1977 Fender Twin Transformers

[desertmouth90]

So apparently these are 1977 fender twin transformers.

I need to work out how to test them, but should I make a steel string singer with these? For my second ODS I was planning on making it a 50w and getting custom transformers wound or I could use these and make it a bastardised 124.

What you think?

[martin manning]

If they are '70's, I would read the dates as Choke, 725, 25th Week of 1977; PT, 336, 36th Week of 1973; OT, 405, 5th Week of 1974. [edit: had one mixed up]

Start by ID'ing all the leads and making sure that each winding has a reasonable DCR and no continuity to any other winding or to the core. Then put some low voltage AC into the PT and OT and check the voltages on the other windings for the correct ratio.

[Ten Over]

The OT and PT are likely from the eighties. The 21009 OT was used on the Twin Reverb II.

[Stevem]

A 77 twin would have been the 135 watt model and had a UL output transformer so it would have had two more primary leads.

Also if your planing on a 50 watt amp with just two output tubes and a single channel then that PT is going to have a ton of extra heater current which will keep your filament voltage At a unsafe high level, and along these same lines your B+ will be high, put you could take advantage of that suck that extra voltage down thru a big dropping resistor before the first filter node and have a lot less ripple voltage.

[Ten Over]

A 77 twin would have been the 135 watt model and had a UL output transformer so it would have had two more primary leads.

Not that it's relevant to the current thread, but Fender made 100W Twins for the first part of 1977 and then made 135W Twins for the later part of 1977.

[desertmouth90]

A 77 twin would have been the 135 watt model and had a UL output transformer so it would have had two more primary leads.

Also if your planing on a 50 watt amp with just two output tubes and a single channel then that PT is going to have a ton of extra heater current which will keep your filament voltage At a unsafe high level, and along these same lines your B+ will be high, put you could take advantage of that suck that extra voltage down thru a big dropping resistor before the first filter node and have a lot less ripple voltage.

The 50watt transformers will be custom wound. not using these

[Stevem]

I don't know about the early 77 100 watt twin stuff because I have 76 master volume 70 watt PR and its my understanding that Fender started both models off with UL at the same time.

[Ten Over]

I don't know about the early 77 100 watt twin stuff because I have 76 master volume 70 watt PR and its my understanding that Fender started both models off with UL at the same time.

Let's see it.

Here are 45W Pro Reverb transformers with '77 date codes.

Transformers copy.jpg

[nuke]

Production of ultra-linear and straight-up P-P amps overlapped, at least the production of transformers did anyway.

I would tend to suspect that transformers were ordered early ahead of production of the UL series amplifiers, and thus some UL amps have transformers that pre-date the end of regular push-pull. Been a while, but I recollect seeing such many years ago.

Ultra-linear was a great way to play the "power game" and a pair of 6L6GC that would make 40-60-watts push-pull (60 kinda pushing the truth) would make 70 watts in ultra-linear configuration, and the Twin and Super-Six went up to 135 watts in ultra-linear configuration.

Watts sold amps back then.

Today, a gigging guitarist playing small-to-medium venues is lucky to be allowed any stage volume at all other than in-ear monitors.

[solderhead]

Warning: Old Man Shouts at Clouds.

I know that almost everyone in the world of small-volume guitar amp builders prefers to buy off the shelf, and that very few of us bother to dig into transformer design. The result is that when most of us see an OT that has screen taps we reflexively refer to that topology as "ultralinear" because that's what we're familiar with -- but at the risk of being pedantic, those Fender amps that we all like to refer to as "UL" were never actually UL amps. Fender actually designed their amps to run in "distributed load tetrode mode," the difference from "UL" being the location of the taps with respect to the total number of turns on the screen winding, and the effects that the location of those taps has on power output and distortion.

When winding an OT that has screen taps, if you vary the percentage of primary turns from B+ to the screen tap, and generate curves of power output vs. primary turns, and distortion vs. primary turns, you'll find that the results are non-linear and there are distinct/different operating points that provide the highest power or the lowest distortion. Depending on what you're trying to accomplish, you'll choose the transformer design with the percentage of turns on the screen tap that suits your needs.

"Ultra-Linear" was defined by Hafler and Keroes in their 1955 patent as a specific set of design parameters where the screen load is 18.5% of the plate load impedance - measured from the power supply source (B+) terminal, or 43% of the number of turns on the plate winding.

Why does this matter?

H&K designed HiFi amps. They chose their operating points at the optimal point for lowest distortion in the HiFi application -- they chose the nadir on the distortion vs. turns curve. They patented their idea and they used UL as a marketing term for their amplifier products.

Fender wasn't as interested in linear operation, Fender was interested in power output. For this reason Fender's choice of operating points were *far* lower on the curves as Fender's transformer designers sought to maximize power output without paying attention to distortion levels in the guitar amp application. They chose the relative maxima on the power output vs. turns curve.

This explains why UL HiFi transformers like Hammonds aren't necessarily the best choice for guitar amps, as they're optimized to minimize distortion. They're a great choice for a HiFi amp as they choose the traditional low distortion operating point.

Years ago we had a long discussion at MEF where we hashed this out. That thread still exists at MEF, but unfortunately MEF seems to be down at the moment:
http://music-electronics-forum.com/t33707/

I know, I know --- I'm the only guy in the universe who cares about this, and I have to roll my eyes every time that somebody mis-applies the word "ultralinear" to Fender amps. Fender DL OTs have never been ultralinear, though they are screen-tapped OTs, and they share that characteristic with UL OTs. The difference is that the location of the screen taps differs -- A LOT. IMO this isn't only pedantics -- as if you're the kind of guy who's actually into designing an amp for a particular purpose these differences will matter.

[martin manning]

But "Ultra Linear" is a much better marketing term. FWIW I try to say DL regardless of the technical details. Have you measured the reflected primary impedance and DL fraction on a Fender DL OT?

[solderhead]

Have you measured the reflected primary impedance and DL fraction on a Fender DL OT?

I did do that years ago and posted a graph of my observations in the MEF thread that I mentioned previously. I had copied Fig. 5 from the original H&K 1950s patent that showed their definition of the UL operating range as a hashed vertical bar on the plot. I had added a green vertical bar to the left on the plot that referenced my test results from a late 70s Twin Reverb that I had on the bench. Fender's operating points were very close to pure tetrode mode.

Sadly, MEF is down again and I can't pull up the relevant thread. I've looked on my drives and I can't find my image. But when I google for this topic, the first hit is a blog post that references my previous cloud-shout about the UL/DL nomenclature. His blog post actually includes my artwork from the MEF post and credits me with the image.

https://ppamps.blogspot.com/2014/01/ult ... -late.html

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.

[solderhead]

Here's my old graphic that was copied and posted on the blogspot site. The Fender DL operating point is the green bar. As I recall there was a preliminary image that had a mistake in it that was corrected later on. I don't know which one he copied, and I can't find my notes right now.

the author of that blogspot post independently verified his own measurements that matched my graphic well enough that he borrowed my graphic for his blogspot page, so I think this graphic is the correct version:

Ra-a = 2.26k
Screen tap at 12.5% of the winding



What this DL business tells us is that the people who pontificate that the Fender UL Twins sound bad because they are ultralinear amps don't really know what they are talking about. The operating point chosen by Fender was almost pure pentode (tetrode) mode.

[LOUDthud]

Thanks to @Solderhead for posting that graph. 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.

[martin manning]

At this link http://www.r-type.org/articles/art-003a.htm#004 you can see that the difference between 20% and 43% DL doesn't have a huge impact on THD, and it varies some with tube type.
If I wanted to operate 6L6GC at 500V B+ with the load line set for maximum output power (200% Pa max on the LL) and have it pass through the Vg1=0 knee at Vg1 = 0, I would set the DL fraction at 10%. I wouldn't be surprised if the accountants were only too happy to get rid of the choke in the power supply too.