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Post subject: Re: Twin and Early Showman Output Transformers.
Posted: Fri Sep 13, 2019 4:09 pm
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TA -- Always happy to share any information I have -- although I don't actually have any of the transformers I'm seeking information on -- if I did, I could simply measure them and call it a day. Ergo, the search for any and all who could help provide a clue.

I've surely seen significant differences in gain from one model to the next, or between different versions of the same model, but in my experience -- assuming passive component issues are eliminated as a contributor -- they always come down to either tube issues, or circuit differences.

As an example, when I test small signal tubes, I test not only for µmho (transconductance), but also for AC Gain. A transconductance test is perfect for Med Mu tubes where the primary concern is a strong output voltage across a (relatively) low impedance circuit, while gain is secondary. Think -- 6SN7, 6CG7, 12BH7, and the like. With high mu tubes however, circuit impedance is typically high, so gain is more important than an ability to dump current into a (rather) low impedance load -- which is by definition what a transconductance test does. When testing 12AX7 tubes for gain then, I've seen perfectly good tubes (as established by transconductance test) have as much as a 20% spread in gain. A player would definitely notice this much spread in gain, calling the higher gain tube very hot, or achieving overdrive earlier. Fill the amp with all high gain tubes and another with all lower gain tubes, and the difference is significant.

Circuit differences have their impact as well. Look at the early Bassman piggyback amps, with huge losses through the tone stack, requiring 3 gain stages to sufficiently drive the power amp (PA) section. Later tone stacks with less aggressive compensation (AB165) could easily get away with just two stages.

As for transformer characteristics, the only characteristic that can affect what might be considered as relating to gain would be the turns ratio, which presumably would be a fixed characteristic within a given transformer model.

All else being equal, one transformer with a higher primary DC resistance will produce slightly less power output for a given level of excitation current due to the copper losses involved. But at lower power output levels (before copper losses become notable), power output will be relatively unaffected.

The amount of primary inductance primarily affects a transformers ability to transform power at low frequencies. Higher inductance means a bigger core and more primary turns to prevent saturation at high power levels at low frequencies. But as long as the turns ratio remains the same as in a smaller transformer, then mid frequency power remains nearly identical. The difference between the Band Master and Bassman transformers is a perfect example.

Gain and power are intertwined, but distinctly different. Two amplifiers can have the same input sensitivity to achieve full power output -- say 1.0 vac at 1 kHz. But if one amplifier is a 20 watt amplifier, and the other is a 100 watt amplifier, then using an 8Ω output as a constant, the 20 watt amplifier has a gain of 12.6, while the 100 watt amplifier has a gain of 28.3, or 225% more gain. Fender was famous for taking the same "front end" and tying to different power amplifier sections. In the above example then, if the 20 watt example is a Deluxe, and the 100 watt is a Single Showman, there would be a great perception of different gain levels, as even though the preamp sections would be identical, it would take a notably lower volume setting on the Showman to achieve the same sound level as produced by the Deluxe.

You may already know all of this and more, and if so, then please excuse my covering some basics here.

I may in fact have a line on information covering the 45945 transformer. If so, I will certainly try to get the lam and stack sizes for you -- but how much more I can coax out of the individual with the information remains to be seen. At this point, I'll just be happy to know what the heck the secondary impedance was designated to be for this transformer!

Dave


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Post subject: Re: Twin and Early Showman Output Transformers.
Posted: Fri Sep 13, 2019 4:31 pm
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dcgillespie wrote:
I may in fact have a line on information covering the 45945 transformer.


If you do and the amp's owner agrees to cooperate, be sure to secure all pertinent data -- amp model, output tube type, speaker array, and approximate build date. Photos would be nice as well.

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: Sat Sep 14, 2019 5:55 am
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Absolutely will!


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Post subject: Re: Twin and Early Showman Output Transformers.
Posted: Mon Sep 16, 2019 11:48 am
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The AC gain of tubes is all over the map. I wanted to build a standalone tester with a dedicated oscillator to test and compare tubes, but wasn't sure how to proceed.
I've done enough cork sniffing to recommend Telefunken for clean, Amperex for blues and EI for rock, but could never figure a way to measure the differences in any meaningful way.
My Dad's old college books point to plate coatings as a big variable in shot noise and current carriers, but I only thought of comparing current carrying capability, not transconductance, but max current.
But AC gain measurement may give me the insight I'm looking for....

"The amount of primary inductance primarily affects a transformers ability to transform power at low frequencies. Higher inductance means a bigger core and more primary turns to prevent saturation at high power levels at low frequencies. But as long as the turns ratio remains the same as in a smaller transformer, then mid frequency power remains nearly identical. The difference between the Band Master and Bassman transformers is a perfect example."

Saturation at low frequencies is where I have found distinct differences in amps. I first got a clue of this when I watched the speakers in a SF Twin without a baffle. They were waffling back and forth 1/4 to 1/2", not making a sound.
When I measured gain across the spectrum, it was as you describe, pretty much the same midrange gain. But the amp had its highest gain at 30-40Hz. This subsonic response robs the midrange of power, it also has to ride on the bass wave, causing distortions.
While I don't have your historical perspective, I can measure the direct correlation between inductance and low end response.
The earlier twins had low end response, but the saturation characteristics of the smaller transformers squeezes the mud out and the low end gain drops off as the volume went up.
Both Bass and guitar players have relayed the problem to me of bass farts onstage as the low guitar harmonics argue with the bass line.
I usually mod the preamp cathodes to reduce the bass, but the real answer is to replace the SF tranny with a reissue... an expensive Heresy.


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Post subject: Re: Twin and Early Showman Output Transformers.
Posted: Mon Sep 16, 2019 2:33 pm
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Man I wish I had an old Twin with the big transformer to mess around with. Not to change anything mind you, but just to measure the heck out of.......

Interesting comments TA. The low frequency power handling ability of Fender's premium output transformers (identified as those used in BF and later amps that employ 4 mount legs) is generally quite good. For example, the 125A13A 4Ω Bassman transformer (and its 125A9A 2Ω cousin) are capable of passing a 40 Hz sine wave at 50 watts RMS, with the waveform only just starting to deform (but not clip) in the last watt or two. This is sufficient to pass at full power the low E string of a conventional 4 string Bass guitar. By 35 Hz, maximum power falls to 46.5 watts RMS, and at 30 Hz, maximum power has dropped to 42 watts RMS, representing the needs of the low B string on a 5 string bass. For a transformer that was designed before 5 string basses had even been thought up, that is rather impressive performance -- and it's hardly performance can only be achieved in a test environment conditions. This was observed using a Fender 125P5D power transformer with GZ34 rectifier, in the conventional Fender BF power amp circuit that employs 5.5 db of NFB, and with the output tube quiescent current set and balanced at 50 mA per tube.

For optimum LF power handling capacity of an output transformer, having the output tubes operate with balanced quiescent (no signal) current levels and at an optimum overall quiescent current level is crucial. Unbalanced tubes invite early saturation since the unbalanced plate currents end up working to magnetize the core. When that happens, the size of the core is effectively reduced, reducing the LF power handling capability of the transformer. No doubt this is why Fender stubbornly hung onto using 5881 tubes, because the darn things were so well matched even at random, right out of the box. Later, CBS recognized the DC Balance issue and of course changed the old bias control in BF amps to a DC Balance control. Great for maximizing the LF performance capability of the output transformer, but unless you had tubes spec'd to Fender's bias point, you were out of luck. General replacement tubes could either red plate at one extreme, or be so cold as to cause gross cross-over distortion. As well, operating the tubes at 30-32 mA as so many do is really dipping to the realm of cross-over distortion. Good for tube life, but doesn't really produce a sound players like, and also limits power output as well. For tubes that are capable of routinely handling peak current levels of over 200 mA each in Class AB output stages, bumping up the quiescent current to 45 mA or even 50 mA means little to tube life in the long run.

Fender's 125A29A/125A30A transformers are well capable of passing undistorted 35 Hz waveforms at 100 watts RMS, making them stellar performers as well. And, the same biasing comments apply to their LF performance as well. Much later, Ed Jahns' transformers were even better yet, with at least his 70 watt and 135 watt transformers being able to handle 35 Hz signals at those power levels with low distortion.

While it means little to this conversation, the really remarkable thing is that all of these transformers can also pass a near full power 20 kHz sine wave as well with little if any deformation of the waveform, which is remarkable for transformers designed for this application. By comparison, Fender's lesser transformers start falling apart above 2 kHz, and are quite poor at 20 kHz. I don't have any first hand experience with the large high power Twin transformer, but suffice it to say that the BF and later premium transformers are hardly limiting factors in the amplifiers they are used in.

Which then brings up EQ. Besides that provided by the tone stack, for years, Fender rolled off frequencies below 40 Hz at a rate of 6db/octave -- accomplished by those designs using a 500 pF cap at the input of the phase inverter stage, and of course at a little lower frequency in the combo designs that typically used a 1000pf in that location. But speakers that "breathe" of their own accord with no signal applied indicates gross LF instability -- invariably caused in guitar amps by power supply decoupling caps that have gone south, although anything is possible of course. Still, it is a sign that something is definitely very wrong with the amplifier.

Still no word back from the one tech who says they can provide some confirmation regarding the 45945 transformer, but I remain hopeful.

Dave


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Post subject: Re: Twin and Early Showman Output Transformers.
Posted: Wed Sep 25, 2019 10:42 am
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Here are some records of performance and transformer specs you can glean on.
Image
These are three other models that had their bass boosted during the SF era
Image


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Post subject: Re: Twin and Early Showman Output Transformers.
Posted: Sat Nov 30, 2019 9:42 am
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Hi all...

My 1963 brownface showman has a huge output transformer stamped 45268. The tube chart specifies 4x5881

As such; I’ve always run my amp at 4ohms just like my ‘63 brownface bandmaster.

I’d thought my amp would have been 8ohm and a ‘showman 12’ - but everywhere I’ve googled says the 45268 is a 4ohm transformer

Would you all be confident that it’s 4ohm?

Thanks in advance for your comments


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Post subject: Re: Twin and Early Showman Output Transformers.
Posted: Mon Dec 02, 2019 12:54 pm
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Lewis, what is the static DC resistance across the output (secondary) leads?


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Post subject: Re: Twin and Early Showman Output Transformers.
Posted: Mon Dec 02, 2019 2:59 pm
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Lewis81 wrote:
Hi all...

My 1963 brownface showman has a huge output transformer stamped 45268. The tube chart specifies 4x5881

As such; I’ve always run my amp at 4ohms just like my ‘63 brownface bandmaster.

I’d thought my amp would have been 8ohm and a ‘showman 12’ - but everywhere I’ve googled says the 45268 is a 4ohm transformer

Would you all be confident that it’s 4ohm?

Thanks in advance for your comments


My 6G14 schematic specifies the 45550 output tranny for a single-speaker application (ie: an 8Ω load), driven with 6L6GC's. I'm betting that your 45268 iron is the 4Ω equivalent.

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: Mon Dec 02, 2019 6:22 pm
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Retroverbial wrote:
Lewis81 wrote:
Hi all...

My 1963 brownface showman has a huge output transformer stamped 45268. The tube chart specifies 4x5881

As such; I’ve always run my amp at 4ohms just like my ‘63 brownface bandmaster.

I’d thought my amp would have been 8ohm and a ‘showman 12’ - but everywhere I’ve googled says the 45268 is a 4ohm transformer

Would you all be confident that it’s 4ohm?

Thanks in advance for your comments


My 6G14 schematic specifies the 45550 output tranny for a single-speaker application (ie: an 8Ω load), driven with 6L6GC's. I'm betting that your 45268 iron is the 4Ω equivalent.

Arjay


+1

The info I have says the 45268 tranny is for a 5F8 or 5F8A twin with 4 5881 tubes and 2 speakers. Not conclusive, necessarily, but I'd sure like to see a picture of your tube chart. Never the less, I believe that is a 4 ohm output tranny.

Sure would love to see pictures of the amp and tube chart.

Regards,


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Post subject: Re: Twin and Early Showman Output Transformers.
Posted: Mon Dec 02, 2019 6:32 pm
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vinyl wrote:
Sure would love to see pictures of the amp and tube chart.

+1

The speaker enclosure as well.

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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