Choosing an Inductor for a Bass Control

[loudthud]

Hammond sells several small chokes in the right range.

154E, 20H, 1666 Ohms, 20mA
155C, 60H, 2750 Ohms, 8mA
156C, 150H, 3700 Ohms, 8mA

[trobbins]

g1, were you able to change the measurement frequency for the inductance test, or use the phase angle or Q to get an idea of where the self resonant frequency was?

It looks like the excitation voltage across the inductor could get up to a few volts. The inductance may change a fair bit on an old iron-core, which I guess isn't an issue except when trying to correlate a meter reading where the meter excitation voltage isn't known.

[MarkusBass]

Hammond sells several small chokes in the right range.

154E, 20H, 1666 Ohms, 20mA
155C, 60H, 2750 Ohms, 8mA
156C, 150H, 3700 Ohms, 8mA

Somewhere in the description there was information that this was humbucking type of inductor. I don't know what that means but most probably there were 2 windings. I would investigate it because Hammond chokes may not be correct for this task.

Mark

[Mike Sulzer]

It is a complicated circuit. I think the bootstrapping at the grid makes the input impedance at the cathode look fairly high. Then the feedback from the plate in the cut position alters the location of the pole making the cut/boost more symmetrical. But I would not bet my life on it.

1) yes, it´s cut/boost.
It cuts when shunting highs or lows to ground from 6J7 plate, boosts when grounding 6J7 cathode.

2) it is not symmetrical, neither in amount of boost or cut nor, much more important, *frequency* : when boosting C and L are bypassing 10k ohms; when cutting they re bypassing whatever impedance is on 6J7 plate, which is not plain 100k since they are in parallel with 6J7 Rp, ots internal plate resistance.
Since it´s connected as a triode, it´s relatively low.

Don´t know its value but a similar 12AX7 triode stage has 100k in parallel with internal 68k (read it in some datasheet) so effective plate impedance is around 40k.
Don´t think we have a way too different value there, but in any case it´s easy to measure.

3) There is also another "invisible" resistor there, one in series with 6J7 cathode.
Again can be calcuated but measuring is easy and accurate for that tube, instead of a generic one.

Momentarily disconnect the tone control by lifting one end of the .5 cap and the wire going to .1-6N7 grid.
Momentarily add a 500k resistor from grid to ground so it does not lose bias.

Then inject 100mV 440hz tone in 6J7 grid, measure audio on its plate, I guess you´ll have around 5X gain.

* Measure max gain/boost (and internal cathode resistance):

Ground .5uF cap , what does plate signal rise to?

Say it rises 4X , then max boost is 4X=12dB
And internal resistance is 10k/4=2500 ohms.

4) Now you can also trace tone control curve and turnover frequency:
It will start rising above 10k-.01uF or 1600 Hz, a very reasonable value.
It will probably shelve at some 4x that, or 6 kHz, again very reasonable for a guitar.

The inductor "should" probably boost below, say, 800 or 1000Hz to usefully match the Treble control range, so calculate what inductance value shows some 10k impedance at 800 or 1000Hz and there you have it.
I bet it´s quite a hairy inductor.

I´m analyzing this from a purely functional point of view and quite certain that the original designer must have travelled a similar path.

5) * Now analyze Cut:
it will work about the same, but it will be stronger (cutting is easy, boosting not so) and will happen at a different frequency.

Not sure about this, it depends on personal taste, way back then people were used to cutting highs, what simple tone controls do, but must have cherished their hard to get lows (poor speakers and low power) so I don´t see them cutting them off.

Notice that Fender tone controls which became way more popular were basically boost only.

6) another path is to compare it to old Gibson amps, some used a Cut-only Bass control (Matchless amps copied that) , wouldn`t be surprised at all that said passive control turnover frequency was similar to what was used here.

Designers usually have some favourite "pet ideas" and tend to repeat them all over the place, we might call them their "signature".

[Chuck H]

Somewhere in the description there was information that this was humbucking type of inductor. I don't know what that means but most probably there were 2 windings. I would investigate it because Hammond chokes may not be correct for this task.

Mark

That was me. I posted a reference to a thread where a guy looked into it and found that Thordarson made a humbucking inductor just for use in tone control circuits. AFAIK it's never been determined if the unit in the Gibson is, or is not humbucking. Gibson does mount it outside the chassis and it looks to be shielded, so maybe it's not. I don't think the Hammond units are shielded so they would certainly pick up more hum and be more interactive with guitar proximity.

I made a humbucking inductor once by stacking two coils wired in series with inverse physical winding direction and sandwiching a thin copper plate between them to minimize mutual inductance. IIRC this arrangement maintained about two thirds the inductance of the coils. It wasn't done to buck hum though. It was done to stop the EMF from an amp attenuator from feeding back with single coil pickups. Basically, the radiant field from each coil cancelled that of the other. Like a reverse humbucker.

If a humbucking inductor is necessary I think Bob will have to roll his own.

[J M Fahey]

Maybe he can wind a same specs inductor on a ferrite potcore, by their very geometry they are relatively unaffected by eternal magnetic fields, unless you do something gross like sitting them on top of an EI transformer core.

Where it seems to be placed looks like it´s quite a few inches away from large transformers.

[g1]

The humbucking thing is a red-herring (temat zastępczy Mark ). Not your fault Chuck as the guy that posted that info on the vintage amps forum was kind of confusing. The humbucking thing was from a different article about tone controls, not about the Gibson. I can see no evidence physically of this coil having another winding.

g1, were you able to change the measurement frequency for the inductance test, or use the phase angle or Q to get an idea of where the self resonant frequency was?

It looks like the excitation voltage across the inductor could get up to a few volts. The inductance may change a fair bit on an old iron-core, which I guess isn't an issue except when trying to correlate a meter reading where the meter excitation voltage isn't known.

I haven't used this meter a lot, and only on capacitors. The quick test I did would have been at the 1Khz default. I can check at 100Hz and 100Khz when I test further. It has phase angle and Q, but you will need to tell me how to use them to find self-resonant frequency. And any other relevant tests you can think of with this DE-5000 meter. As many have searched for values for this coil, it would be good to get as much documentation as possible on-line. Thanks.

Edit: added meter manual

[bob p]

As a hijack, I'd be interested in hearing you review that meter after you've had some time to get to know it. I have one of the B&K 878B meters that needs to be replaced, because somebody failed to discharge a cap before taking a measurement and trashed the meter. The repair cost is almost equivalent to the repurchase cost, so I'd be interested in hearing about other options.

[bob p]

As many have searched for values for this coil, it would be good to get as much documentation as possible on-line.

YES!!!

Getting good documentation on the coil would be a godsend for those who are trying to restore one of these amps, or for someone who has the masochistic personality traits that would predispose him toward a cloning effort. I'm sure that everyone who has participated in those many threads on this subject would be happy to find as much documentation on the circuit as possible.

But why stop there?

What if someone wanted to follow Juan's recommendation, and design a circuit that has the same tone and feel of the original, using modern circuits instead of going through the headaches of rolling their own inductor? That's where knowing the frequency response behavior of the tonestack would be particularly helpful. If we're trying to get as much documentation as possible on-line, my dream request would involve having documentation of the frequency response curves of the original amp. That might make it possible for someone to fashion a modern circuit that performs the same task, which could take the form of a Baxandall stack or perhaps the gyrator circuit previously suggested.

[g1]

As a hijack, I'd be interested in hearing you review that meter after you've had some time to get to know it.

I don't really have a point of reference aside from comparing to the LC function on my multi-meter, this dedicated tester is much better.
Here's a teardown which may provide some useful info:
https://www.youtube.com/watch?v=ji-UT7HJm0Q

[g1]

Ok, I finally got to do more measurements. If someone wants to figure out the self-resonant freq. from the data, the manual for the meter is in post #37 if needed. Measurements were done at 100Hz, 1K, 10K, and 100Khz.
DC resistance measured 1228 ohms.

100Hz 1K 10K 100K
L 24.5H 17.56H 2.187H 23.6mH

phase angle 76 81.5 -73 -82.6

Q 4 6.82 3.3 7.8

D .248 .146 .302 .128

Rp 62K 750K 454K 116K

[nickb]

Ok, I finally got to do more measurements. If someone wants to figure out the self-resonant freq. from the data, the manual for the meter is in post #37 if needed. Measurements were done at 100Hz, 1K, 10K, and 100Khz.
DC resistance measured 1228 ohms.

100Hz 1K 10K 100K
L 24.5H 17.56H 2.187H 23.6mH

phase angle 76 81.5 -73 -82.6

Q 4 6.82 3.3 7.8

D .248 .146 .302 .128

Rp 62K 750K 454K 116K

Ooohh data

Based on that here is a lumped model that is a pretty good fit at 1 & 10Khz. SRF= 4.5KHz. R2 represents core losses which in practice are frequency dependant.

[Jimmy84]

Hi g1
First of all I would like to thank you for making measurements and sharing with us!
I would like to clarify some points.
In your first measurement (post #30) you say it has 2500ohm DCR.
When you remeasured at post #41, you say it is 1228ohm. So which DCR is correct?
When you did a measurement, did you set a bass potentiometer to the meddle position to exclude any circuit influence to the measurement?
Would you be so kind as to make some measurements of output transformer as well and its core dimensions?
Thank you a lot in advance!

[g1]

I will try Jimmy84. I do not have the unit at home so it may take some time. Maybe next week.

[SoulFetish]

If you'll forgive me going off topic for a sec, have there been any bob p sightings around the intertoobs at all?
It's been a while, and I was wondering if there was any way of reaching out to him to see how he's doing?