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Post subject: Re: Road worn 1974 Super Reverb restoration
Posted: Sun Aug 16, 2015 7:27 am
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Bummer. Several ppl I know have used the new Tungsol (Russian) 7581A. They really like it. Can be biased from 20-30 watts no problem (max is 35 watts per tube). They sound pretty good at 20 watts...

Just an FYI.

http://www.dougstubes.com/tung-sol-7581-a.html


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Post subject: Re: Road worn 1974 Super Reverb restoration
Posted: Sun Aug 16, 2015 11:49 am
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I never manage to find that story about different laminations... But you're right Arjay, I shouldn't rely on anecdotal evidence.
To build some real evidence, I'm gonna buy an LCR meter and start specing some of these OTs.
By using a small air core inductor against the laminations, I should be able to plot a comparative inductance to evaluate the differences in laminations.
The OT laminations are one of those components that are the most obtuse to examine in a guitar amp, but they are a critical tone shaping component that is directly in the signal path. I want to find out more about them.
In my limited experience of designing transformers and reverse engineering dead ones, I've found.....
A small difference in permeability or material used makes a large difference in performance.
The thickness of the laminations and the way they are stacked makes a big difference in the brightness of the transformer.
Some OTs use segmented secondary windings. That is, part of the secondary is wound against the core and the second part wound outside the primaries.
Marshalls use segmented primaries on some of the dead OTs I have disassembled.
A layered set of windings makes for a more efficient coupling than a scramble wound aftermarket tranny.
A DC bias increases permeability.. To what extent, I don't know
Now I want to unlock the laminations....any suggestions?


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Post subject: Re: Road worn 1974 Super Reverb restoration
Posted: Mon Aug 17, 2015 7:27 am
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TimsAudio wrote:
Trick question there Arjay...
The early silverfaces used the old BF OT. The story I got it that the later CBS OT was the same part number, but used different laminations.

The permeability was greater and produced better transfer characteristics for more gain and slightly more power. It also produced more subsonic bass response that gobbled the goodness out of the midrange. It would have sounded real good in the showroom with all that bass, but it was also the main reason the SF amps were panned by the pros of those days. As Leo described it,"Yea we got some bad iron".

If you compare the response curves of the BF and SF versions, they have similar schematics, but very different response curves. I can only attribute the differences to the "bad iron"


Are you saying that Schumacher used different lammy material and/or spacing in the later OT made for the Super Reverb? Or a different supplier of the transformers?

Never heard of this.

Thanks!


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Post subject: Re: Road worn 1974 Super Reverb restoration
Posted: Mon Aug 17, 2015 7:45 am
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BMW2002Ti wrote:
Or a different supplier of the transformers?


Better Coil & Transformer (EIA prefix code "831") was also a designated supplier of trannies to Fender, beginning sometime in late '66 or early '67.

Arjay

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Post subject: Re: Road worn 1974 Super Reverb restoration
Posted: Mon Aug 17, 2015 9:06 am
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Yeah, maybe that is it. From my understanding, BC&T did not have as tight a QC as Schumacher (who really did --- for all the trannies they made?).


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Post subject: Re: Road worn 1974 Super Reverb restoration
Posted: Mon Aug 17, 2015 9:16 am
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I've had a few blown BC&T output trannies cross my bench over the years but percentage-wise, probably no more than the more common Schumacher-Woodward iron. As for the quality, I never observed any measurable discrepancy between the two. But I didn't (and still don't) have the specialized test rigs and equipment that Tim mentioned so my report can only be regarded as an unsubstantiated anecdote.

Arjay

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Post subject: Re: Road worn 1974 Super Reverb restoration
Posted: Mon Aug 17, 2015 9:38 am
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Looks like Arjay has the mfr info that could point to the source of the change.
This is comparative measurements that I have done.
http://s1083.photobucket.com/user/Darle ... r.jpg.html
I can't find any other component that cpold explain the differences in enhanced bass response other than a change in the OT.
I have measured the resistance of the coils with no major differences, so I am suspecting a change in laminations would be the culprit.


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Post subject: Re: Road worn 1974 Super Reverb restoration
Posted: Mon Aug 17, 2015 9:56 am
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Excellent documentation.

Were you able to correlate the data with transformers of a specific manufacturer?

Arjay

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Post subject: Re: Road worn 1974 Super Reverb restoration
Posted: Mon Aug 17, 2015 7:17 pm
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TimsAudio wrote:
Looks like Arjay has the mfr info that could point to the source of the change.
This is comparative measurements that I have done.

http://s1083.photobucket.com/user/Darle ... r.jpg.html

I can't find any other component that cpold explain the differences in enhanced bass response other than a change in the OT.

I have measured the resistance of the coils with no major differences, so I am suspecting a change in laminations would be the culprit.



Nice data. BTW... did you take into account the plate voltage differences between the Super Reverb phase inverter of the AA763 circuit and the AC568 (and later)? Could effect the frequency response.

I know the circuitry around the PI in Bassman amps makes a world of difference in the performance of these amps. Taking all the iterations of the SF models back to the classic BF PI circuitry (with bias pot revision).. really helps these SF Bassmans.


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Post subject: Re: Road worn 1974 Super Reverb restoration
Posted: Mon Aug 17, 2015 8:49 pm
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I would have to look up my paper records to find the chassis number, but I am going on the assumption that plate voltages will affect gain over an even spectrum, but not add bass. That is, frequency neutral.
That's the job coupling devices do. like capacitors and transformers.
To get that low 20Hz bass response in the SF would require larger value capacitors than the BF. I don't really see this in the schematic comparison, so I'm pointed to the tranny laminations to account for the difference
The subsonic response across many models at the same era convinced me that it wasn't a mistake, but a policy shift after CBS took over. Or as Arjay suggested, a change in manufacturer who didn't get it right right outta the gate. The excessive bass was reduced during the later, ultralinear era.

The reason I see this as important is because while the subsonics can't be heard well, they rob a lot of available power from the midrange. Since guitar music is mostly midrange, an amp that focus' its power in the midrange makes a guitar sound better.


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Post subject: Re: Road worn 1974 Super Reverb restoration
Posted: Wed Aug 19, 2015 6:39 am
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OK back to simplicity for a minute then feel free to carry on about the metallurgy atomic alignment etc.....
LOL X12 !

Anyone know where I might find a correct terminal board for the speaker wiring ??? apparently 1973 was the last year they bothered with one (little round guy where all the wires meet in the middle).
without having to buy all this
http://www.ebay.com/itm/SPEAKER-HARNESS ... 46336c6342

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Post subject: Re: Road worn 1974 Super Reverb restoration
Posted: Wed Aug 19, 2015 7:00 am
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TimsAudio wrote:
The subsonic response across many models at the same era convinced me that it wasn't a mistake, but a policy shift after CBS took over.


CBS "took over" on 5 January 1964. Which, according to your assertions, makes any subsequently-built Fender amp susceptible to the phenomenon you've hypothesized. And those amps were all blackfaces -- right up until the silverface cosmetics were introduced beginning in the spring of 1967. I can't say I've ever heard a blackface Fender that didn't sound like a blackface. And the '67 silverface Super Reverb I once owned (my first new Fender amp) sounded nearly the same as my then-bandmate's '66 blackface Super Reverb -- his had 10K5 Oxfords, mine had the CTS alnicos (which is what I attributed the minor sonic differences to).

Arjay

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Post subject: Re: Road worn 1974 Super Reverb restoration
Posted: Wed Aug 19, 2015 8:12 am
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I don't have specific tranny numbers for the different curves. The tranny supplier change matches the era that the bass is increased.
I wish I had documented them, But its not too late, they're still out there and still come across my bench.
Mostly, I do the response curves as an excuse to run the amp for a while after repair. I know most of you would just pick up a guitar..
But I was an unwashed stereo & video tech making the transition to music equipment. It soon became evident that there was more to a guitar amp than 0dB and 20-20khz. So I started out not knowing what I was looking for. It was only after a few years and 100 graphs that patterns started to emerge and I was able to draw any conclusions from the data. This conversation allows me to make another one
You don't hear much difference because its below our range of hearing, but you can actually see the low frequency cone waffling away the power. The scope shows the midrange riding on the large subsonic waveform. This makes it hard to get good midrange tone at higher power.


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Post subject: Re: Road worn 1974 Super Reverb restoration
Posted: Wed Aug 19, 2015 12:17 pm
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here she is as of today
still only two original CTS speakers 50th strat into Route 66 comp only and then into the S/R
Forgive the missed notes, I was trying to keep the phone from falling off the table so...................

https://youtu.be/dps_ZLJIWhc

I did order new 25uf 25V lytics for the board today and a 100uf 100V for the bias board
might as well make it fresh across the board eh !

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Post subject: Re: Road worn 1974 Super Reverb restoration
Posted: Wed Aug 19, 2015 12:34 pm
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TimsAudio wrote:
You don't hear much difference because its below our range of hearing, but you can actually see the low frequency cone waffling away the power.


That's one of the difficulties when trying to assess the quantitative results of such a comparison -- those differences, while measureable, may be less perceptible to the human ear than they are when viewing them on a spectrum analyzer. But you've done good work here and given many of us food for thought.

Arjay

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