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Post subject: Twin and Early Showman Output Transformers.
Posted: Wed Sep 11, 2019 8:18 am
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Arjay or anyone,

I saw this recent post on another site. Was wondering if anyone know the history behind these transformers?

Thanks!


Back in the 60's, I was a teenager, and played bass (EBO) in a pretty good garage band in Indy (the epicenter of the garage band phenomenon back in that day), and was huge into Fender at the time, having scratch built modified (one channel) Twin Reverb amps for the lead and rhythm players, a single Channel Showman for myself, and another amp for the PA. Blackface circuits were everything to me and I knew them by heart as I'm sure others do as well. But after HS, the bass was put up, the equipment sold, and I went dormant with regards to playing.

Fast forward to recent times when the passing of my all time favorite bass player (Joe Osborn), and a local opportunity to start playing again stirred me to dig out my bass after almost 50 years of storage. Very happy with a 70's SF Bassman I purchased and modified for my needs, but in doing all that work, it also stirred up my interest in vintage Fender amp design again. Lately, and with much research, I have been (if only for my own satisfaction) working to make sense out of all the various power and output transformers used during the '60 - '61 period for their 40 and 80 watt amplifiers.

Specific to this post, particularly the output transformers for the final Twin (not TR) and the earliest Showman amplifiers, which is rather intricate as otherwise identical and not so identical transformers moved through the various part numbering systems Fender used, and the various other amplifiers Fender used them in -- complicated further by the fact that the secondary impedance was rarely labeled on Fender schematics.

Any credible information anyone can provide to my questions would be greatly appreciated (the internet is packed with inaccuracies), credible meaning either first hand knowledge, or knowledge of well proven fact. My questions relate to only 5881/6L6GC amplifiers, and not the rare 7355 based Showmans:

1. High power Twins primarily used output transformer part number 45268, which ultimately morphed into its final (and somewhat smaller) form as the well known and long running 125A29A/022889 (4Ω) transformer. But before the 45268 transformer was put out to pasture, it too was almost surely given its own 125AxA part number, as that part numbering system came into being well before the 125A29A transformer came to be. Does anyone know what that part number might be?

2. Of the no less than four output transformers used in early 6L6/5881 based Showman amplifiers, three are of interest here:

A.) 45946 is reported to be a 16Ω transformer, but I have not been able to place an amplifier with that part number transformer together with the speaker cab the amp was sold with to verify this. Can anyone verify this?

B.) 45945 is likely an 8Ω transformer. But like the 45946, I've not been able to place together an amp with this transformer and the speaker cab it was sold with to verify. Anyone?

C.) If 45945 is an 8Ω transformer, then 125A4A is the likely successor (same transformer) part numbering wise -- if the 125A4A can be verified as an 8Ω transformer itself. If it is, then it ultimately morphed into its final and somewhat smaller form as the 125A30A/022897 8Ω transformer for the BF Single Showman and Vibrosonic Reverb amplifiers. Can anyone verify this?

The 45946, 45945, and 125A4A are all physically large, being the same size as the Twin's 45268.

Since I was out of the Fender loop for a few decades, I thought some of the more current tech/builders here at AK might be able to help fill in the gaps. The little information I've been able to glean from magazine articles, modern transformer manufacturers, or charts listing this type of information are either incomplete, or inaccurate enough to raise question.

Thanks in advance for any help you can provide!

Dave


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Post subject: Re: Twin and Early Showman Output Transformers.
Posted: Wed Sep 11, 2019 8:36 am
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Triad trannies are really not my area of expertise but I can say with certainty that I've never seen one in a blackface Fender whether Showman, Dual Showman, or Twin Reverb. The last examples I recollect were those used in the brownfaces. I'm not saying that Triads weren't used, only that I have never seen one. IMO if the guy with the questions can find an original Schumacher 125A29A (4Ω secondary) or 125A30A (8Ω secondary) he should grab it and be thankful. Those trannies are some of the best iron ever made.

Arjay

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Post subject: Re: Twin and Early Showman Output Transformers.
Posted: Wed Sep 11, 2019 9:02 am
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Much thanks, for quick reply. Those Triads were very good iron, too. Though not has hefty as the TR or Showman Schumacher irons. I have not played on one of those early 1960s BF four by 6L6GC or 5881 Fender amps. One day...


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Post subject: Re: Twin and Early Showman Output Transformers.
Posted: Wed Sep 11, 2019 9:06 am
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I've played a blond 5881 Showman before (actually a "Double" Showman) but it had Fender's standard 125A29A O/T. Sounded great!

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Twin and Early Showman Output Transformers.
Posted: Wed Sep 11, 2019 12:55 pm
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Retro -- Thanks so much for your reply -- I'm the individual that made the original post on another forum that BMW so kindly spread to this forum for me. While I've never personally come across the transformers I'm researching (45945, 45946, and 125A4A), I have no doubt that the 125A29A and 125A30A transformers are superior, even though slightly smaller in physical size relative to their older brethren. The latter transformers are wound using interleaving and (almost certainly) a higher grade steel that allows for no doubt at least equal performance to the old larger transformers on the low end, and almost certainly far superior performance on the high end. The result is that both of the latter transformers will pass a clean sine wave from 35 Hz to 20 kHz at 100 watts RMS (clean meaning without obvious visual distortion). That surpasses the capabilities of many vintage mid-grade high fidelity transformers, and almost surely, most of the garden variety replacement guitar amp transformers produced today.

My quest however is not about finding one of these transformers, or even a 125A29A or 125A30A transformer for that matter, as thankfully I already have two of each. My quest is about determining the secondary impedance of the three transformers listed for two reasons: -- The information is missing from available schematics -- but it would be nice to know how other circuit elements were altered (NFB for example) for the various impedance levels provided, plus, in my searching, it appears that the general understanding is that as long as you've got JBL D120F, D130F, or D140F Fender speakers connected to an old (primarily Blond) Showman, you're good to go. Nothing could be further from the truth as the JBL Fender speakers were supplied in both 8Ω and 16Ω versions, and while the speaker cabs usually had just one speaker, sometimes there were two (Double Showman). Add in mix and match between speaker and amp cabs, replaced speakers, and replaced transformers, and who knows what is what anymore? Truth is, sussing out the information on these transformers has proven difficult enough that it seems to have left the general public tech domain.

Gathering the facts is complicated by the fact that at the time (60-61), there were a total of (I now know) 5 different transformers (by part number) used in the early 5881/6L6GC based Showmans, and to further complicate things, the part numbering system was also changing over to the new 125AxA system at the time as well. Therefore, some of these transformers may be physically the same transformer but reassigned a new part number, while others of course are simply different transformers. And to your point, the final Blond/Brown units were in fact shipped with 125A29A or 125A30 transformers, bringing the grand total to 7 transformers actually used in the pre BF Showmans -- and this doesn't even include the two transformers dedicated to the rare 7355 based Showmans!

Within the 5881/6L6GC amplifiers then, the only transformers that remain in question are the:

1. 45945 -- This would seem to be an 8Ω transformer, as most pics I've seen of complete amps presented in original condition have this transformer operating with a single 8Ω JBL -- but not all pics depict that.

2. 45946 -- Is "reported" to be a 16Ω transformer -- but installations I've seen that appear to still have this transformer installed as original equipment use a NFB resistor that would not be consistent with a 16Ω transformer.

3. 125A4A -- is "reported" to be an 8Ω transformer, and of the three is possibly the most common transformer seen in Showman amplifiers. But it too has been seen in supposedly original condition amps with 16Ω speakers.

So it's a mess. The high power Twin is not nearly so convoluted, using just two transformers in its life, both of whose secondary impedance are fully verified. My only question with that amplifier is if the 45268 transformer -- the main transformer used in the twin -- was ever given a new identity in the 125AxA numbering system -- I don't see any evidence that it ever was.

All of these transformers -- the three questioned for the Showman, but also the Twin's 45268 are physically identical, and notably larger than the Classic BF 4 tube transformers. As I've stated before, the Blond era amps have a notable and dedicated following. Besides chasing this information down to quill my own curiousity, I would think those devoded to the pre BF Showman amps would find the information quite useful as well.

Again, any and all help would be greatly appreciated!

Dave


Last edited by dcgillespie on Wed Sep 11, 2019 7:21 pm, edited 1 time in total.

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Post subject: Re: Twin and Early Showman Output Transformers.
Posted: Wed Sep 11, 2019 6:51 pm
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Let me interject just briefly, before Arjay answers --- if he does. To find the true secondary and primary impedance of these older transformers, you really need a signal generator and a scope (or at the least, a good DVM).

I've found what is labeled by those old manufacturers and what is the actual working impedances, often don't matchup. Triads are great transformers. Prolly a lot of the "tweed era" sound is due to them. Along with the tube topology and circuitry.


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Post subject: Re: Twin and Early Showman Output Transformers.
Posted: Wed Sep 11, 2019 10:57 pm
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Steve, I think the most significant factor of Fender's "tweed" sound is the downstream location of the tone stack. A secondary but nonetheless still important reason is the configuration of the NFB circuit. Some of this was still applicable as Leo moved his amp line into the brownface era and he was still experimenting with the boost/cut treble control and pushing tubes ever harder.

Dave, you've got a lot on your research plate and I hope you're able to sort it out. I wish I had more to contribute but most of my data (empirical and anecdotal) is focused on the blackface/silverface amps. Drop by as your quest yields results and let us know what you've learned. BTW, JBL built a D-series speaker for Fender in the mid '60s which was marked 8-16Ω. The actual DCR was about twelve ohms.

Arjay

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Post subject: Re: Twin and Early Showman Output Transformers.
Posted: Thu Sep 12, 2019 1:05 am
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Arjay, yeah, tone stack and GNFB is biggest contributor, to tweed tone. I have three amps with those Triad transformers. Very nice tone. But, how much is the iron and how much is the circuitry? Mostly, circuitry.


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Post subject: Re: Twin and Early Showman Output Transformers.
Posted: Thu Sep 12, 2019 2:15 am
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I think I'd concur with that. Circuitry has more of an impact than mere components, though each supplies some element in the amp's sonic composition.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Twin and Early Showman Output Transformers.
Posted: Thu Sep 12, 2019 7:02 am
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Triad archive, may be helpful although part numbers will not match to Fenders #'s...Careful deductions can be made though and substitute iron can be ID'd.
http://www.triadtransformers.com/catalogHome.html


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Post subject: Re: Twin and Early Showman Output Transformers.
Posted: Thu Sep 12, 2019 7:05 am
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The debate between circuit or component producing an audible sonic signature will always be with us I suppose -- particularly with guitar amps since there is little or no corrective circuitry involved to minimize the issue. In reality, there are examples where circuit configuration can be shown to influence how well a particular passive component operates (primarily transformers), and examples where the type of passive components used can be shown to influence how well a circuit operates. Modern manufacturing has improved the tolerance and construction of passive components today such that to my mind, as long as appropriate components of sufficient quality are used, the circuit is even more of the dominant factor than ever before.

Arjay -- Thanks for your support. As for the JBLs rated for 8-16 Ohm operation, that is yet another factor I chose not to mention in my requests so as not to muddy up the subject even more than it already is.

And talk about mud -- try this one on: Imagine a tech today getting in a debut Tolex Twin, and a debut Bassman piggyback amp at the same time, that are both untouched time pieces. The Twin had previously already morphed into its high power 4 output tube design, but now uses SS rectification. The Bassman of course is still a 2 output tube design that still retains a rectifier tube, but now drives a single 12" speaker in its separate speaker cab. Imagine his puzzlement then when he realizes that both of these amplifiers came from the factory with the same 45548 output transformer installed (which is physically the same size as the standard 125A13A Bassman output transformer used all throughout the 60s), and that both of these amps work just fine with it!!

When you add in all the little details like the JBL speakers rated for 8-16 Ohm operation, and well out of the norm transformer cross-installations like that noted above, it's a wonder that even Fender themselves could keep it all straight in those fast changing times, let alone some poor tech trying to make any sense of it all -- back in the day, but especially today. As is always the case however, knowing what the proper output load should be for a given transformer and circuit design always makes for the best vacuum tube amplifier performance. And so for the early Showman, the quest continues for those last three transformers.........

Dave


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Post subject: Re: Twin and Early Showman Output Transformers.
Posted: Thu Sep 12, 2019 8:03 am
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Dave, if you haven't already, you might check out the Steel Guitar Forum. A lot of those guys played (or still play) Fender Showman's and you might find a tech-minded player who could shed additional light on your search.

Arjay

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Post subject: Re: Twin and Early Showman Output Transformers.
Posted: Thu Sep 12, 2019 11:19 am
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I have listened to a number of miracle transformations with a simple change of output transformers.
OTs are only 70 - 80% efficient and a simple change in laminations or interleaved windings makes an incredible difference. I'm a true believer there. Its some of the real
Black magic in an amp.

You can determine a transformer output impedance with a 12 volt transformer and a voltmeter.
The voltage ratio is the same as the turns ratio.
Hook up the 12 volts to the red/blue primary wires and measure the loaded voltage on the primary. Now measure the voltage on the secondary.
For tube amps in general, a voltage ratio of 20:1 is 8 ohm, 40:1 for 4ohm and 10:1 for 16 ohm.

The core size indicates power level at low frequency. The variations may be to control bass saturation characteristics.
The inductance of the primary windings indicates core size and lamination material. Over the decades, Fender OTs ran from 6 Henrys to 19 Henrys and back to 6.

After your descriptions of on-the-fly production changes, I can see why Fender omitted details on the schematics.
If he was a mapmaker, it would read, Here there be Monsters. Good luck with your search.


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Post subject: Re: Twin and Early Showman Output Transformers.
Posted: Thu Sep 12, 2019 12:20 pm
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Thanks TA -- Transformer theory and fundamentals I'm well versed on -- and would also add that one thing Leo did to help aid in the efficiency of his transformers -- which I agree can have a huge impact on performance (try playing low bass notes through a Band Master!) -- was that he had a transformer for every occasion. He always wound for the need, never offering a tapped secondary for multi-use in any of his transformers. Manufacturing costs are minimized that way, but performance is also maximized as the use of a single size secondary wire helps to keep winding capacitance at a minimum. Then with the use of interleaving and better steel in his later transformers, the 125A9A, 125A13A, 125A29A, and 125A30A became truly stellar performers.

Dave


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Post subject: Re: Twin and Early Showman Output Transformers.
Posted: Fri Sep 13, 2019 10:19 am
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Performance is why and how I have been studying OTs in Fenders. It's interesting to hear about your pursuit from a historical perspective.
The schematics didn't account for the huge variations in gain I measured across the same models of different periods. I have focused on the OT and the specs, particularly inductance and primary resistance, bear out the differences.
I have these for many of the BF, SF, and reissue transformers. None for pre BF.
Would you be able to share some of these specs, like core dims, etc. for these early OTs that you do have?
Like you, I'm trying to get in the old man's head, just from a different angle.


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