Choosing an Inductor for a Bass Control

[bob p]

I'm trying to restore an old Gibs on amp from the 1940s that someone else has hacked on. I've got it pretty much back together except for the tonestack, and I've got a question about that that I'd like to run by you.

Here is the relevant stage in the circuit:


The tone stack uses an inductor in the boost-cut circuit for the bass control, and no value is indicated on the schematic. There's only a house number that doesn't translate to anything.

My question is what value of inductor should be chosen for the job. I've crunched some numbers, and I'm getting answers in the range of 5 Hy. In addition, that answer makes the part particularly difficult to find. I'm wondering if any of you would have a different opinion with respect to the value of the inductor.

I got the answer by calculating the reactance of the treble side of the circuit, setting both sides equal, and solving for L.

Opinions?

[Chuck H]

I'm somewhat formula impaired. Not to mention that it's easy to miss a specific parameter in some circuit designs.

Notice that the signal for the tone stack is operating as negative feedback off a (relatively low impedance) floating cathode circuit. Without doing any math I'm immediately reminded of the Torres midrange control for guitars that operates with a similar source impedance and control pot value. The inductor used in that circuit is 1.5H and manages the mid cut to preserve the LF. The Torres circuit uses taps on a Xicon TL021 transformer (available through Mouser) to achieve this value. That's probably where I'd start and just see how it sounds.

[bob p]

Thanks for your insights, Chuck.

FWIW I already checked Mouser for a 42TL024 transformer, which Steve Bench says has a series inductance of 5H if you place all of the elements in series. (That 5H value matches my calculations.)

Mouser Lists two options:
42TL024 - Obsolete
42TL024-RC - Special Order

Unfortunately the Xicon data sheets don't say anything about inductance, so I have to take the gamble that Steve's old measurements on the obsoleted transformer that is no longer available actually carry over to the new Xicon product which has replaced it.

Another problem is that Mouser doesn't actually carry either of these Xicon transformers. The Mouser site lists the first TL024 as Obsolete and the second TL024-RC as a non-stock item that has to be special ordered in quantity. I haven't pursued a special order from Mouser. I've been looking around at other suppliers and I can't seem to find them in-stock.

But that's sort of putting the cart before the horse, as I'm not even sure that it's the right part. I'm thinking that the suggested value of 1.5H would give me f3=800Hz for the bass control, which would make it more like a midrange control. I'm thinking that I need much higher L, but I'd really like someone to check my calculations.

Regarding the Torres kit: Torres doesn't seem to have his midrange kit in stock either.

Super Midrange and Tone Switch - OUT OF STOCK

Even if it was in-stock, I'm not sure I'd want to pay $50 to $75 to source a $2 transformer.

aargh.

update: Mouser does seem to have the TL021 in stock. Is that the one that Torres uses in his mid control kit? I'm wondering where you got the 1.5H value, since the inductance is not listed in the Xicon data sheet.

[TigerAmps]

Another option is: Q-filter

I will warn you - it will take a few weeks for him to wind it up for you.
I used a 3H in an Acoustic 126 and mounting it flat in the bottom of the chassis, it was silent.

Orientation of the inductor (or transformer) will make a huge difference in the amount of hum induced.

Otherwise - these are hard to come by unless you are willing to wind them your self.

[Leo_Gnardo]

Orientation of the inductor (or transformer) will make a huge difference in the amount of hum induced.

I've found this to be definitely true. Also applies for those Ampeg bass amps with midrange toroid inductors. If minimum hum/buzz is a goal, you have to experiment, and maybe have to mount the inductor in an odd place with extension leads.

I was lucky some 15 years ago to find a surplus shop that was selling mini pc mount inductors for a pittance, $1 each. Stocked up but now running short on some popular values after having repaired plenty of Acoustic, Mesa/Boogie and some other amps. Somebody, somewhere must have these little pot-core or toroid units available. I'll bet Mesa does - but I'm also sure they don't want to become the go-to site for everybody that wants a PC mount inductor. Antique/CE offer some intended to be replacements for wah pedals - these may be useful in some applications. And it's always worth a look at Apex Jr surplus shop, you never know what you'll find.

[g1]

Is that an EH-185?
If it is I have one on a way way back burner that I can check sometime next week.

[MarkusBass]

Do you have the core that was used in the amp? The size of the core could tell a little bit about the inductance. Ampeg used 0.8H for mid control. I'm afraid that for bass control you need more than 5H. I did some tests and I needed 10H, or even 15H to get proper bass control. Are you sure that other values on the schematic are correct (assuming that "500M" means "500k")? Just for test you may use a power supply choke from a tube amp. Some of them are 10H. What about other Gibson schematic from that era? Maybe the inductor value is specified there.

Mark

[rjb]

Maybe roll your own?
I have a hunch these are the cores used for Q-Filters, but there aren't many in stock.
Not enough to meet the $25 minimum order.
Sorry I haven't found an alternate source. Oh well.

https://www.cwsbytemark.com/index.ph...&cPath=206_231

[Mike Sulzer]

Is that an EH-185?
If it is I have one on a way way back burner that I can check sometime next week.

Must be. That sure looks like a piece out of a EH-185 schematic. You probably can measure the inductance without disconnecting it if you have the right instrument.

[Mike Sulzer]

Do you have the core that was used in the amp? The size of the core could tell a little bit about the inductance. Ampeg used 0.8H for mid control. I'm afraid that for bass control you need more than 5H. I did some tests and I needed 10H, or even 15H to get proper bass control. Are you sure that other values on the schematic are correct (assuming that "500M" means "500k")? Just for test you may use a power supply choke from a tube amp. Some of them are 10H. What about other Gibson schematic from that era? Maybe the inductor value is specified there.

Mark

Both the EH-185 and EH-195 schematics list it as a GQ13. The question has been asked here before and on other forums with no definite answer. Values of 10 to 20 H are mentioned. I would take the simple minded approach and assume that it is 13 H, but I like the ideas of trying a power supply choke.

[MarkusBass]

I would take the simple minded approach and assume that it is 13 H.

Yes, I don't know why I didn't think of this possibility. Instead, I just simulated the circuit with different values of the inductor. The bass pot is set to 0.1 of the total value. As you can see with 5H the circuit changes the frequency response even at 3-4 kHz. Starting from 10H the frequency response becomes reasonable and with 13H looks quite good. But I wonder what size was the core to get 13H inductor. Does anyone has a photo of the amp guts?

Mark

[bob p]

g1 -- yes, that's it. If you could get an actual inductance measurement from the part that would give the right answer.

Mark -- I don't have the original core. The amp had been hacked on, the tone circuit ripped apart and the original part is gone. I have no way to verify the numbers on the schematic are correct as the tonestack circuit on this amp is gone. Maybe we could corral g1 into verifying that the numbers when he looks inside of his amp. It's easy to confirm just by looking at the schematic that M needs to be interpreted as the Roman numeral, not as the SI abbreviation... otherwise the parts values in the amp just don't make sense.

I'm not surprised that the values could be as high as 20H. Before doing any math I was silly enough to guess that a garden variety inductor in the mH range would work, but doing the simple approximation comparing reactances implied the result would be in the range of whole numbers. At least that put me in the right ball park.

Thanks for the simulation. That helps to remove some of the guesswork.

[trobbins]

The shunt capacitance may be quite influential, allong with ESR, for the actual frequency response. The ESR would likely be quite high for fine wire with those anticipated H values.

[Enzo]

Resistance would affect the Q rather than the frequency, wouldn't it?

[trobbins]

Yup, frequency response 😉