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Post subject: Re: Transformer Inductance vs Bass in old BF/SF Amps
Posted: Sat Jan 23, 2016 7:42 pm
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The only possible component of a loudspeaker that *might* affect a quantifiable electrical specification as it ages would be the motor, ie: the magnet. And while it's true that the flux field will diminish over time, it would likely require a hundred years or more before such a loss would be measureable.

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Post subject: Re: Transformer Inductance vs Bass in old BF/SF Amps
Posted: Sat Jan 23, 2016 7:55 pm
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Maybe voice coil and air gap changes with time & use?


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Post subject: Re: Transformer Inductance vs Bass in old BF/SF Amps
Posted: Sat Jan 23, 2016 9:25 pm
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BMW2002Ti wrote:
Maybe voice coil and air gap changes with time & use?


If you think about that long enough you'll eventually discover the folly in such a hypothesis.

Arjay

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Post subject: Re: Transformer Inductance vs Bass in old BF/SF Amps
Posted: Sat Jan 23, 2016 10:49 pm
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Retroverbial wrote:
BMW2002Ti wrote:
Maybe voice coil and air gap changes with time & use?


If you think about that long enough you'll eventually discover the folly in such a hypothesis.

Arjay


Actually, I suspect it is possible that changes in how pliable the surround and spider are may well affect impedance.

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Post subject: Re: Transformer Inductance vs Bass in old BF/SF Amps
Posted: Sun Jan 24, 2016 1:01 am
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Prove it.

Arjay

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Post subject: Re: Transformer Inductance vs Bass in old BF/SF Amps
Posted: Sun Jan 24, 2016 2:05 am
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Retroverbial wrote:
Prove it.

Arjay


Search for:
SRS_Loudspeaker_Parameters
from Purdue.

The relevant part states that "Q(ms) (which is the mechanical Q of a driver) has its main effect on impedance" Q(ms) is determined by losses in the suspension (spider and surround).

I thought I read that decades ago. Had to google it to make sure.

If you can decipher a google url, this is the search result.

https://www.google.com/url?sa=t&rct=j&q ... Ww&cad=rja

My search was "does the weight of a speaker cone affect impedance"

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Post subject: Re: Transformer Inductance vs Bass in old BF/SF Amps
Posted: Sun Jan 24, 2016 7:47 am
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Great substantiation.

There's a wealth of other usable data contained in that report as well.

Thanks for posting.

Arjay

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Post subject: Re: Transformer Inductance vs Bass in old BF/SF Amps
Posted: Sun Jan 24, 2016 11:32 am
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Arjay, that's what the guy was saying. His inductance and impedance was different between new irons and speakers versus same model equipment, after several years of use. Not sure how that changes the tone and response, but maybe the reason measured values change over time.
:?: :D


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Post subject: Re: Transformer Inductance vs Bass in old BF/SF Amps
Posted: Sun Jan 24, 2016 1:42 pm
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Probably so. While the measurement(s) of each component within an older amp may vary only slightly from spec, the cumulative effect is bound to be audible to some degree. It's why we all love the tone from a '64 Deluxe Reverb while that of a '77 model may be likened to a batch of toll house chocolate-chip cookies that's slightly under-baked.

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Post subject: Re: Transformer Inductance vs Bass in old BF/SF Amps
Posted: Sun Jan 24, 2016 9:06 pm
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OK, I edited my update to include response and impedance curves.
I also clarified the test instructions to include disconnecting the speaker and ensuring that the secondary of the OT is open.
Any load lowers the inductance readings. Likewise, the capacitors seem to charge up a little and lower the readings. Take inductance measurements quickly.
I use dummy loads for response curves. The speakers throw off the measurements.
But just for kicks, I included an impedance curve for the speaker array and an individual driver.
Its interesting to see the resonant peak disappear in the array. I didn't know it did that.
Here's another look
Image


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Post subject: Re: Transformer Inductance vs Bass in old BF/SF Amps
Posted: Sat Feb 20, 2016 6:03 pm
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Update Feb 20,2016
I have been looking for a SF Super Reverb for comparison, but as it happened, next up was a
Reissue Super Reverb. It’s a PR469, S# CR254103
OT marks are A036485 EIA 606 MS 2-10
Core size 3.75 X 3.125 X 1.25
Primary resistance 96 Ohms
Primary Inductance 6.88 H
This tracks closely with the original BFSR that I previously examined. The tone balance tracks closely as well. I still need a SF version to document.

I also ran into a few Twins to measure. Two SF and a 65 Reissue AB763. I still need a BF for comparison, but these proved enlightening anyway..
Silverface Twin S# A980135 Master/Pull Boost Ultra linear OT 135W
OT marks are 013691
Core size 3.75 X 3.125 X 1.75 Double stack laminations
Primary resistance 74 Ohms
Primary Inductance 19 H

Silverface Quad Reverb S# A98069 Master/pull boost
OT marks are 022889
Core size 3.75 X 3.125 X 1.5 Double stack laminations
Primary Resistance 70 Ohms
Primary Inductance 16.7 H

Reissue 65 Twin Reverb-Amp AB763 S# AC 038939
OT marks are 91032086, B037612 EIA 606 MS 2-27
Primary resistance 79 ohms
Primary Inductance 7.8 H

I know its too soon to make any definitive conclusions, but I’m seeing a trend to high inductance in the SF and low inductance in the BF and Reissues. However, an incomplete check on another reissue Twin yielded a mid value.

Reissue Twin PR186
Primary Resistance 76 Ohms
Primary Inductance 10.4 H

Other 50-watt amps I’ve tested:
Mesa Nomad 55 80 Ohm, 15H
Mesa DC-5 82 Ohm 14H
Peavey Valve King VK112 128 Ohm 15.7H
Marshall DSL40C 138 Ohm 9.76H
Crate Vintage Club 50 180 Ohm 9H

A high resistance, to me, indicates a smaller diameter wire being used in the primary. This allows the core size to be smaller and saturate out the bass better.
We’ll keep updating this thread as I get more data.
In the meantime, here’s a few pics of the Quad-Reverb. I never knew they existed until it showed at my doorstep. It uses a stock Twin SF chassis with 16W utah 12” speakers @ 4 Ohm.

Image

Image

Image

Image

Image

Image

The grille looks a little funny as the previous owner put 1' foam in to tone it down. I cut 4" holes in front of each driver to let a little treble out. Now it sounds balanced and smooth, even up close.
I've been wanting to try this mod for a while and it does tame the amp down for small venues.


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Post subject: Re: Transformer Inductance vs Bass in old BF/SF Amps
Posted: Sat Feb 20, 2016 9:26 pm
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Albert Collins was a big fan of the Quad Reverb. He's the only QR champion I can think of.

Arjay

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Post subject: Re: Transformer Inductance vs Bass in old BF/SF Amps
Posted: Sat Feb 20, 2016 9:55 pm
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That must be the loudest combo amp ever made. And the heaviest...

:lol:


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Post subject: Re: Transformer Inductance vs Bass in old BF/SF Amps
Posted: Sat Feb 20, 2016 10:54 pm
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BMW2002Ti wrote:
That must be the loudest combo amp ever made. And the heaviest...

:lol:


I'd bet the Super Six was heavier. I know it was more cumbersome!

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Post subject: Re: Transformer Inductance vs Bass in old BF/SF Amps
Posted: Sat Feb 20, 2016 11:24 pm
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thompal wrote:
I'd bet the Super Six was heavier. I know it was more cumbersome!


+1000

So damn heavy they didn't even bother with a top-mounted handle -- it had a pair of them on one side instead. It should've had an OSHA warning label affixed to the cab specifying "TWO-MAN LIFT".

:lol:

Arjay

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