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Post subject: Transformer Inductance vs Bass in old BF/SF Amps
Posted: Sat Dec 05, 2015 2:00 pm
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Output transformers have a huge effect on how an amp sounds. I have compared gain/frequency response of many classic Fender amps and found that, model for model, the Silverface amps have considerably more bass response than their Blackface predecessors. I haven’t found any differences in the schematics that would account for the amount of subsonic response in the Silverface models.
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So I’m working on the theory that the source of the differences is in the output transformer.
The size and composition of the transformer core controls how much low end response it will couple to the speaker. In the old days, this was important to lower bass response that causes muddy tone and focus the limited amount of power available on the midrange.. Some pickers still find this useful.
The way it works is to limit the size or effectiveness of the iron core. As the primary magnetizes the core, the iron can only hold a finite amount of magnetic flux before it saturates the core and can’t raise the magnetic flux any higher. Saturation occurs at the lowest frequencies first. So a 50 watt amp may only put out 20 watts at 40Hz.because the transformer is choking on the power saturation.. This is where the differences in the BF/SF lay.
Now the cores may be the same relative size, but the permeability of the laminations can be changed without any visible difference. Permeability is the amount of flux the iron can hold. Silicon steel alloy is common low grade alloy. Nickel alloy has high permeability. There are many grades in between. I’m trying to establish if the laminations were changed to enhance bass response across the model line. Its also possible that core size increases are the reason.
While I have established to myself that the bass increase is real, I can only guess why it was done. All that bass does sound good in the showroom an in your living room. It could have been a sales decision. Or it could have been a cost decision. Good laminations are more expensive.
So in order to get any farther than guessing, I bought a LCR meter to measure the inductance of the transformer primaries. I am expecting to find that the subsonic response will be paired with a high primary inductance.
Now the coils of the transformer are dictated by ratios and current carrying capability of the wire.
Output transformers with 8 ohm secondaries are 20:1 ratios, usually, and the norm is three turns per volt. The primary wire is chosen to be the largest gauge that will fit in the core. So the inductance of the coils is dictated by outside requirements. So I’m thinking that if I measure significant differences in inductance, I can attribute it to differences in the permeability of the laminations.
Initial results have been as predicted. A Mesa with lots of bass measures 15Henries. Then a Marshall with no bass response was 5 Henries. So I think inductance correlates to bass response, generally.
Now I need to get a hold of some BF/SF amps to measure and compare.
I also need to establish some kind of test procedure. I found that if the secondary is connected to anything, it lowers the inductance. I’m wondering if I would get a more meaningful reading with or without a load.
I’m measuring resistance of the primaries to establish coil characteristics.
I’m measuring inductance of the primaries
I should measure core size.. I’m not sure whether overall size or core size is more significant.
Taking these measurements should be simple for qualified techs. It can be done on most amps by pulling power and rectifier tubes and making the measurements at the tube sockets after discharging..
The plate to plate measurements can be made between Pins 3 of the output tubes. Plate to B+ can be done between Pin 3 of the output tubes and pin 8 of the rectifier tube.
I’m not sure which measurement is more significant. I’m doing both to compare the differences in inductance between coils and total inductance. This brought up another problem. If you measure the resistance of the individual primaries, they add up to the total measured resistance. If you make the same measurement in inductance, the total inductance is much higher than the sum of the individual coils. What’s going on here?
Another thing I have to figure out is the Inductance meter itself. On the scope, I see it puts out a modified square wave to charge the coils. But in the 20Henry range, the wave goes away. It still measures a known inductance properly, but I was trying to figure out the frequency it measures at. This threw me. It is 896Hz on all scales except 20H. What is going on here?
So if I can get some inductive advice on how to proceed, I’d be grateful. I’m not big on the math, but I’m all about uncovering Leo’s secrets. This seems to be a big one.
I want to develop a procedure to get some meaningful inductance tests. With enough data, I can possibly correlate it to account for the bass response differences in SF/BF amps.
.

.


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Post subject: Re: Transformer Inductance vs Bass in old BF/SF Amps
Posted: Sun Dec 06, 2015 8:15 am
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Good information Tim ,

Thank's


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Post subject: Re: Transformer Inductance vs Bass in old BF/SF Amps
Posted: Sun Dec 06, 2015 10:10 am
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Tim, great write-up. I wonder if anyone has written on the history of Schumacher transformers. Esp those supplied as OEM for Fender amps.

:?:


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Post subject: Re: Transformer Inductance vs Bass in old BF/SF Amps
Posted: Sun Dec 06, 2015 7:01 pm
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I haven't had access to any of that kind of data. I haven't seen much hard data except for what I can generate myself.
The sentiment that I get from musicians is that the BF amps sound better than the SF amps. So I'm pursuing this angle to find out why.
If anyone else has an Inductance meter and can measure BF & SF OTs, I would appreciate any data. Thanks, Tim


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Post subject: Re: Transformer Inductance vs Bass in old BF/SF Amps
Posted: Mon Dec 07, 2015 12:52 am
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Tim,

One thing I've read about transformers and chokes, is the material and wiring technique has allow for more consistent inductance and lower DC resistant --- in most high-end modern trannies.

Effect on the tone, seems to be a better saturation spec. Less localized heating. Cleaner bass and high notes. Less distortion. Cleaner transients.

Maybe, the alloys and wiring technique was changing more rapidly in the SF era, than the older tweed and BF era. Dunno, but circuitry mods seemed to increase in the SF and later era. Combination of material and circuitry changes maybe what we are looking at here.

Wonder why no one has written a book on tranny manufacturer history in the 20th Century. I'll post a Q, on audio board.


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Post subject: Re: Transformer Inductance vs Bass in old BF/SF Amps
Posted: Mon Dec 07, 2015 3:16 am
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TimsAudio ,

How do you sketch and read those wave forms with frequency, what test equipment do you use ?
I wish to learn and to use it .

Any pictures of your setup ?


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Post subject: Re: Transformer Inductance vs Bass in old BF/SF Amps
Posted: Mon Dec 07, 2015 7:05 am
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I'd like to know precisely which circuit revisions those silverface amps were. Virtually every model started out using the same design as the previous blackface models and it wasn't until approximately six months into the 1968 model year when the original AB763, AA1164, and AA165 designs were "adjusted" to new layouts.

Absent that information, these comparisons are meaningless.

Arjay

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Post subject: Re: Transformer Inductance vs Bass in old BF/SF Amps
Posted: Mon Dec 07, 2015 10:34 am
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While I have only shown individual examples, I have multiple examples of most of these models graphed and they generally agree.
The documentation is inadequate because I wasn't looking for transformer performance at the time. It is a result of the data that I have collected that causes me to focus now on the transformer laminations as the prime suspect. Plainly, I need more accurate data.
I haven't seen any changes in the schematics that would increase subsonic response that I have measured. It would take a change to .22 coupling caps or something major like that to effect such a difference. Most of the SF changes are minor.
So that is why I'm focusing on the OT. Here, a small change can make a big difference. And changes can't easily be seen. That is why I am trying to get a better idea of what is going on by measuring the comparative inductance of various BF & SF amps. I am also going to have to measure comparative core size to see if that had any effect. Another issue is the change from a paper to plastic bobbin. This allows much closer proximity to the core for greater inductance. In disassembling blown OTs, I was always struck by the fact that they didn't attempt close contact with the core. The paper bobbon is unnecessarily thick and then they put in a wooden wedge to tighten the laminations, causing more of a gap. I realize now that they were intentionally reducing the inductance to starve the bass. I should wind a couple coils on different bobbins with the same core and see how they compare.

It will take me years and luck to penetrate these mysteries on my own. I was hoping that maybe the Fender community might be able to help. Is there a way to build a database collectively here?
Besides settling on a test procedure, a target list of models and serial number ranges will ne necessary to correlate any data into meaningful conclusions. You have indicated three chassis, are the any significant others?


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Post subject: Re: Transformer Inductance vs Bass in old BF/SF Amps
Posted: Mon Dec 07, 2015 11:07 am
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Specifics on the circuit sub-types for each graph may help. Like AA864, instead of BF Bassman.

As for the air gap, my impression is that some air gap is needed in push-pull outputs to compensate for mis-matched sections (or tubes) --- to a certain percentage.


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Post subject: Re: Transformer Inductance vs Bass in old BF/SF Amps
Posted: Mon Dec 07, 2015 11:31 am
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TimsAudio wrote:
You have indicated three chassis, are the any significant others?


I suggest that you research every model that Fender ever made by name then explore each circuit revision that these amps incurred during their individual production runs. As an example, there are at least four different circuits for a silverface Super Reverb Amp, probably the same for other silverface models as well (Vibrolux Reverb, Pro Reverb, Twin Reverb, Bandmaster Reverb, Bassman, etc).

When most of these models became silverfaces they were still blackfaces on the inside. And the circuit identification printed on the tube chart cannot be trusted because FMI/CBS never updated them to reflect a newer circuit revision. As an example, a silverface Super Reverb built in 1970 would still have "AB763" printed on the tube chart but the layout was in fact the AA270 revision.

The only accurate method of comparing sonic performance between seemingly identical output trannies is to use a blackface amp as your datum point, then measure it against a silverface model with the same circuit revision. Once that data is noted and correlated, then start measuring the newer silverfaces (of which I'm certain that you'll observe variances to one degree or another).

From a personal perspective, I know that the amps that CBS tampered with do in fact sound different than the original FEIC designs but that tampering did not commence until mid-year 1968. However, many of the models started appearing with silverface cosmetics nearly a year prior and every one I've ever owned or played performed pretty much identically to the blackface versions. And I'm not talking three or four amps here -- I mean seventy-five or eighty over a span of some fifty years.

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: Transformer Inductance vs Bass in old BF/SF Amps
Posted: Sat Jan 23, 2016 12:32 pm
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Update January2016.
A blackface Super Reverb came in for a baseline examination. Its in good shape with a cap job in '04. I'm not sure that the reverb tank is original.. Installed tubes are 2 X 6L6 Philips ECG and Superior preamp tubes. They don't look or sound like anything special. They all have a wraparound plate like you see in 12AT7s.
My testing procedure has simplified to the point where chassis removal is not necessary for measurement.
1) Remove power tubes. Discharge pin 3 with a resistor to ground.
2) Remove Speaker plug and insert open 1/4" plug to ensure an open secondary.
3) Measure DC resistance pin 3 to pin 3
4) Measure inductance pin 3 to pin 3. Measure quickly before caps charge.
5 Measure lamination stack dimensions.
6) Measure lamination thickness. measure 10 lams for accuracy /10

Documentation Form
Brand: Fender
Model: Super Reverb
Chassis: AB763
Serial #: A25090
Power Transformer: 125P5D, 022798, CSA TEST, 606-720
Output transformer: 125A9A , 022855, 606-719
OT dimensions: 3.75" W X 3.25" H X 1.25" thick.
OT lamination thickness: .0185"
OT Resistance Blu - Brn : 97 Ohms
OT inductance Blu - Brn: 6.6 Henries
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Interesting, the array loses the resonant peak...
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Nice double layer reverb bag.
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Last edited by TimsAudio on Sun Jan 24, 2016 8:31 pm, edited 1 time in total.

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Post subject: Re: Transformer Inductance vs Bass in old BF/SF Amps
Posted: Sat Jan 23, 2016 2:44 pm
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So how does this '67 Super Reverb sound and how do the tranny measurements compare with the data you've already accrued?

Arjay

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Post subject: Re: Transformer Inductance vs Bass in old BF/SF Amps
Posted: Sat Jan 23, 2016 2:53 pm
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Not sure how much the impedance (at say, 1kHz) plays in your measurements. And whether you are using a 2-ohm dummy load or the OEM speakers. But, my bet is the actual impedance of those 4 vintage CTS speakers is prolly not 2-ohms, at 1kHz. Maybe best to test them first, before measuring any secondary values off that OPT.


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Post subject: Re: Transformer Inductance vs Bass in old BF/SF Amps
Posted: Sat Jan 23, 2016 5:29 pm
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Actually, many speaker manufacturers rely on a 1KHz tone to determine the design impedance of a voice coil. Thus, that Super Reverb's speaker array likely measures close to its 2Ω specification when reproducing a one-thousand Hertz test tone.

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: Transformer Inductance vs Bass in old BF/SF Amps
Posted: Sat Jan 23, 2016 5:39 pm
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Arjay, I was thinking more of how the impedance changes, as the speakers get older. I brought this up, because on another board someone was doing similar measurements. Comparing new and used irons and speakers. Some older parts varied quite a bit from OEM nominal values.

:)


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