're thinking about the same Voxes, those would have been 1965-66 series of UL-7xx and UL-4xx amps, where the 7 prefix refers to a guitar amp and the 4 prefix refers to a bass amp in the model numbers. The trailing digits referred to the amp's power output in watts. Those amps came in 120W versions (4xKT88), 60W versions (2xKT88), and 30W versions (4xEL84). I think they all had essentially the same solid state preamp. Schematics are difficult to come by -- I'm not sure that they even exist today for the entire line of amps. All that I've ever found have been fragments.

The 4120/7120 120W high power amps appear to have used some sort of Distributed Load ("DL") topology (no idea where the taps are); the mid power 760 60W amp did not; the low power 430/730 30W amps did not.

It seems that Vox only used DL power sections for the 120W amps and the 60W and 30W amps never got that treatment. Fwiw I read at The Vox Showroom that the "UL" in those amps' names referred not to ultralinear performance, but instead due to having been UL certified for USA sales. I don't know if there's any truth in that, but the owner of The Vox Showroom reportedly has an old catalog that supports the claim that the "UL" nomenclature was used to indicate UL certification for US sales.
https://voxshowroom.com/uk/amp/710.html
I'm confused by that, as Vox (Jennings Musical Industries in the UK) never sold their amps into the US market, instead they licensed their name for American sales (to Thomas Organ Company in the US).

Although the UL-certification story doesn't make sense to me since Thomas Organ and not Jennings was using the "Vox" trademark in the US, I do recall reading that Vox Jennings was not happy with the solid state offerings by Thomas Organ and the ongoing licensing arrangement. Maybe someone else can fill me in on what Jennings' plans were for the USA trademark and USA sales at that time. Nonetheless, the schematics I've found do support the idea that in this line of amps "UL" does not refer to Ultra-Linear, as the 30W and 60W amps in the UL series didn't have screen taps on the OT. Only the 120W amps had screen taps. Maybe the "UL" really does refer to UL certification? I don't really know.

I would be very interested in hearing if anyone has data about the VOX DL operating points for the 120W amps. I've never seen one of those amps in real life, and my chances for doing so are almost zero as most of them were destroyed by VOX as defective returns due to a preamp schematic error that caused a catastrophic manufacturing error. Only a few amps survived and those are in the hands of collectors now. I've never seen one come up for a public offering.

There are some videos of those amps on YouTube. They have that characteristic Rubbery to Peppery sound.

[jbrrrrr]

There's nothing that I hate more than trying to google an arcane topic and finding that the relevant links provided are nothing more than links to my old forum posts form years past. That kind of search result implies that I know more about this than anyone else -- which is certainly NOT the case. The only thing more infuriating to me is when some AI bot references those posts and gets the facts completely ass-backwards and begins to propagate misinformation.

Maybe even as frustrating: realizing that all information beyond a certain point is lost in decades of old forum posts that are no longer accessible - attachments that were hosted on sites that aren't operating any longer, comments that have been retroactively overwritten by privacy software, etc. Based on the last couple of years of amp building research, it seems fair to say that a significant portion of the higher level research that has been done and shared is no longer available - and with MEF going down, *hopefully* not for good, but you never know - that's one less outlet of info housing the invaluable real life experiences that have been documented for people like me who just weren't around to comprehend the value of it when it was shared.

I generally have this very pessimistic view of the longevity of anything shared on the internet - I've become a kind of internet-hoarder, keeping extensive documentation on concepts I find interesting, saving schematics and images, forum posts, exporting whole pages to PDF, etc. As the only thing I think we really have available once a site goes down permanently is the Internet Web Archive/Wayback Machine which is pretty hit or miss in terms of what it actually archives, until there's a more reliable way of actually saving this information for everyone long-term, I'd encourage anyone to keep local copies of stuff they find useful.

At least if people have their own local versions of the things they've found interesting - well archived and searchable, maybe 5-10 years from now, we might be able to resurface some gem from @solderhead or others that answers a question comprehensively. I hate the doom and gloom of it but it's a real thing - there's portions of my early adult years on the internet that are just gone because I didn't realize MySpace wouldn't be around for 50+ years.

[solderhead]

I hate to think about how much information has become lost because forum posts now point to pages that have gone 404 with dead links. Right now I'm thinking about how much information will be lost forever if MEF were to go under -- that site had become the archive for many other sites that ceased to exist, with blueguitar.org being the most recent site that had to be archived there. If MEF goes down that would be a major loss.